The Greek wordskepsis means investigation. Literally, a“skeptic” is an inquirer. Not all ancient philosopherswhom in retrospect we call “skeptics” refer to themselvesas such. Nevertheless, they all embrace ways of life that are devotedto inquiry. Ancient skepticism is as much concerned with beliefas with knowledge. As long as knowledge has not been attained, theskeptics aim not to affirm anything. This gives rise to their mostcontroversial ambition: a life without belief.

Ancient skepticism is, for the most part, a phenomenon ofPost-Classical, Hellenistic philosophy. The Academic and Pyrrhonianskeptical movements begin roughly in the third century BCE, and endwith Sextus Empiricus in the second century CE. Hellenistic philosophyis a large-scale conversation, not unlike philosophy today. Theskeptics (among them Pyrrho, Timon, Arcesilaus, Carneades,Aenesidemus, and Sextus Empiricus) do engage with Pre-Socraticphilosophy, Socrates, Protagorean relativism, Plato, and perhapsAristotle. But their contemporary and principal interlocutors areEpicureans, Stoics, Cynics, and Megarian logicians (cf. Long 2006, ch.4 and 5).

• 1. The Central Questions
• 2. Skeptical Ideas in Early and Classical Greek Philosophy
  • 2.1 Early Greek Philosophy
  • 2.2 Plato
  • 2.3 Aristotle
• 3. Academic Skepticism
  • 3.1 Arcesilaus
  • 3.2 Carneades
  • 3.3 Later Academic Skepticism
• 4. Pyrrhonian Skepticism
  • 4.1 Early Figures: Pyrrho and Timon
  • 4.2 Aenesidemus, the Ten Modes, and Appearances
  • 4.3 Agrippa, and the Five Modes
  • 4.4 Sextus Empiricus
• 5. Ancient and Modern Skepticism: Transitions
  • 5.1 Augustine: Re-Conceiving Skepticism in a Theological Framework
  • 5.2 Skepticism About Induction
  • 5.3 Subjectivity, and Other Minds
  • 5.4 External World Skepticism
• Bibliography
  • Primary Sources
  • Secondary Literature
• Academic Tools
• Other Internet Resources
• Related Entries

1. The Central Questions

The core concepts of ancient skepticism are belief, suspension ofjudgment, criterion of truth, appearances, and investigation.Important notions of modern skepticism such as knowledge, certainty,justified belief, and doubt play no or almost no role. This is not tosay that the ancients would not engage with questions that figure intoday’s philosophical discussions. Ancient debates addressquestions that today we associate with epistemology and philosophy oflanguage, as well as with theory of action, rather than specificallywith the contemporary topic of skepticism. They focus on the nature ofbelief, the relationship of belief to speech and action, and themental states of inquiry (on the role of ancient skepticism throughoutthe history of Western philosophy up to today, cf. Lagerlund 2020; onthe relation between ancient skepticism and philosophical debatestoday, cf. Machuca and Reed 2018, Vlasits and Vogt 2020, and Machuca2022).

From the point of view of the ancient skeptics, assertions areexpressions of dogmatism. And yet, the best-known ancient skeptic,Sextus Empiricus, wrote extensively. Can the skeptics say anythingmeaningful about their philosophy without asserting anything about howthings are (Bett 2013, 2019)? Skeptical writings have a peculiarformat, one that comes with its own challenges: the skeptics aim todescribe their philosophytout court, as they practice it,without laying out any particular theories or doctrines. Skepticalideas have been charged with a family of objections: they might beself-refuting, inconsistent, self-contradictory, and so on (Castagnoli2010). Another line of objection is associated with Hume, namely that“nature is always too strong for principle.” As Hume putsit, “a Pyrrhonian cannot expect that his philosophy will haveany constant influence on the mind” (part 2 of section 12,“Of the Academical or Sceptical Philosophy,”AnEnquiry Concerning Human Understanding, London, 1748). It is onething for skepticism to be coherent. It is another thing for it to belikely that anyone, no matter how much they rehearse skepticalarguments in their mind, will succeed in adhering to it (Johnson2001), as ancient Pyrrhonist philosophers claimed to be able todo.

Like later epistemologists, the ancient skeptics start from questionsabout knowledge. But discussion quickly turns to beliefs (Fine 2000).The Greek term translated here as belief,doxa, can also betranslated as opinion. The root ofdoxa isdokein,seeming. In a belief, something seems so-and-so to someone. But thereis also an element of judgment or acceptance. The relevant verb,doxazein, often means ‘to judge that something isso-and-so.’ Hellenistic discussions envisage three attitudesthat cognizers take to impressions (how things seem to them): assent,rejection, and suspension of judgment (epochê).

Suspension is a core element of skepticism: the skeptic suspendsjudgment. However, if this means that the skeptic forms no beliefswhatsoever, then skepticism may be a kind of cognitive suicide.Arguably, belief-formation is a basic feature of human cognitiveactivity. It is not clear whether one can lead an ordinary human lifewithout belief, or indeed, ancient opponents of the skeptics say,whether one can even survive. Perhaps even the simplest actions, suchas eating or leaving a room without running into a wall, involvebeliefs (Annas-Barnes 1985, 7; Burnyeat 1980). It is also hard to saywhether someone who succeeded in not forming any beliefs couldcommunicate with others, whether they could engage in philosophicalinvestigation, or whether they could even think at all.

The ancient skeptics are well aware of these objections. The mostwidely discussed charge is that they cannot act without belief(Apraxia Charge). In response, the skeptics describe their actionsvariously as guided by the plausible, the convincing, or byappearances. The notion of appearances gains great importance inPyrrhonian skepticism, and poses difficult interpretive questions(Barney 1992). When something appears so-and-so to someone, does thisfor the skeptics involve some kind of judgment on their part? Or dothey have in mind a purely phenomenal kind of appearing? The skepticalproposals (that the skeptic adheres to the plausible, the convincing,or to appearances) have in common their appeal to something less thanfull-fledged belief about how things are, while allowing somethingsufficient to generate and guide action.

However, the claim that ancient skepticism is about belief, whilemodern skepticism is about knowledge, needs to be qualified. Ancientskepticism is not alone in being concerned with belief. Descartesspeaks repeatedly of demolishing his opinions (for example,Med 2:12,AT 7:18; cf. Broughton, 2002,33–61). Contemporary epistemology often pays equal attention tothe notions of both belief and knowledge. The distinctive focusof ancient skepticism on belief becomes clearer once we consider athird concept that figures centrally in ancient discussions: the criterionof truth. The skeptics and their opponents discuss how onerecognizes a true impression as true. Are there some evident things(some kind of impressions), which can be used as standards orcriteria, so that nothing is to be accepted as true if it is not inagreement with these evident things? The Stoics and Epicureansformulate theories that conceive of such criteria. The skepticsrespond critically to their proposals. Accordingly, the conception ofa criterion of truth assumes as central a role in ancient debates asdoes the notion of knowledge in modern discussions. This debateincludes in-depth analysis of sense perception and its relation tobelief. According to Epicurus, all sense perceptions are true, butjudgments based on these perceptions are true or false (Striker 1977,Vogt 2017). The Stoics explore differences between sense perception,illusion, and hallucination (Vasiliou 2019). Their account of thecriterion of truth starts from perceptual impressions that qualify, orfail to qualify, as cognitive (Shogry 2018). The Stoics propose thatwe should accept only cognitive impressions, and accordingly we shouldonly form beliefs based on a subset of true perceptualimpressions.

Discussion of the criterion of truth arguably also covers some of theground that is later discussed in terms of certainty. The Stoics saythat a particular kind of impression is the criterion of truth: thecognitive impression. Cognitive impressions make it clear throughthemselves that they reveal things precisely as they are. This notionis an ancestor to the later conception of clear and distinctimpressions, and thus, to discussions of certainty.

Consider next the notion of doubt. Doubt is often considered thehallmark of skepticism. So how can it be that ancient skepticism isnot about doubt (Corti 2010, Vogt 2014a)? Insofar as ‘todoubt’ means no more than ‘to call into question,’the ancient skeptics might be described as doubting things. However,skeptical investigation as Sextus Empiricus describes it does notinvolve doubt (I shall focus here on Pyrrhonism; on Cicero’s useofdubitari, see Section 3.3). Skeptics find themselvesstruck by the discrepancies among impressions. This experience isdescribed as turmoil (Machuca 2019). They aim to resolve thisdisturbance by settling what is true and what is false among them. Butinvestigation leads them to suspension of judgment, which brings itsown peace of mind (Outlines of Skepticism [=PH]1.25–30). Where in this account should we locate doubt? Is theinitial turmoil the ancient skeptic experiences a kind of doubt? Arethe ancient skeptic’s investigations a kind of doubting? Shouldwe describe suspension of judgment as a kind of doubt? All threestages may resemble doubt, at least insofar as the ancient skepticshave not settled on answers to the questions they investigate. But allthree stages are also different from doubt as it is conceived in laterepistemology. The ancient skeptics do not describe themselves asmaking an active effort at doubting what ordinarily they wouldbelieve, as some philosophers in the Cartesian tradition have it.Instead the ancient skeptics find themselves in turmoil because ofdiscrepancies in how things strike them. Moreover, the progressionthat ancient skeptics describe differs from the doubt-belief modelthat later thinkers tend to employ. The ancient skeptics improve theirpsychological condition by moving from turmoil to suspension ofjudgment, not by removing doubt. It seems best, then, to refrain frominvoking the modern conception of doubt as at all fundamental in thereconstruction of ancient Greek skepticism.

Some of the distinctness of ancient skepticism lies in the fact thatit is developed by philosophers who genuinely think of themselves asskeptics. In later epistemology, skepticism is largely construed fromthe outside. In particular, early modern skepticism is, for the mostpart, conceived by philosophers who aim to refute it. But ancientskepticism is explored by skeptics, and that is, by philosophers whointend their lives to be reflective of their philosophy (Cooper 2012ch. 5.5–7; Bett 2013b). Socrates raises the challenge that itmight be truly bad (for one’s life, for the state of one’ssoul, and so on) to base one’s actions on unexamined beliefs.For all one knows, these beliefs could be false, and withoutinvestigation, one does not even aim to rid oneself of false belief,which is admittedly a bad thing for one’s soul. Only an examinedlife is worth living (Cooper 2007). Once we take this challengeseriously, as the ancient skeptics do, we embark on a kind ofinvestigation that is seen as directly relevant to our lives. Ourbeliefs are assumed, at this pre-skeptical phase, to be guiding ouractions. Confidence in unexamined views seems misplaced. Othersregularly disagree with us. In favor of each view, some argumentscan be adduced, some practices invoked, some experiences cited. Theseconflicting arguments, practices and experiences need to be examined.But that just raises further views that are in conflict. As aconsequence, suspension of judgment on every such question looksrationally mandatory. But it is also rational to persist ininvestigation. The skeptic is committed to a search for the truth, onvirtually all questions, even if this search repeatedly andpredictably leads to suspension of judgment (Cooper 2012).

2. Skeptical Ideas in Early and Classical Greek Philosophy

2.1 Early Greek Philosophy

The early Greek philosophers develop distinctions between reality andappearances, knowledge and belief, and the non-evident and theevident. These distinctions form the framework in which skepticism canbe conceived. The idea that truth is seen and knowledge gained fromsome perspective outside of the ordinary ways of mortal life, and thatmortals rely on something lesser, be it the hear-say of fame, orsigns, or appearances, runs through much of early Greek thought.However, few early Greek thinkers seem to have had skeptical orproto-skeptical inclinations. Xenophanes and Democritus are perhapsthe most prominent apparent exceptions.

Xenophanes famously insists that all conceptions of the gods areanthropomorphic and culturally contingent (DK 21B14, B15). TheEthiopians pray to gods who look like Ethiopians, the Thracians togods who look like Thracians (B16). If horses and cows had hands, thehorses would draw pictures of gods that look like horses, and the cowswould draw gods as cows (B15). Xenophanes puts forward a number oftheological theses of his own. But he says that no man will know theclear truth about such matters. He makes a point that has lastingrelevance in discussions of skepticism: even if someone succeeded insaying something that actually is the case, he himself would not knowthis. Thus, all is belief (120: B34) (cf. Sassi 2011 oninterpretations of Xenophanes that influence the history ofskepticism).

Atomism—a theory which thrives in Hellenistic times as thephysical theory of Epicureanism, and is thus an interlocutor ofskepticism—leads into difficult epistemological questions. Theatomist can argue that sense perceptions are explicable as complexevents, initiated by objects each one made up of a lot of atomsfloating in the void, from which atoms proceed and traverse theintervening space, and affect the senses. It is certain objects, madeup by the atoms proceeding from the objects in question (filmy images)that we actually perceive. We neither perceive ‘realreality’ (atoms and void), nor even macroscopic objects andtheir properties (for example, a square tower). Democritus seems tohave argued along these lines (SEM 7.135–9; cf. fr. 9,SEM 7.136; Theophrastus,De Sensibus 2.60–1,63–4), and accordingly his atomist view of perception can beseen as grounding a kind of proto-skepticism.

Democritus’ student Metrodorus of Chios says at the beginning ofhis bookOn Nature “None of us knows anything, not eventhis, whether we know or we do not know; nor do we know what ‘tonot know’ or ‘to know’ are, nor on the whole,whether anything is or is not” (Cicero,Acad. 2.73;trans. Lee (2010) = DK 70B1; SEM 7.48, 87–8; Eusebius,Praep. evang. 14.19.9). This formulation reflects awarenessof the fact that a simpler statement than the one reported in Cicero,such as “there is no knowledge,” can be turned againstitself. In particular, Metrodorus recognizes the role thatunderstanding concepts plays in any such statement. Does its proponentknow something, merely by virtue of understanding what the terms sheuses in her philosophizing refer to? Sextus presents Metrodorus’pronouncement as related to an enigmatic idea that he ascribes to twoother philosophers, the Democritean Anaxarchus of Abdera and the CynicMonismus (SE M 7.87–88). Both are said to have likened existingthings to a stage-painting. This comparison, which Burnyeat capturesin the catch-phrase “all the world’s astage-painting,” is open to a range of interpretations (Burnyeat2017). For Monismus, Burnyeat argues, it is likely to have had amoralistic upshot, along the lines of “all is vanity.” InDemocritean philosophy, and insofar as later skepticism recalls thesaying, it is bound to be a proposal in epistemology (or possiblyepistemology and metaphysics).

The 5^th century sophists develop forms of debate which areancestors of skeptical argumentation (Bett 2020b). They take pride inarguing in a persuasive fashion for both sides of an issue andin developing an agonistic art of refuting any claim put forward.Further, the sophists are interested in the contrast between natureand convention. The formative roles of custom and law were discussedby some of the earliest Greek authors (consider Pindar’s“law is king” and its many interpretations, for example inHerodotus). The sophists explore the idea that, if things aredifferent for different cultures, there may be no fact of the matterof how those things really are. The skeptics engage with both legs ofthe distinction between nature and convention. Pyrrhonian skepticismemploys an argument to the effect that, if something is by natureF, it isF for everyone (affects everyone asF) (see sections 4.2 and 4.4). Pyrrhonism further associatesconvention with appearances, so that the sceptic, by adhering toappearances, can lead an ordinary life (see section 4). However, thecontrast between nature and convention does not figure importantly inancient skepticism, and there is no skeptical school that wouldconfine itself to ‘moral’ skepticism, or skepticism aboutvalues.

Sextus Empiricus, whose writings provide the most detailed extantaccount of Pyrrhonian skepticism, emphasizes how strongly the skepticsdepart from all other philosophers. As he presents it, thePre-Socratics who put forward some of the views cited above are whathe calls dogmatists. They make claims about nature, reality,knowability, and so on. The second-most detailed extant account ofPyrrhonian skepticism is offered by Diogenes Laertius, in hisLives of Eminent Philosophers 9.61–116. It contains alarge number of references to early Greek poets and Pre-Socraticphilosophers, suggesting that these early thinkers formulate ideasthat are similar to skeptic ideas (§§61–73). It iscontroversial whether the skeptics Diogenes has in mind claim thisancestry. Alternatively, other philosophers may have charged theskeptics with sharing ideas with non-skeptical thinkers, therebydeparting from their non-dogmatic approach (Warren 2015). Either way,two observations seem relevant. Contrary to a classic assessment ofDiogenes’ report by Jonathan Barnes (1992), the sections of textin which connections are made between early Greek thought andskepticism may be worth exploring. Moreover, scholars may have paidtoo little attention to skepticism’s ancestry in poetry (Clayman2009). Pyrrho seems to have referred to Homer as a proponent of ideashe approves of, ideas about change, the status of human rationalityand language, and more.

2.2 Plato

The Socrates of Plato’sApology tries to solve apuzzle. The Delphic oracle says that no one is wiser than Socrates.But Socrates does not think himself wise. He calls into question thetruth of neither the oracle’s pronouncements nor hisself-perception. Accordingly, he must figure out how both areconsistent with each other. In order to do so, Socrates talks tovarious groups of experts in Athens: politicians, poets, andcraftsmen. As it turns out, all of them think that they know somethingabout great and important things, but in fact, it seems clear toSocrates, they do not. When asked they cannot provide reasons forbelieving the things they claim to know that are rationallysatisfactory. Socrates knows that he does not know about these mostimportant matters (megista; 22d); his interlocutors, itappears, do not know that they lack this knowledge. In this respect,Socrates is wiser than everyone else who has any general reputationfor wisdom. In the course of recounting his conversations with others,Socrates says something enigmatic: “About myself I knew that Iknow nothing” (22d; cf. Fine 2008). Some readers (ancient andmodern) take Socrates to profess that he knows nothing. But thecontext of the dialogue allows us to read his pronouncement asunproblematical. Socrates knows that he does not know about importantthings. He advocates the importance of critically examiningone’s own and others’ views on important matters,precisely because one does not know about them (Vogt 2012a, ch. 1).Such examination is the only way to find out.

Socrates’ commitment to reason—examination as the way tofind out—inspires the skepticism of the Hellenistic Academy(Cooper 2004b, Vogt 2013). One is bound to lead one’s life basedon one’s beliefs, he assumes. Therefore, one ought to examineone’s beliefs, and abandon those that one finds to be false. Oneought to do so because otherwise one might lead a bad life.Socrates’ questioning is rooted in a concern with the good life.Insofar as it is, one might think that the Socratic roots of ancientskepticism lead one toward a kind of limited, wholly moral skepticism.However, Socrates’ examinations are not confined to valuequestions. While ethical questions may be the starting-point, theyimmediately lead to questions about the soul, the gods, knowledge, andso on. For Socrates and his Hellenistic followers, value questionscannot be insulated from questions of psychology, physics, andepistemology.

Another strand of skeptical thought begins with questions about thenature of philosophical investigation. In theMeno, Platoformulates a famous puzzle. How is investigation possible? We cannotinvestigate either what we know or what we do not know. In the formercase, there is no need to investigate, and besides, if we really doknow we already have in mind everything that investigation couldreveal to us about the matter. In the latter case, we would not knowwhat to look for, and we would not recognize it if we found it(80d-86c). So there is no room for investigating anything. Socratescalls this an eristic argument, thus drawing attention to the factthat this is a puzzle that sophists have put forward (cf.Plato’sEuthydemus).

Plato’s solutions to this puzzle are difficult to assess (Fine2014). Learning is recollection, says one proposal (this is the onethat Socrates himself immediately offers). We already know, but onlyin some implicit way, what it takes investigation to come to knowexplicitly. This is the famous Anamnesis theory (81a-d). If we give upon investigation, we shall be lazy people, says another argument(81d-e). A third solution of the puzzle arguably says that one of itspremises is false. It is not the case that, for everything, we eitherknow it or are entirely ignorant of it. Rather, there is a thirdstate, namely belief (83a-86a). Investigation can begin frombeliefs.

TheMeno explores a mix of these solutions. Plato developswhat he calls a hypothetical method (86c-100b). That is, theinterlocutors in some sense begin from their beliefs (for example,“virtue is good”). But they do not endorse them. They setthem up as hypotheses, and employ these hypotheses in investigation(on the relationship between Pyrrhonian skepticism and Plato’sand Aristotle’s notions of hypothesis, cf. Corti 2011). Aspurious dialogue, theSisyphus, discusses a similar puzzle,though with respect to deliberation. Deliberation either involvesknowledge or is mere guesswork; either way, deliberation isimpossible. Here, too, we may ask whether the distinction betweenknowing and not knowing is exhaustive, or whether attention to beliefchanges the picture (Fine 2021, ch. 8).

Plato discusses and re-formulates several of the metaphysicalconsiderations that back up proto-skeptical and early skepticalintuitions. The relevant passages are spread out over a number ofdialogues, among which passages in thePhaedo,Republic,Theaetetus andTimaeus areperhaps most important. In these dialogues, Plato develops some of hisown metaphysical ideas. He also engages critically with metaphysicaltheories that he does not ultimately adopt. However, in order toexplore these theories he formulates them in detail, often invoking aPre-Socratic ancestor as a proponent of a given idea. Thesediscussions are a great source of inspiration for Pyrrhonian skeptics,who are interested in what may be called a metaphysics ofindeterminacy (Bett 2000, Vogt 2021).

In contrasting the Forms with the perceptible realm, Plato discussesproperties. For example, for all sensible items,A is tallrelative to someB and short relative to someC.There is no such thing as anything’s being tallsimpliciter. When we call something tall, we measure itagainst something else, look at it from a particular perspective, andso on. As we might say, “tall” and “short”are relative predicates. But perhaps many more, or indeed all,predicates work that way, even where this is less obvious.Plato’s arguments lead to the question of whether it isconceivable that all our predications are, in this particular sense,relative. If this were the case, it might quite fundamentally upsetour conception of the world as furnished with objects that haveproperties. Such considerations lead to another idea about propertiesin the perceptual realm. If a fence is low and high, a cloud is brightand dark, a vase beautiful and ugly, and so on, then it seems that,perhaps quite generally, perceptual things areF andnot-F. Only the Form ofF isF (forexample, only the Beautiful is beautiful). While the relevant passagesare difficult to interpret, it is clear enough which line of thoughtcomes to influence later skeptics. The skeptics engage with the ideathat, if something appears to beF and not-F, it isnot really (or: by nature) eitherF or not-F.

In theTimaeus, Plato argues that an account of the naturalworld can only be ‘likely’: it is aneikôslogos. Most generally speaking, the idea here is that certainexplananda are such that theorizing about them can do no more thanmirror their, comparatively speaking deficient, nature. This idea hasancestors in Xenophanes and Parmenides, and it plays a crucial role intheTimaeus (Bryan 2012). Academic skeptics employ variousnotions of the plausible and the convincing, thus developing furtherthis tradition, albeit no longer with the assumption that differentdomains require different kinds of theorizing.

In theTheaetetus, Plato explores the kind of culturalrelativism that is associated with some sophists. In his examinationrelativism is extended to a general theory, not restricted to thedomain of values. Socrates (as main speaker of the dialogue) ascribesrelativism to Protagoras, who is famous for saying that “man isthe measure.” Socrates reformulates this claim as follows: whatappears toA is true forA, and what appears toB is true forB. On this premise, Socrates argues,there is no rational way to prefer our perceptions while awake to ourperceptions while asleep, or similarly, to prefer sober to intoxicatedor deranged perceptions. In each state, our perceptions are true forus. Socrates analyzes relativism in several steps, pointing to evermore radical implications. Along the way, he envisages a moderatemetaphysics of flux, where objects do not have stable properties. Buteventually he points out that relativism is committed to an evenstarker revisionist metaphysics, radical flux. For it to be possiblethat what seems toA is true forA, and what seemstoB is true forB, there cannot be a stable worldthatA andB both refer to. Rather,“everything is motion” (Tht. 179c-184b). Theskeptics employ versions of some of the arguments in theTheaetetus, without, however, arriving at relativistconclusions. Schematically speaking, the relativist says that,ifX isF forA andF*forB, thenX isF-for-A andF*-for-B. Pyrrhonian skepticism of the variant foundin Sextus reacts by continued investigation into whetherX isF orF* (or both or neither).

2.3 Aristotle

Aristotle engages, at several points in his works, with theMeno Problem. For example, Aristotle points out that, forsuccessful investigation to proceed, one first needs a well-formulatedquestion. One needs to know the knot in order to untie it. In order toknow what to look for and recognize it when one finds it, one needs tofirst think one’s way through the difficulties involved, andthereby formulate a question. If one does, then one shall be able torecognize the solution once one hits upon it (Met. 3.1,995a24–b4). These ideas are highly relevant to Hellenisticdiscussions. The skeptic is an investigator, and one anti-skepticalcharge says that, if indeed a skeptic knew nothing, they could noteven formulate the questions they investigate. Interpreters haveemphasized a contrast between Aristotle and the skeptics. ForAristotle, formulating puzzles and thinking one’s way throughthem puts one in a better position, such that one gets clear about howmatters are. This may be different for the skeptics: engagement withdifferent angles of philosophical problems leads them to suspension ofjudgment (Long 2006, Code 2010). Other scholars argue that theskeptics’ way of thinking through puzzles improves theircognitive condition even though they do not settle for an answer to agiven question (Vogt 2012a, ch. 5). Also, scholars point to modes ofthinking in Plato and Aristotle that are not primarily concerned withresults. In contemplation, a cognizer may think again and againthrough the same kinds of matters, and yet doing so presumablyimproves their cognitive condition (Olfert 2014).

InPosterior Analytics I.1, Aristotle says that all teachingand learning comes about through things we already know. When wephrase questions, we already have ‘That-Knowledge’ and‘What-Knowledge.’ For example, when we ask questions abouttriangles, we need to know that there are triangles (otherwise wewould not have questions about their properties). We also need to havea notion of what triangles are (we draw a triangle, not a square, whenwe phrase a question about its properties). Another way in whichAristotle addresses theMeno Problem conceives of particularperceptions as the starting-points of investigation. Complex cognitiveactivities arise from simpler ones. Many particular perceptions leadto memory, to experience, and eventually to expert understanding(Met. 1.1,An. Post. II.19). With thegeneralizations of memory, experience, and expertise comes the abilityto investigate. With respect to skepticism, the important point hereis that the starting-points of investigation are not themselves inneed of justification.

Like Plato, Aristotle engages with the Protagorean claim that, asAristotle puts it, all seemings (dokounta) and appearances(phainomena) are true (Met 4.5). If this were so,Aristotle says, everything would have to be true and false at the sametime. Aristotle argues that earlier thinkers arrived at such viewsbecause they identified being solely with the perceptual (4.5,1010a1–3). Caught up in this assumption, they did not see who orwhat was going to judge between conflicting sense perceptions. Forexample, it seemed unsatisfactory to dismiss the views of sick and madpeople simply on the grounds that they are in the minority, therebyconsidering as true what appears to the greater number of people.Similarly, Aristotle reports that these earlier thinkers looked at theways in which things appear differently to different kinds of livingbeings, and to one person at different times (4.5,1099b1–11).

InMetaphysics 4.4, Aristotle notes that some people considerit possible for the same thing to be and not to be, and for someone tobelieve so (he refers to a range of positions, all of which in someway are related to denial of the Principle of Non-Contradiction; seeCastagnoli 2010, I.5.4). Against this, Aristotle says it is thefirmest of principles that things cannot be and not be at the sametime. To deny this shows a lack of training. With adequate training,one recognizes for which things proof should be sought, and for whichit ought not to be sought (see alsoAn. Post. I.3). It isimpossible that there be demonstration for everything. Otherwisedemonstration would go onad infinitum. Scholars often referto this point when discussing the skeptical modes of argument. Theskeptics might be guilty of what, from Aristotle’s perspective,would be a mistake of exactly this kind (on this theme in laterAristotelian logic, cf. Malink 2020).

Aristotle continues in a way that is highly relevant to discussions ofskeptical language and action. A person who wishes to deny that thingscannot be and not be at the same time has two options. Either they saynothing, or they talk to us. In the first case, there is no need forus to refute them. This person is like a plant—they do not talk.In the second case, either their utterance signifies something, or itsignifies nothing. If it signifies something, then they say thatsomething is so-and-so (which Aristotle takes to be self-defeating forthem). If it signifies nothing, then it does not qualify as speech.Even though they make an utterance, the person is in effect notspeaking with us (or to themselves). Aristotle also explains theplant-metaphor in terms of action (1008b10-30). A person who believesnothing is like a plant because they cannot act. Pursuit and avoidancetestify to the fact that people have beliefs. (On Aristotle andskepticism, see the papers collected in Irwin 1995.)

3. Academic Skepticism

3.1 Arcesilaus

With Arcesilaus (316/5–241/0 BCE) and his role as leader of theAcademy (266/268 BCE), Plato’s Academy turns skeptical.Arcesilaus does not refer to himself as a skeptic—thisnomenclature is a later designation. However, Arcesilaus stands at thebeginning of a re-orientation in the history of Platonically inspiredphilosophy. He rediscovers Socrates the examiner, arguably based onhis reading of Plato’s dialogues (Thorsrud 2018).Socrates’ commitment to investigation, to the testing andexploring of one’s own and others’ beliefs, and hispassion for weeding out falsehoods, are the starting-points of hisAcademic skepticism (Cicero,Acad. 2.74, 1.46). Throughoutthe history of this skeptical school, these traits, and thecorresponding commitment to a life guided by reason, remain alive(Cooper 2004b, Vogt 2013). When, as we shall see below, Arcesilausdefends a skeptical life without belief, this is because, as hethinks, reason itself, if properly and faithfully followed, leads usto live that way. To Arcesilaus, the skeptical life is a life livedfollowing reason, a life basedon reason—just as thecompeting Stoic and Epicurean lives are alleged by their proponents tobe. Arcesilaus engages with the epistemologies of these contemporariesof his. In particular, the Academics call into question that there isa criterion of truth, as both Epicureans and Stoics, beginning in thegeneration before Arcesilaus, claim there is. One interpretivequestion that concerns Arcesilaus, but also later Academic skeptics,is especially vexed. According to Seneca (Letter 88,44), the Academics paradoxically claim to know that nothing can beknown. Scholars disagree on how this and similar reports fit togetherwith the picture of Academic skeptics as open-minded inquirers (Allen2017). According to Sextus Empiricus, the Academics skeptics say thateverything is inapprehensible (PH 1.226, cf. PH 1.3). Is“everything is inapprehensible” the single thing that canbe apprehended? Does so-called “non-apprehension”(akatalêpsia) amount to the general claim that there isno knowledge? Alternatively, the Academics may dialecticallyappeal to the Stoic notion ofkatalêpsis, and arguemerely that the Stoic notion of apprehension is not compelling (Allen2022).

Like Socrates, Arcesilaus did not write anything. His views must beunearthed from Sextus’ comparisons between Pyrrhonian andAcademic skepticism, from Cicero’s discussions in theAcademica, and from a range of shorter (and sometimeshostile) reports. Major themes in Arcesilaus’ philosophy are (i)his dialectical method, (ii) discussion of whether there is acriterion of truth, and (iii) his defense of the skeptic’sability to act.

(i) Method. Arcesilaus embraces what scholars call a dialecticalmethod (Couissin 1929 [1983], Perin 2013, Thorsrud 2018). This methodis inspired by Socrates. It proceeds by asking one’s real orimaginary interlocutor what they think about a given question, thenplunging into an examination of their views, employing their premises.Can they explain their position without running into inconsistencies,and without having to accept implications that they want to resist? Asa consequence of this method, it sometimes appears as if a skeptic,while examining someone’s view and its consequences, makes apositive claim: “so, such-and-such is not so-and-so.”However, within a dialectical exchange, this should be read as“according toyour premises, such-and-suchfollows.” This method remains a key ingredient of Greekskepticism. While the different skeptical schools develop variants ofthe dialectical method, skeptical argument is often characterized bythe fact that skeptics think of themselves as engaging with“dogmatic” interlocutors. In the skeptical tradition, asarticulated for example by Sextus Empiricus (see section 4.4.),“dogmatists” are philosophers who put forward, and defend,positive answers to philosophical questions about reality, knowledge,ethical values, etc. They need not do so dogmatically or rigidly orwithout consideration of alternatives in order to count, in skepticalterms, as “dogmatists.”

(ii) The Criterion of Truth. Zeno, the founder of Stoicism, androughly 20–30 years Arcesilaus’ senior, was for a time astudent at the Academy. He was still in the Academy when he formulatedkey Stoic doctrines. Like Arcesilaus, he claims Socratic ancestry.Zeno is inspired by some of the same ideas that inspire the skeptics.In particular, he engages with the Socratic idea that knowledge isintegral to virtue. Contrary to Arcesilaus, Zeno aims to give accountsof knowledge and virtue, and holds them up as ideals that human naturepermits us to achieve. For him, knowledge is very difficult to attain,but ultimately within the reach of human beings. From the point ofview of Arcesilaus, Zeno’s claim to Socratic heritage is almostoffensive: Zeno seems to be too optimistic about our cognitive powersto be following Socrates (Frede 1983). Scholars have traditionallyenvisaged an exchange of arguments between Zeno and Arcesilaus, whereeach modified his views in the light of the other’s criticism.However, Zeno most likely formulated his views between 300–275,and Arcesilaus argued against him c. 275 to 240, when Zeno (who diedc. 263) was probably already retired (Brittain 2006, xiii; Alesse2000, 115 f.; Long 2006, ch. 5).

The core of the dispute between Arcesilaus and the early Stoicsconcerns the question of whether there is a criterion of truth. Thenotion of a criterion is introduced into Hellenistic discussions byEpicurus, who speaks about thekanôn (literallymeasuring stick) andkritêrion. For Epicurus, acriterion is that evident thing, viz., the content of asense-perception, against which claims about the non-evident aretested. For example, physics advances claims about non-evident things,such as atoms and void. These are not accessible to the senses, andaccordingly, do not count as evident. Perception rules out variousphysical theories. For example, a physical or metaphysical theoryaccording to which there is no movement can be dismissed because it isin disagreement with the evident.

Zeno argues that a certain kind of impression—namely a cognitiveimpression (phantasiakatalêptikê)—is the criterion of truth (cf.Shogry 2018). Zeno’s conception of cognition(katalêpsis, literally grasping, apprehension), and therelated notion of a cognitive impression, aim to solve anepistemological problem. Belief-formation aims at the truth; there isa norm inherent in the practice of believing that one should onlybelieve truths. It is not transparent to us, however, which of ourbeliefs, or claims to the effect that such-and-such is true, aresuccessful in their aim and hit the truth. Zeno argues that someimpressions are cognitive. A number of Stoic definitions of thecognitive impression, formulated by Zeno and his successors, aretransmitted. Scholars approach them in at least three ways. First,some scholars consider one of the Stoic definitions (SE M 7.247–52) ascanonical (Nawar 2014, Shogry 2018). Two, scholars at times examinespecific exchanges between Stoics and skeptics in the assumption thatsome clauses in Stoic definitions respond to skeptic objections (forexample, Nawar 2017). Three, scholars have also pointed out that, hereas elsewhere, the Stoics offer a multitude of definitions, withoutprivileging one of them as “the” definition; differentdefinitions may speak to different dialectical contexts or theoreticalfoci (Vogt 2022).

With some simplification, a cognitive impression reveals its contentand that it is kataleptic (cf. Cicero,Academica2.77–8). For example, when I look at my computer screenwhile typing this, I may very well have a cognitive impression thatthis is my computer screen. When I look up and out of the window, Ihave an impression of a friend walking across campus that is probablynon-cognitive. This impression might be true. But since I see her fromsuch a distance, it is pretty surely not cognitive. That is, not alltrue impressions are cognitive, but all cognitive impressions aretrue. We should only assent to cognitive impressions, and so informing our beliefs, hold only those certified beliefs (cf. Brittain2014 on the Stoic view that assent to cognitive impressions is notcompelled).

Arcesilaus calls into question whether there are impressions of thiskind. His main point seems to be that there could be an impressionthat is phenomenologically indistinguishable from cognitiveimpressions, but nevertheless misrepresents the matters it gives animpression about. To use an example that may derive from Carneades(see section 3.2), there is no impression of a given egg such that noimpression of any other egg could be phenomenologicallyindistinguishable from it (Shogry 2018 and 2021, Machek and Veresforthcoming). Assuming that some clauses in Stoic definitions respondto specific skeptic challenges, the following clause, added to ashorter definition, appears to speak to these cases: “andof such a kind ascould not arise from what is not”(Long and Sedley (1987) [= LS] 40; DL 7.46, 54; CiceroAcad.1.40–1, 2.77–8; SEM 7.247–52). Absent acriterion of truth, Arcesilaus’ skeptic suspends judgment abouteverything (PH 1.232). Reason itself, Arcesilaus thinks,demands such suspension.

(iii) Action. If skeptics suspend judgment, argues their dogmaticopponent, they are not able to act. Stoic philosophy conceives ofthree movements of the mind: impression, assent, and impulse(Plutarch,Col. 1122a-d). All three figure in action. Theagent assents to the impression thatA is to be done; theirassent is an impulse for the actionA; if there is noexternal impediment, the impulse sets off the action (Inwood 1985). Itis a cornerstone of Stoic philosophy that there can be no actionwithout assent, and so without the belief that the action done isto be done. The Stoics aim to avoid the kind of determinismaccording to which actions are not ‘up to’ the agent(Bobzien 1998); for them, assent, but not impression, is up to theagent. In response to the Apraxia Charge, Arcesilaus seems to haveargued that the skeptic can act without having assented (Plutarch,Col. 1122A-d), and so without believing that the action doneisto be done. However, this is not his complete response.From the point of view of the Stoics, skeptical action, if performedwithout the relevant kind of assent (that is, assent that it is up tothe agent to give, and that is a rational acceptance of theimpression), is like the action of a non-rational animal, or like theautomatic movement of plants when they grow and flourish. Arcesilausis robbing people of their minds (CiceroAcad. 2.37–9;Obdrzalek 2013). But Arcesilaus need not and does not go so far as tocompare human agents with non-rational agents. As human beings,skeptics have rational impressions. They perceive the worldconceptually, and think about it. Arcesilaus does not suggest thatskeptical action is causally set off by impressions, or in the way,whatever that is, that animal actions are set off. This would be aproblematic proposal, for it would disregard that the skeptic has ahuman mind. Given the complexity of human thought, the skeptic islikely to have several, and often competing impressions. If allimpressions triggered impulses, the skeptic would be inactive due to akind of paralysis. The second component of Arcesilaus’ reply,thus, is that the skeptic, in acting without assenting, adheres to thereasonable (eulogon) (SEM 7.158; 7.150; Striker2010). That is, Arcesilaus aims to explain skeptic action as rationalagency (Cooper 2004b). Arcesilaus disputes the dogmatic claim thatsome impressions can be identified as true, and the related claim thatone can only act on thebelief that some impression is true.But he does not argue that there are no differences betweenimpressions which agents could take into account. His agents arerational: they think about their options, and go with what looks, inone way or another, more plausible.

Arcesilaus defends skeptical action also against Epicurean critics(Plutarch,Col. 1122A-d), again by showing on the basis ofthe Epicureans’ own premises that skeptical action is possible.Can the sceptic explain why, when leaving a room, they go through thedoor rather than running into the wall? Arcesilaus seems to haveexploited the Epicurean view that, while all sense-perception is true,belief can introduce falsehood. Like the Epicurean, the skeptic cankeep apart the perception and a view formed on its basis. By notassenting to the perception, thus adding belief (“here is thedoor”), the skeptic guards against the source of falsehood,namely belief. But a skeptic has perception of the door available tothem, which is enough for not running into walls.

3.2 Carneades

Like Arcesilaus, Carneades (214–129/8 BCE) refrains from writingand philosophizes in a Socratic spirit. Carneades led an embassy ofthree philosophers from Athens to Rome in 156/5 BCE. Aside from hisofficial role, he is said to have given two speeches, arguing forjustice one day and against justice the next. Whether or not this ishistorically correct (Powell 2013), the traditional take onCarneades’ speeches is not that he aims to overthrow justice.Instead, the upshot is taken to be that he wants to show that thesupporters of justice—including Plato and Aristotle—do nothave the successful arguments they think they have to show whatjustice is and what it requires (Lactantius,Epitome 55.8, LS68M). Like Arcesilaus, Carneades (i) engages with Stoic epistemology.His account of skeptical action includes (ii) a detailed proposalregarding the criterion. As part of his less radical skepticism,Carneades seems (iii) to allow for a certain kind of assent, andperhaps for belief.

(i) The Stoic-Academic Debate. Chrysippus, the third major Stoic(after Zeno and Cleanthes), and his student Diogenes of Babylon,revise Zeno’s epistemology, defending it againstArcesilaus’ arguments (Brittain 2006, xiii). In response totheir arguments, Carneades continues the exchange with the Stoics thatArcesilaus began (SEM 7.402–10). His first moveaddresses the link between mental states and action. People in statesof madness, he argues, act just as easily and naturally on theirimpressions as other people, even those who act on cognitiveimpressions (if there are any). From the point of view of exhibitedbehavior, there does not seem to be a difference: any and allimpressions, even those the Stoics think clearly arise from somethingthat is not, are in all respects relevant to action completely on apar. Cognitive impressions, if there are any, have no superiority.

In a second argument, Carneades points to objects that are similar toone another: can the wise person discern any two eggs, two grains ofsand, and so on? The Stoics have multiple replies. It is conceivablethat, in some contexts of action, the wise person assents to what isreasonable (eulogon) (DL 1.177), without having a cognitiveimpression of how things are. Or, if faced with the task to identifygrains of sand while lacking a cognitive impression, the wise personcan suspend judgment. However, the wise will train themselves so as tobe able to perceive minute differences (Cicero,Acad. 2.57),where it might be important to do so. This point is backed up by Stoicphysics: no two items in the universe are identical, and theirdifferences are in principle perceptible (Sedley 1982, 2002).Carneades replies that even if no two things were exactly alike(consistent with his general line of argument, he does not take astance on such questions), a very close similarity could appear toexist for all perceivers (Cicero,Acad. 2.83–5); thatis, the impressions of two items, though in fact these items mightdiffer from each other, could be indistinguishable (Nawar 2017, Shogry2018 and 2021). Discussion continues with a move on the part of theStoics: they add to their definition of the cognitive impression“one that has no impediment.” Sometimes an impressionis—as it were, by itself—cognitive, but is unconvincingdue to external circumstances (SEM 7.253). It is a difficultquestion whether this addition harms the Stoics more than it helpsthem. If the initial conception of a cognitive impression hangs on theidea that something about its phenomenological nature, or somethinginternal to the impression, marks it as cognitive, the Stoics give upon a crucial assumption if they grant that sometimes there are“impediments.” If, however, cognitive impressions aredifferentiated by a causal feature (the way they are caused by the‘imprinter’ which causes the ‘imprint’), thefurther addition might help (Frede 1983, Nawar 2014), since theimpediment might need to be removed before the causal connection couldbe confirmed (Hankinson 2003).

(ii) Carneades’ Criterion. Even though Carneades further pursuesa discussion begun by Arcesilaus, he does not simply continue withinthe framework of Arcesilaus’ skepticism. The distinctiveness ofhis position is best seen in the context of his criterion: thepersuasive (pithanon). The notion of the persuasive can beunderstood in two distinctively different ways. Persuasiveness mightbe a causal feature, so that a persuasive impression sets off aphysiological process of being moved in a certain way. But there mayalso be a rational kind of persuasiveness. Carneades construespersuasiveness in rational terms. For him, the persuasive is theconvincing, or perhaps even the plausible.

Carneades develops a three-stage criterion: (1) In matters ofimportance, skeptics adhere to the persuasive. (2) In matters ofgreater importance, they adhere to the persuasive and undiverted. Apersuasive impression is undiverted if there is no tension between itand its surrounding impressions. (3) In matters that contribute tohappiness, skeptics adhere to persuasive, undiverted, and thoroughlyexplored impressions. A persuasive impression is undiverted andthoroughly explored when it and the surrounding impressions areclosely examined without its persuasiveness being diminished (SEM 7.166–84). Consider an example. A skeptic looks in adark room for a rope. Before they pick up what appears to them to be arope, they look closely and poke it with a stick. Coiled objects canbe ropes, but they can also be snakes. The persuasive impression thatthis is a rope must be examined before the skeptic adheres to it(M 7.187).

The three-stage criterion is put forward in the context of action.However, Sextus describes Carneades’ criterion as a criterion oftruth, not a criterion of action (M 7.173). Carneades mighttake himself to offer more than a practical criterion. His discussionsof the persuasive come close to a general epistemological theory(Couissin 1929 [1983], Striker 1980, Bett 1989 and 1990, Allen 1994and 2004 [2006], Brittain 2001). Cicero renders the Greekpithanon asprobabile (and sometimes asverisimile), which modern editors sometimes translate in terms ofwhat is probable or likely to be true. Some scholars think thatCarneades is an early thinker about likelihoods, and argue that hedevelops a fallibilist epistemology (Obdrzalek 2004).

(iii) Assent and Belief. Does adherence to persuasive impressionsinvolve belief? Carneades coins a term for the kind of adherence hehas been describing: approval (Cicero,Acad. 2.99). Hedistinguishes it from assent in the sense of Stoic and other dogmatictheories, which establishes a belief that something is in actual facttrue; but he nevertheless describes it as a kind of assent (Cicero,Acad. 2.104). Carneades’ disciples disagree on whetherapproval is any kind of genuine assent. That is, they disagree onwhether, in approval, one forms a belief. Philo and Metrodorus thinkthat Carneades allows for some kind of belief, close to or identicalwith belief as the Stoics understand it. Clitomachus disagrees, andCicero follows Clitomachus (Acad. 2.78, see also 2.59, 2.67).Scholars continue to debate these issues (Allen 2022), and the basicproblem remains unchanged. It is not clear whether there is aplausible notion of belief according to which belief falls short of‘holding to be true’ (or according to which, though somekind of ‘holding true’ is involved, the relevantaffirmation-as-true is weaker than in beliefs as Stoics and otherdogmatist epistemologists conceive of them). In any event,affirmations as true, at least of the full and flat-out (“inactual fact”) sort the Stoics think of, are precisely what theskeptic does not make.

Another approach to Carneades’ stance toward belief is to askwhether he might invoke Platonic considerations. Consider thatSocrates, when asked in theRepublic what he thinks the goodis, refuses to reply because he thinks beliefs without knowledge areshameful (Rp. 506c). In response to this, his interlocutorspoint out that there is a difference between putting forwardone’s beliefs as if one knew them to be true, and putting themforward with the proviso that they are merely beliefs (Vogt 2012a, ch.2). The shamefulness of mere belief might disappear through thisproviso. A passage in Cicero’sAcademica suggests thatCarneades invokes this thought. According to Carneades, the wiseperson can hold beliefs if they fully understand them to be beliefs(2.148). Along similar lines, it has been suggested that Carneadesmight conceive of a hypothetical mode of believing (Striker 1980[1996, 112]), perhaps engaging with a move in Plato’sMeno. Investigation cannot get off the ground if we do not,in some sense, begin with our beliefs about the matter underinvestigation. But how can we do so without endorsing our beliefs, notknowing whether our views are true? Plato’s answer at this pointis: by hypothesizing our beliefs. Today we would insist thathypotheses are not beliefs. However, it is conceivable that Carneadesargued along these lines, and that the details of his vocabulary gotlost or confused in doxography.

3.3 Later Academic Skepticism

Carneades was an enigma to his students and immediate successors.Clitomachus (head of the Academy from 127 to 110 BCE) seems to haveattempted the impossible: to adhere closely to Carneades’philosophy, even though he never understood what Carneades truly meant(Levy 2010). The cornerstone of his adherence lies in the view thatCarneades argues for suspension of judgment and against beliefsunderstood as Stoics understand them. Philo of Larissa, anotherstudent of Carneades, interprets his teacher as allowing for tentativebeliefs in the skeptic’s life. With Philo, the skeptical era ofPlato’s Academy comes to an end. Philo’s philosophy seemsto divide into two phases. In Athens, and as head of the Academy, hestays relatively close to Carneades. Moving to Rome later in hiscareer, he develops a markedly different position. He argues onlynarrowly against the Stoic criterion and their conception ofcognition. One can apprehend things and so come toknowthem—one just cannot apprehend them in the way in which theStoics construe cognition (PH 1.235). The fact that there isno apprehension in the sense of the Stoics does not mean that there isno knowledge (Acad. 2.14). This move shifts the discussion inseveral important ways. First, Philo can be interpreted as a kind ofexternalist: one can know something without knowing that one knows it.Absent Stoic cognitive impressions, we are not able to identify whichinstances of ‘holding-true’ qualify as knowledge; but wenevertheless have some knowledge (Hankinson 2010). Second, thisproposal is a step toward modern skepticism, which is not concernedwith criteria of truth, but with knowledge.

Cicero’s skeptical philosophy in his own philosophical writingsis again distinctively different. In line with his notions of what isprobable (probabile) or likely to be true (verisimile), Cicero often examines a range of philosophicalpositions, aiming to find out which of them is most rationallydefensible. He thinks it is better for us to adopt a view that islikely to be true, rather than remain unconvinced by either side(Thorsrud 2009, 84–101). Cicero is of the greatest importancefor the transition between ancient and early modern skepticism. As inother fields of philosophy, Cicero’s influence is partly theinfluence of the translator. In transposing philosophical ideas intothe language of a different culture, the ideas change. Cicerosometimes speaks of doubting,dubitari (e.g.,Acad.2.27, 106; however, he often sticks with the earlier language ofassent and suspension). But doubt has no place in Greek skepticism(see section 1).

4. Pyrrhonian Skepticism

4.1 Early Figures: Pyrrho and Timon

When comparing Pyrrhonian and Academic skepticism, two topics standout: Pyrrhonism aims at tranquility; and it assigns pride of place toappearances. Anecdotes about Pyrrho’s life (365/60–275/70BC) convey how unaffected he was (DL 9.61–69). This kind ofideal—a tranquil state of mind—is not part of Academicskepticism, and scholars disagree on its role in Pyrrhonism(Machuca 2006; Striker 2010; Perin 2020; on Democriteaninfluences on tranquility in Pyrrho and Timon, cf. Svavarsson 2013).Insofar as the point of the anecdotes about Pyrrho’s life isthat Pyrrho did not avoid or pursue anything with fervor, or that hedid not despair about things that other people find terrible, theycapture ideas that remain central to Pyrrhonism (for a general accountof early Pyrrhonism, cf. Castagnoli 2013; on Sextus’ takeon Pyrrhonian tranquility, cf. Svavarsson 2015).

The biographical anecdotes portray Pyrrho as a strikinglyunconventional figure, unaffected not just by emotion and belief, butalso by perception—to the extent that friends had to pull himoff the street when a wagon approached (DL 9.62). At the same time,Pyrrho seems to have said that the skeptic adheres to appearances(phainomena) (DL 9.106; Bett 2000, 84–93; on earliernotions of appearances relevant to skepticism, cf. Barney 1992). Thismight suggest that he would not cross the street when a wagon isapproaching, and so appears to him. One story makes Pyrrho appearnot only unusual, but arguably a not so sympathetic character. Passingby a drowning man, he was so unmoved that he simply walked on (DL9.63). This is in rather stark contrast with Stoic notions ofunaffectedness, where the idea would be that, not being disturbed byemotions like fear and panic, the passerby is ideally equipped to helpeffectively. Some biographical details can seem to moldPyrrho’s life to fit the schema of the sage: a traveler to theEast (Flintoff 1980), whose insights are conveyed in brief sayings; anenigmatic figure, exemplary and shocking at the same time. Though itis difficult to assess the testimony on Pyrrho’s travels,scholars increasingly explore resonances between Pyrrho’spronouncements, later Pyrrhonian skepticism, and dimensions ofBuddhist philosophy (Hanner 2020). For example, Mill (2018) arguesthat a tradition in classical Indian philosophy develops a“skepticism about philosophy” that resembles dimensions ofPyrrhonian thought.

Even though he discussed tranquility and adherence to appearances,Pyrrho was arguably no Pyrrhonian skeptic (Bett 2000, 14–62).That is, it is likely that he put forward a dogmatic position, in thesense that he had positive philosophical views about the character ofreality. Pyrrho wrote nothing. Much of what we know about him ispreserved through the writings of Timon, his adherent(325/20–235/30 BCE) (Burnyeat 1980b; Clayman 2009). Themost important piece of testimony is a passage reporting an account byTimon:

It is necessary above all to consider our own knowledge; for if it isin our nature to know nothing, there is no need to inquire any furtherinto other things. […] Pyrrho of Elis was also a powerfuladvocate of such a position. He himself has left nothing in writing;his pupil Timon, however, says that the person who is to be happy mustlook to these three points: first, what are things like by nature?second, in what way ought we to be disposed towards them? and finally,what will be the result for those who are so disposed? He [Timon] saysthat he [Pyrrho] reveals that things are equally indifferent andunstable and indeterminate (adiaphora kai astathmêta kaianepikrita); for this reason, neither our perceptions nor ourbeliefs tell the truth or lie (adoxastous kai aklineis kaiakradantous). For this reason, then, we should not trust them,but should be without opinions and without inclinations and withoutwavering, saying about each single thing that it no more is than isnot, or both is and is not, or neither is nor is not (ou mallonestinê ouk estin ê kai esti kai ouk estinê oute estin oute ouk estin). Timon says that the resultfor those who are so disposed will be first speechlessness(aphasia), but then freedom from worry (ataraxia);and Aenesidemus says pleasure. These, then, are the main points ofwhat they say (Aristocles in EusebiusPE 14.18.1–5 =DC53; tr. Bett 2000 with changes)

In response to the first question, how things are in their nature,Pyrrho makes a metaphysical claim: they are indeterminate (Bett 2000,14–29). There are no stable items, or no items with stableproperties. Scholars sometimes hesitate to ascribe such a position toPyrrho, because it is undoubtedly dogmatic. Perhaps the text can begiven an epistemological reading: things are indifferentiable andunmeasurable and undecidable, because we fail in differentiating,measuring, and determining how they are (Svavarsson 2010; Thorsrud2010). But Pyrrho’s response to the second question may onlyfollow if we adopt the metaphysical reading (Bett 2000, 29–37).Pyrrho infers that our perceptions and beliefs are neither true norfalse. They are not truth-evaluable, presumably because there are nofacts which could be correctly captured. Third, if we understand thesethings, speechlessness (aphasia) follows, and thentranquility (ataraxia). Pyrrho does not say that we shouldcease to speak. He suggests that we adopt a complicated mode ofspeech, constructed around the expressionou mallon(“no more”), which aims to capture the indeterminatenatures of things, when we attempt to say anything about anything(Bett 2000, 37–39; Vogt 2021).

4.2 Aenesidemus, the Ten Modes, and Appearances

Aenesidemus (first century BCE) was discontented with the viewsdiscussed in the Academy at his time, of which he began as anadherent. Philo’s proto-externalism as well as a counterpositionformulated by Antiochus both appeared to him dogmatic. Aenesidemusaimed to revive a more radical skepticism, and left the Academy forthis purpose. Arguably, he is the first Pyrrhonian skeptic.Aenesidemus wrote a treatise, thePyrrhonian Discourses,probably similar in structure to Sextus’Outlines ofPyrrhonism and partially preserved in a summary by Photius:a general account of skepticism, followed by books on particularphilosophical questions (Hankinson 2010). The basic elements ofAenesidemus’ skepticism are: the skeptic puts appearances andthoughts into opposition; this generates equipollence(isostheneia, lit. “of equal weight”) betweenseveral appearances and/or thoughts; suspension of judgment follows;with it comes tranquility; and the skeptic leads a life according toappearances (DL 9.62, 78, 106–7). However, we do not know muchdetail of his views on these matters. Instead, Aenesidemus is famousfor having developed Ten Modes or Tropes—forms of argument bywhich the sceptic puts appearances and thoughts into opposition. Keyquestions about Aenesidemus’ skepticism concern (i) theinterpretation of his Modes, (ii) the relationship of his philosophyto competing theories, (iii) the scope of the Ten Modes, and (iv) theskeptic’s mode of speech.

(i) Conflicting Appearances or Causal Invariance. The Ten Modes arepreserved in Diogenes Laertius (9.78–88), Philo of Alexandria(On Drunkenness 169–202), and Sextus. Diogenes’account of the Ten Modes may postdate Sextus’ (Sedley2015). Sextus gives extensive illustrations, and integrates the TenModes into his general account of Pyrrhonism (PH1.36–163; cf.M 7.345 for ascription of the Ten Modesto Aenesidemus; cf. Annas-Barnes 1985 and Hankinson 1995, 268; thesequence below follows Sextus). Here is the first of the Ten Modes,interpreted in two ways.

10–1:
Arguments concerning oppositions based on the differences betweenkinds of animals.

Conflicting Appearances Interpretation:
X appearsF to animal of kindA (e.g.,humans) andF* to animal of kindB [whereFandF* are opposite or otherwise incompatible properties]. Wecannot judge howX really is, because we are a party to thedispute.

Causal Invariance Interpretation:
For something to be ‘really’F, it would have toconsistently affect different perceivers asF. But differentconstitutions of different animals cause different impressions of thesame thing. For different animals, something isF andF* (whereF andF* are opposite orotherwise incompatible properties). Therefore, things do not seem toreally beF orF*.

The Conflicting Appearances Interpretation is based on Sextus’account (Annas-Barnes 1985). The focus here is on the idea that everykind of animal, perceiver, sensory faculty, thinker, or judger(depending on which mode we consider) is only one of several animals,perceivers, sensory faculties, thinkers, or judgers. The object isperceived or considered from a particular point of view. Everyone is aparty to the dispute, and there is no ‘view from nowhere.’Accordingly, the dispute cannot be decided. The Causal InvarianceInterpretation, on the other hand, suggests that the focus ondecidability is introduced by Sextus. Aenesidemus may (implicitly orexplicitly) have endorsed the following idea: ifX wereF by nature,X would affect everyone asF.IfX affects different people (living beings, sensoryfaculties, etc.) asF andF*,X is bynature neitherF norF*. For example, ifXis harmful toA and beneficial toB, it is neitherharmful nor beneficial in its nature (Woodruff 2010, Bett 2000). TheTen Modes can generally be construed as engaging either with conflictsbetween appearances or with causal invariance:

10–2:
Arguments based on the differences among human beings (differences inbody and in soul).

10–3:
Arguments based on the differences between the senses and on thecomplexity of perceived objects.

10–4:
Arguments based on states (dispositions and conditions of a humanbeing, such as age, motion versus rest, emotions, etc.).

10–5:
Arguments based on positions, distances, and places.

10–6:
Arguments based on mixtures (objects in conjunction with externalthings like air and humidity; physical constituents of sense organs;physiology of thought).

10–7:
Arguments based on the composition of the perceived object.

10–8:
Arguments based on relativity (to the judging subject, tocircumstances, etc.). 10–8 comprises at least 10–1 to 10–7, or all TenModes.

10–9:
Arguments based on constancy or rarity of occurrence.

10–10:
Arguments concerned with ways of life, customs, laws, mythicalbeliefs, and dogmatic assumptions, all of which can be put intoopposition to each other.

(ii) Skepticism, Relativism, Epicureanism. Consider first therelationship between skepticism and relativism (cf. Bett 2000; Vogt2012a, ch. 4). Relativism, as envisaged in Plato’sTheaetetus, looks at a similar range of phenomena. Thingsappear different to different kinds of animals; to different people;and so on. Relativism embraces the intuition that there is (as wewould say today) faultless disagreement. That is, you and I disagree,but neither of us is wrong. Accordingly, metaphysical relativismclaims we must give up the intuition that we both refer to the samething. In theTheaetetus, the world dissolves into radicalflux: there are no stable items with stable properties that we bothrefer to.

The Ten Modes, according to Conflicting Appearances, differ fromrelativism by turning precisely the other way (Annas-Barnes 1985,97–8; Pellegrin 1997, 552–3). They implicitly rely on theintuition that there are stable items with stable properties. Ofcourse, the skeptic is not committed to the thesis that oppositescannot hold of the same thing, and that therefore no two conflictingappearances can be true. However, the modes presuppose a common sensemetaphysics that does not accommodate faultless disagreement. In allcases of disagreement, at best one of us can be right. If we cannotfigure out which view is right, we should suspend. This does not meanthat Pyrrhonians are committed to a common-sense metaphysics. The TenModes are only one of several tools that skeptics have at theirdisposal. They may thus imply a metaphysics that, at other points,skeptics would call into question (cf. Fine 2003b, 352).

Causal Invariance differs from Conflicting Appearances precisely withrespect to the metaphysics that is, even if only dialectically,invoked. Aenesidemus seems to have explored the relationship betweenskepticism and flux. He remarks that skepticism leads to Heracliteanphilosophy. The idea that one thing appears to have contraryproperties (the ones it appears to different animals/persons/senses tohave) leads to the idea that one thing actually has contraryproperties (PH 1.210; cf. Schofield 2007 on the role ofHeraclitus and causal invariance). This remark can be taken as anexpression both of moderate flux and of relativism (Aenesidemus doesnot seem to think of radical flux, where it is no longer even possibleto refer to anything). There is no stable reality of how things are(moderate flux);X isF andF* insofar as,ifX seemsF toA, this is true forA, andF* toB, this is true forB(relativism). This proposal differs from the Causal Invarianceinterpretation of the Modes presented above. There, Aenesidemus seemsto argue that things do not really have stable properties (they areneitherF norF* by nature); he does not say thatthey areF andF* (as relativism says).

A third approach, competing with skepticism and relativism, isEpicurean epistemology. Again, the set of phenomena to be accountedfor is the same. But it is described differently. Epicurus insiststhat we should not even speak of conflicting appearances. Rather, weshould speak of different perceptions. Perceptions cannot refute eachother, because they are of the same weight. Epicurus here uses theterm that is central to Pyrrhonism: equal weight,isostheneia (DL 10.31–2). The fact thatperceptions differ has perfectly reasonable explanations: I look froma distance, you look from nearby; I have a cold, you are healthy; I ama human being, another cognizer is a dog; and so on. These factsfigure in the explanations of how our perceptions are constituted.Accordingly, Epicurus argues, all perceptions, even though theydiffer, are true. They all have a causal history that physics canexplain. The precise interpretation of this proposal is controversial.One might object that the notion of truth employed here is deeplypuzzling. It is not clear what it means to describe all perceptions astrue if they cannot be trueor false.

(iii) Scope. For the greatest part, the Ten Modes seem to be concernedwith perception in a broad sense, so that it includes pleasure andpain, harm and benefit, as well as pursuit and avoidance. To perceivesomething as pleasant or beneficial is to pursue it. Perception andevaluation are also mixed in another way: depending on the frequencywith which we perceive something, it seems more or less amazing andprecious to us.10–10 envisages oppositions that canbe construed with the help of dogmatic theses. The Ten Modes thus fitSextus’ description of what skepticism is: the ability to putappearances and thoughts (phainomena andnooumena)into opposition (PH 1.8, 1.31–33; cf. DL 9.78).

Another issue concerning the scope of the Ten Modes is whether theyaddress general or particular matters. Compare the example of whetherthe tower is round or square (T) to the example of whether honey issweet or bitter (H). (T) is a particular; the question is whether thistower is round or square. (H) can be construed as a particular(“is this bit of honey sweet?”), or as a general issue(“is honey sweet?”). The Ten Modes offer strategies forsuspension of judgment on both kinds of questions.

Scholars have asked whether it is a problem for skepticism if the TenModes appear ‘systematic’ (Sedley 2015). A set ofarguments that aims to be complete, covering domains according to astandardized pattern, may appear to be out of tune with theskeptic’s presumed mode of investigation. Purportedly, skepticsthink through given questions as they arise, arriving at suspension ofjudgment in a piecemeal fashion. If this self-description is to betaken at face value, then modes of generating suspension of judgmentacross the board may appear problematic. In this respect, DiogenesLaertius’ report of the Ten Modes may be superior toSextus’ account. Diogenes begins with a remark that suggeststhat skeptics pick up where other philosophers have already begun tomake an argument (9.78–9). Other philosophers have collected,presumably, ways in which ‘we are persuaded’, say, becausethings regularly appear the same way. Now the skeptics, as it were inresponse to this, add a collection of further cases, where things donot appear the same way to different cognizers. If this is thedialectical set-up, the Ten Modes may not be ‘systematic’in ways that harm skepticism. They may co-opt the patterns of dogmaticreports about cases where appearances are stable (Sedley 2015).

(iv) Language. Aenesidemus contributes an interesting move to thequestion of how the skeptic can speak. Consider the relationshipbetween a skeptic’s state of mind and their utterances. One wayto construe this relationship is that an utterance reflects a state ofmind. This is a background assumption to the idea that, if skepticsuse assertoric language, they hold beliefs. Another option is toassume that language does not have the means to capture theskeptic’s state of mind. On this premise, a skeptic might flagtheir utterances as falling short of doing so. This isAenesidemus’ strategy. He says that the Pyrrhonian determinesnothing, and not even this fact that he determines nothing. ThePyrrhonian puts matters in such terms, he says, because he has no wayto express the actual thought of the sceptic in determining nothing(Photius,Bibl. 169b40–170a14, = 71C(6)-(8) LS).

4.3 Agrippa, and the Five Modes

Almost nothing is known about Agrippa (1^st to2^nd century CE; SEPH 1.164–177; DL9.88–89). However, the modes of argument that Sextus calls theFive Modes are attributed to him. These modes are among the mostfamous arguments of ancient skepticism (Barnes 1990, Hankinson2010).

5–1Diaphônia:
The mode that argues from disagreement. With respect to some matterthat presents itself, there is undecided (anepikriton)conflict, both among the views of ordinary life and the views held byphilosophers. Due to this, we are unable to choose or reject onething, and must fall back on suspension.

5–2Eis apeiron ekballonta:
Arguments that throw one into an infiniteregress. That whichis brought forward to make a given matter credible needs yet somethingelse to makeit credible, and so onad infinitum.Since we thus have no starting point for our argument, suspension ofjudgment follows.

5–3Pros ti:
Arguments fromrelativity.X only ever appearssuch-and-such in relation to the subject judging and to the thingsobserved together with it. Suspension on howX really isfollows.

5–4Hypothesis:
Someone makes an assumption without providing argument. A dogmatist,if thrown back into an infinite regress of arguments, just assumessomething as a starting-point, without providing an argument(anapodeiktôs). We suspend over merehypotheses—they could be false, opposite hypotheses could beformulated, and so on.

5–5Ton diallêlon:
Arguments that disclose acircularity. This mode is used whenthat which ought to confirm a given investigated matter requiresconfirmation (pistis—credibility) from that matter. Weare unable to assume either in order to establish the other. Wesuspend judgment on both.

It is a commonplace to say that, while the Ten Modes, as presented inSextus, are concerned with conflicting appearances, the Five Modes areabout argument or proof. In these modes, the skeptics developstrategies by which to attack theories that the dogmatists defend. Ifthis is how we characterize the Modes, Aristotle’s objection(section 2.3) immediately comes to mind. Do the Five Modes reveal theskeptic’s lack of understanding because they presuppose thateverything is subject to proof? (Barnes 1990; Hankinson 1995,182–92; Long 2006, ch.3) The three so-called formalmodes—Regress, Hypothesis, and Circularity—can beconstrued in this fashion: when employing them, the skeptic can arguethat every premise must be supported by argument; if it is not sosupported, the theory begins from a mere hypothesis (on earlier usesof the termhupothesis in Plato and in medicine, cf. Cooper2002 [2004a]); or it is ultimately circular.

However, the skeptic might not be vulnerable to this objection. First,the Five Modes can be construed as dialectical, invoking dogmatictheories of justification (Striker 2004). Second, they might bebroader in scope.5–1 and5–3explore disagreement and relativity. Skeptical examination oftenbegins with the Mode of Disagreement: different answers to a givenquestion are surveyed, and the conflict between them is observed. Theinterpretation of5–1 hangs, for the most part, onthe question of whetheranepikriton should be translated as‘undecided’ or ‘undecidable’ (Barnes 1990). Itwould be dogmatic to claim that matters are undecidable. ThePyrrhonist must prefer the idea that, up to now, matters have not beendecided. In applying the Mode of Disagreement, skeptics caneither record a conflicting argument that others have formulated orcome up with an argument that disagrees with a view formulated by adogmatist (cf. Sienkiewicz 2019, 47–51). This leads to the question ofwhether something can be found that would decide matters, and thus tothe application of further modes. Scholars have observed that5–3, the Mode of Relativity, does not really fit intothe Five Modes. However, the Five Modes could be designed to supersedeand include the Ten Modes, and5–3 might be viewed ascapturing the common thread of the Ten Modes. With the help of5–3, the skeptic can argue that the premises thattheorists employ are formulated from particular points of view, inparticular contexts, and so on (for a reading that emphasizes the roleof the first mode at the expense of the role of the third, cf.Sienkiewicz 2019).

Third, even the so-called formal modes (5-2,5–4,5–5) might not be narrowlyconcerned with proof, but rather with everything that can lendcredibility to something else. Consider Regress(5–2), the first of the formal modes. The text doesnot actually speak of proof (apodeixis) (this is obscured byBury’s Loeb translation;5–4 is the only placein Sextus’ report of the Five Modes that uses a cognate ofapodeixis). Sextus’ language is wider: the mode dealswith everything that can make something else credible. We might readthis in the context of the Hellenistic view that proof is a species ofsign (PH 2.122). A sign reveals something non-evident. Smokethat reveals fire has, from this point of view, a function andstructure that is similar to a proof. The target of the Five Modesmight be sign-inferences in general. If this is so, then their targetmight include what we would call inductive reasoning and causalexplanations (when Sextus introduces a further set of modes, theCausal Modes, he says that they are not really needed, because therelevant work can be done by the Five Modes;PH1.180–86). Taken together, the Five Modes deny all “proof,criterion, sign, cause, movement, learning, coming into being, andthat there is anything by nature good or bad.” (DL 9.90). Thisis notably more than just proof. Indeed, it is an excellent summary ofthe key topics in Sextus’ discussions of logic, physics, andethics.

4.4 Sextus Empiricus

Sextus’ (ca. 160–210 CE) epithet, Empiricus, indicatesthat he belonged to the empiricists, a medical school (cf.Svavarson 2014 for a brief conspectus of Sextus’ philosophy).The empiricist medical school argued against rationalistic tendenciesin medicine (Frede 1990; Allen 2010). Rationalism in medicine aims togive causal explanations as a basis for therapies. Empiricism, on thecontrary, confines itself to observation and memory. Somewhatconfusingly (considering his name), Sextus discusses differencesbetween Pyrrhonism and empiricism, and says that skepticism is closerto medical methodism than to empiricism (Allen 2010). On the whole, itmay be safest to think of Sextus as rejecting medical rationalism aswell as other trends within medicine that by Sextus’ lights aredogmatic. Methodism follows appearances, and derives from them whatseems beneficial. No explanations are attempted, no underlyingsubstances postulated, and no regularities assumed—these aresome of the rationalistic methods that both methodism and empiricismargue against. Methodism also makes no statements to the effect thatsuch explanations cannot be given, or that underlying substances andregularities do not exist, as Sextus says empiricism does (SEPH 1.236–241). Beyond the fact that Sextus was adoctor and wrote on medicine (M 7.202, M 1.61), his philosophicalthinking seems shaped by engagement with medical writers. Scholarshave long been interested in the relationship between medicine andskeptical arguments that pertain to be therapeutic (Voelke 1990). Morerecently, scholars explore ways in which medical writings, inparticular by Galen, inform Sextus’ thinking, even on specificissues such as how to conceive of and solve sophisms (Schmitt,forthcoming). It has also been argued that the skeptics’application of the modes should be interpreted in light of themethodological commitments of non-rationalist medicine (Sienkiewicz2019, 2021).

Sextus’ philosophical writings are traditionally divided intotwo groups. TheOutlines of Pyrrhonism [PH] consistsof three books.PH 1 is the only general account ofPyrrhonism that survives.PH 2 and 3 discuss questions oflogic, physics, and ethics. The other writings are summarily referredto, traditionally, asAgainst the Mathematicians[M]. In fact, they oppose not just mathematical, but alsoother theorists: the title really meansAgainst theTheoreticians, orAgainst the Learned (Bett 2012). In asense, onlyM 1–6 should go by that title. It is acomplete work, andM 7 does not seem to be its continuation(Bett 2012, “Introduction”).M 1–6 discussgrammar, rhetoric, geometry, arithmetic, astrology, and music theory.They argue against theoretical ‘learning’ in these fields.M 7–11 discuss core questions of the threephilosophical disciplines, logic, physics, and ethics, and couldplausibly be referred to asAgainst the Logicians (M7–8),Against the Physicists (M 9–10),andAgainst the Ethicists (M 11). Scholars disagreeon whetherM is earlier (Bett 1997) or later thanPH(Janacek 1948 and 1972). Scholars also disagree on whether we canevaluate different strands of skepticism within Sextus as more or lesssophisticated. Those who considerPH as later often do sobecause they think it shows greater philosophical sophistication,either by avoiding claims that a certain matter cannot be known(sometimes described as negative dogmatism), as found inM1–6 andM 11, or by streamlining discussions fromM 7–10 (Bett 1997; Brunschwig 1980).

These questions are complicated further by Sextus’ attempt toincorporate diverse material, such as different sets of Modes, intohis skepticism. According to recent interpretations, thedifferent sets of modes are part of an integrated philosophicalapproach (Powers 2010). Morison (2018) argues that the Ten Modes andFive Modes both serve the same purpose: to produce equal and opposingarguments to arguments in support of philosophical or scientificviews. Arguably, two kinds of consistency are at work in Sextus’writings. On the one hand, Sextus aims at the consistency of onephilosophical outlook. On the other hand, he aims at the consistencyof having a response to every objection. These two aims overlapgreatly, but they can also come apart. A given argument might refute aparticular critic. This argument may go back to various earlierversions of Pyrrhonism. Similarly, the critical objection that isrefuted may be traced to dogmatic theories formulated over the courseof several centuries. As a result, a given argument in Sextus may beeffective against a given objection he has in mind. It may thuspreserve consistency in the sense of leaving the skeptic unharmed bydogmatic criticism. But at the same time, this argument may haveimplications that are in tension with the way in which Sextus explainsskepticism in other passages. Such tensions are particularly importantwith respect to the way in which Sextus uses core concepts. Forexample, it is not clear that Sextus uses the notion of appearances(phainomena) in a consistent fashion (PH1.8–9; 1.15; 1.22; for the view that Sextus employs the notionconsistently throughout, see Barney 1992). At times, he draws on thecontrast between appearances and thoughts (noumena). But forthe most part, the term refers to all cases where something seemsso-and-so to the skeptic, either perceptually or in thought. In somecontexts, Sextus draws on the idea that appearances are impressions,invoking the dogmatic assumption that impressions are passive. Inother contexts, he does not envisage appearances as entirely passivelyexperienced (Vogt 2012b).

It is thus no surprise that the interpretation of Sextus’Pyrrhonism is quite controversial. This applies in particular to thequestion of whether the skeptic has any beliefs, or beliefs of anykind. In the past 40 years, scholars have paid attention to thisquestion more than to any other interpretive issue. Insofar as thetexts may contain different strands of Pyrrhonian argument, exegesisis to some extent shaped by the philosophical interests we bring tothe texts. Two ideas are particularly prominent here. First, somescholars find in Sextus an account of action that challenges standardancient and modern theories of agency. These theories might portrayordinary agents as all-too-rational, as if every action involved anactively formed belief that such-and-such is good. Scholars explorehow far we can draw on Sextus, asking whether a life guided byappearances (as Sextus says the skeptic’s life is) might afterall be rather ordinary (Frede 1979 [1997]). Second, one might on theother hand embrace those aspects of Sextus’ texts that makePyrrhonism look radically different from ordinary life. From thisperspective, Sextus’ writings invite reflection on the questionof whether it would be possible to live without belief (Burnyeat 1980[1997]; Barnes 1982 [1997]; Burnyeat 1984 [1997]).

PH 1, which figures most prominently in scholarlydiscussions, is atour de force. Sextus gives a generalaccount of what skepticism is, including skeptical investigation,suspension of judgment, the skeptic’s end, action, and language;he gives lists and illustrations of various sets of Modes; he explainsthe so-called skeptical formulae (phônai), such as“I determine nothing,” “non-assertion,”“maybe,” and so on; and he compares skepticism torelevantly similar philosophies.

Sextus emphasizes that the skeptic is an investigator. Others eitherarrive at theories (dogmatism) or at claims about inapprehensibility(negative dogmatism—that the matter investigated is beyondone’s capacity to decide, and so is unknowable). But the skepticcontinues to investigate (PH 1.1–4). Investigation isdescribed as setting appearances and thoughts into opposition(PH 1.8) (Morison 2011 offers a reconstruction of skepticismthat takes its starting–point from this description), and as theapplication of the various sets of Modes (PH 1.36–186).Skepticism does not have teachings, but it is an approach inphilosophy (Smith 2022). Many of the thoughts the skeptics arrive atare expressed in the skeptical formulae (PH 1.13–15;187–209). The starting–point (archê) ofskepticism is divergency—anômalia. Theproto-skeptics are disturbed by the discrepancies they encounter, andbegin to investigate (PH 1.12). They hope to gain quietude by settlingwhat is true and what is false. But then they have a surprisingexperience. Encountering disagreement where several views appear to beof equal weight (isostheneia), they find themselves unable todecide things, give up, and experience tranquility (ataraxia)(Striker 1990 [1996]; Nussbaum 1994). The skeptic’s end(telos) is tranquility in matters of belief (katadoxan) and moderate affection (metriopatheia) in mattersthat are forced upon us (PH 1.25–29). That is, skepticscan free themselves from those kinds of turmoil that come with holdingbeliefs. They cannot free themselves from freezing, thirst, or pain.But they suffer less than others, for they do not add the belief(prosdoxazein) that, for example, pain is bad. The skepticmust explain how, without belief (adoxastôs), they canbe active. Sextus says that skeptics follow appearances, and that is,that they adhere to the fourfold ways of life (PH1.21–24). Nature supplies them with perception and thought;necessary affections compel them (for example, thirst guides them todrink); they go along with traditions and customs; and they can dotechnical things by having been instructed in skills. The notion ofappearances is also central to Sextus’ account of how theskeptic can speak. Without making assertions, a skeptic reports(apangellein) like a chronicler (historikôs)what appears to them now (PH 1.4).

I shall discuss the following aspects of Sextus’ skepticism: (i)investigation and tranquility, (ii) concepts and inference rules,(iii) belief, (iv) the formulae, (v) appearances, (vi) language, (vii)action, and (viii) the so-called special arguments (that is, argumentsthat do not explain the nature of Pyrrhonism, but engage with specificdogmatic theories in logic, physics, and ethics).

(i) Investigation and tranquility. Investigation must aim at discoveryof the truth, otherwise it is not genuine investigation. However, askeptic seems to mechanically apply the skeptical Modes, in order togenerate suspension of judgment and tranquility. Scholars disagree onwhether the skeptics genuinely aim at the truth (Palmer 2000;Striker 2001; Perin 2006; Veres 2020b), while they (also) aim attranquility. Note that this objection, unlike the other problemscentral to contemporary engagement with ancient skepticism, was notraised in antiquity. If ancient skepticism is approached in thecontext of the larger study of ancient philosophy, we might first ofall note that the skeptics in a sense agree with Socrates, Plato,Aristotle, Epicurus, and the Stoics. All these philosophers defend, inso many formulations, a life of reason, of contemplation, of wisdom,or of inquiry as the best or at least a very good human life (onaffinities between the skeptics’ commitment to inquiry andAristotle, cf. Olfert 2015). The objection that skeptical inquiryseems insincere, then, may not have come up in antiquity in quite theway in which it is discussed today because a commitment to inquirywould be common ground among most philosophers. Further, we mightobserve that aiming at the truth includes two aims: to accept truths,and to avoid falsehoods. The Modes are tailored to keep us fromassenting to something that could be false. Insofar as theskeptic’s effort to avoid falsehoods expresses a valuation forthe truth, the skeptic might be a genuine investigator (Vogt 2012a,ch. 5; Olfert 2014).

A related objection calls into question the actual practice ofskeptical inquiry. Does Sextus rely on the assumption that, in anygiven case of putting several arguments into opposition, thesearguments are equally persuasive for the skeptic? This seemsunrealistic: at least in some cases, skeptical inquirers are bound tobe more strongly attracted to one view than to another. How then dothey arrive at suspension of judgment? One skeptical strategy is toremind oneself that additional arguments will be formulated in thefuture (PH 1.33–34, 89, 96–97; 2.38–41;3.233–34). Another strategy is to consider that differentarguments are persuasive to different people (Svararsson 2014).Relatedly, skeptics may find themselves in a position comparable to astudent who takes a seminar on freedom and determinism: it is possibleto be more attracted to one view rather than another, and at the sametime be aware that as far as the arguments are concerned, there isunresolved disagreement among several views, to the effect thatneither of the views seems compelling in ways that warrant assent (cf.Vogt 2012a, ch. 5, on how the skeptical expression “as far asthe argument is concerned” bears on this question; for generaldiscussion of this expression, cf. Brunschwig 1990).

(ii) Concepts and rules of inference. If skeptics do not assent, thenhow can they understand the terms philosophers use (M8.337–332a)? Even more radically, how can they even think(PH 2.1–12)? This objection, which Sextus says iscontinually raised against the skeptics, proceeds on the assumptionthat possession of concepts involves the acceptance of assumptions.For example, in order to examine a given theory of proof, the skepticmust have a notion of what proofs are. This involves assumptions: forexample, the assumption that a proof contains premises and aconclusion. Sextus’ response to this objection invokes theEpicurean and Stoic theories of preconceptions. Human beings are notborn with reason (Frede 1994, 1996). The acquisition of reason is anature-guided process of concept acquisition. At a given age, childrenhave completed this process. They have become rational, which meansthat they can perceive and think in a conceptual way. Only now, theyhave rational impressions to which they can assent. The acquisition ofpreconceptions did not involve assent, simply because the child wasnot yet rational (Brittain 2005).

Sextus invokes dogmatic ideas about the acquisition of reason (or: theabilities of conceptual thought) in his response to the Apraxia Charge(PH 1.23–4). Skeptics are, first of all, active becausenature has equipped them with perception and thought (Vogt 1998,2010). More generally, the skeptics’ ability to think andinvestigate depends on the fact that they have acquired concepts aspart of growing up. This process did not involve assent, andaccordingly, Sextus argues that the skeptic’s ability to thinkdoes not violate suspension of judgment (cf. Brunschwig 1988; Vogt2012a ch. 6; Grgic 2008; Fine 2011). It is conceivable, though, thatthe skeptic’s ability to understand involves some knowledge,namely a kind of knowledge that does not entail any belief (Corti2009, part III; Corti 2015). This option appears counterintuitivegiven today’s premises in epistemology, according to whichsomeone who knows that p also believes that p. The relevant notion ofbelief in skeptical discussions,doxa, as well as therelevant ancient notions of knowledge, however, may behave quitedifferently (cf. Vogt 2012a and Moss and Schwab 2019 on belief;Burnyeat 1980c, Frede 2008 and Schwab 2016 on knowledge). Insofar asdoxa is an inherently deficient activity and attitude, andinsofar as knowledge is conceived in elevated ways, knowledge withoutbelief is a rather intuitive option within ancient epistemology. Ifunderstanding concepts and arguments involves knowledge that does notentail beliefs, the skeptics may be taken to have such knowledge.Modern critics raise the further question of whether the skeptic mustendorse logical laws (such as the Principle of Non-Contradiction) andrules of inference. In particular, they ask whether a skeptic iscommitted to the logical validity of the conditionals they formulatewhen arguing against the dogmatists (Sorensen 2004). Sextus records noancient version of this complaint, and accordingly no directresponse. Relatedly, one may ask whether Sextus’ method, inparticular regarding the Five Modes, is systematic in a way that is intension with suspension of judgment (cf. Sienkiewicz 2021, who defendsSextus against this charge).

(iii) Belief. Bury, in his Loeb translation, translatesadoxastôs as “undogmatically,” for example,when Sextus speaks (PH 1.15) of skeptics as saying that“nothing is true.” This translation suggests that Sextusbans dogmatism from the skeptic’s life, where this still leavesroom for other, non-dogmatic beliefs. Butadoxastôsmeans non-doxastically or ‘without belief’ (cf. Burnyeat1980 [1997]). As noted above, the skeptic’s end is tranquilityin matters relating to belief—kata doxan. A skepticlivesadoxastôs. And even more confusingly, theskeptics assentadoxastôs, when they act.

Contemporary interest in Pyrrhonian skepticism was much spurred byMichael Frede’s paper “The Sceptic’s Beliefs”(1979 [1997]). Frede argues that ancient skepticism was traditionallydismissed too easily as vulnerable to the Apraxia Charge, the chargethat, without belief, the skeptic cannot act. The skeptics seem to beconfident that they have replies to this objection. Thus, it seemsuncharitable not to look closely at these replies. Further, insofar asthese replies respond to the charge that without belief one cannotact, we should focus on what the skeptics say about the role of beliefin their lives. Frede citesPH 1.13, and claims that in thispassage we find a distinction between two kinds of belief:

When we say that the skeptic does not dogmatize, we are not using‘dogma’ in the more general sense in which some say it isdogma to accept anything (for the skeptic does assent to theexperiences forced upon him in virtue of this-or-that impression: forexample, he would not say, when warmed or cooled, ‘I seem not tobe warmed or cooled’). Rather, when we say he does notdogmatize, we mean ‘dogma’ in the sense in which some saythat dogma is assent to any of the non-evident matters investigated bythe sciences. For the Pyrrhonist assents to nothing that isnon-evident. (PH 1.13; trans. Burnyeat (1984) [1997] withchanges)

Following Frede, several scholars focus onPH 1.13 whendiscussing skeptical belief (with the notable exceptions of Barnes1980 [1997] and Barney 1992). They take it to be obvious that, in thisparagraph, Sextus distinguishes between two kinds of belief, one whichhe bans from the skeptic’s life, and one which he allows intothe skeptic’s life. Barnes (1982 [1997]) employs a distinctionbetween rustic and urbane skepticism. The rustic skeptic suspends onall matters. The urbane skeptic suspends on scientific matters, butholds ordinary beliefs. The clause “non-evident mattersinvestigated in the sciences” inPH 1.13 might be takenas a point of reference for the urbane interpretation. However, Barnespoints out that this cannot be right. Everything can be considered asa non-evident matter, even such things as whether honey is sweet.

Against Barnes, Frede argues that the relevant distinction must bedrawn between two kinds of assent, such that “having a viewinvolves one kind of assent, whereas taking a position, or making aclaim, involves another kind of assent, namely the kind of assent thesceptic will withhold” (1984 [1997], 128). Sextus characterizesskeptical assent in three ways. He speaks of forced assent (PH1.23–24), involuntary assent (PH 1.19), andadoxastôs assent (PH 2.102). Frede does not explore thedetails of how Sextus uses these notions. The core of his proposal isthat Sextus allows for a kind of assent that does not involve a claimas to how things are in actual fact.

In (1979 [1997]), Frede is predominantly concerned with theskeptic’s reply to the Apraxia Charge. In (1984 [1997]) hisfocus is on skeptical pronouncements such as “nothing can beknown.” His distinction between two kinds of assent, andaccordingly two kinds of belief, is explored with respect to suchsentences. Frede writes that “[t]o be left with the impressionor thought that p […] does not involve the further thought thatit is true that p” (133). This is the sense in which, on hisinterpretation, the skeptic might think “nothing isknown.” The thought counts as a belief, but not as a claim that,in actual fact, nothing is known by anybody. Contrary to Frede’sinterpretation, one might argue that to believe simply is to holdtrue, at least according to the notions of belief that the skepticsinvoke in discussions with their contemporary critics (Vogt 2012b). Itis thus not clear that Frede’s distinction is genuinely onebetween two kinds of beliefs (Burnyeat 1980 [1997]). Perhaps it is adistinction between two different propositional attitudes, only one ofwhich is belief. As Striker (2001) points out, there is a danger thatdebates over this issue become merely terminological. We might thusdraw a distinction between two issues. It is one thing to disagreewith Frede on what should or should not be called belief, and anotherto dispute whether he identifies and characterizes a phenomenon in theskeptic’s mental life. As Frede argues, skeptics find themselveswith a rather persistent thought, without having accepted it as truein actual fact. This appears to capture a core element of skepticism:the way in which the skeptic thinks such thoughts as “everythingis inapprehensible.”

(iv) The Skeptical Formulae.PH 1.13, the passage in whichscholars find a distinction between two kinds of belief, occurs in achapter entitled “Does the Skeptic dogmatize?” One anglefrom which we might disagree with Frede is to insist thatPH1.13 addresses the status of the core thoughts of skepticalphilosophy, rather than the question of skeptical belief. Consider therest of the chapter:

Not even in uttering the skeptical formulae about unclearmatters—for example, “In no way more,” or “Idetermine nothing,” or one of the other formulae which we shalllater discuss—do they dogmatize (dogmatizein). For ifyou dogmatize, then you posit as real the things that you are said todogmatize about; but skeptics posit these formulae not as necessarilybeing real. For they suppose that, just as the formula“Everything is false” says that it too, along witheverything else, is false (and similarly for “Nothing istrue”), so also “In no way more” says that it too,along with everything else, is no more so than not so, and hencecancels itself along with everything else. And we say the same of theother skeptical formulae. Thus, if people who dogmatize posit as realthe things they dogmatize about, while skeptics utter their ownphrases in such a way that they are implicitly cancelled bythemselves, then they cannot be said to dogmatize in uttering them.But the main point is this: in uttering these formulae they say whatappears to themselves and report their own feelings without any belief(adoxastôs), affirming nothing about external objects.(PH 1.14–15; trans. Annas-Barnes with changes)

When explaining inPH 1.13 how the skeptic does notdogmatize, Sextus may have a particular issue in mind: that someskeptical formulae look like doctrines, and have traditionally beenturned against themselves due to their dogmatic surface-structure. Forexample, “all things are indeterminate” looks like astraightforward dogmatic statement. There is a long history ofskeptical attempts to explain the nature of such pronouncements sothat they no longer undermine themselves. Sextus arguably mentionsseveral solutions to this problem (PH 1.13–15 and1.187–209; cf. Pellegrin 2010). InPH 1.15, Sextusidentifies the following as his main point: the skeptic merely reportswhat appears to them. Along these lines, Sextus calls indeterminacy anaffection of thought (pathos dianoias;PH 1.198), astate that the utterance “all things are indeterminate”aims to capture. The other solution mentioned inPH1.14–15 is somewhat more problematic: the skeptical formulaecancel themselves out. That is, one can say them and convey somethingthrough them. But then, once one has made a point, they as it wereturn back upon themselves and eat themselves up—as fire firstburns combustible materials and then destroys itself. This idea becamefamous through another comparison Sextus uses (invoked by Wittgenstein1922, 6.54): the skeptical pronouncements are like a ladder thatone climbs up; once one is on top, one can throw the ladder away(M 8.481). Scholars disagree on whether Sextus in some senseadmits that these statements are self-refuting (McPherran 1987), orwhether he defuses their self-refutational structure (Castagnoli 2010,III.14).

(v) Appearances (phainomena). While it is difficult toestablish a clear distinction between two kinds of belief in Sextus,there is a comparatively more explicit distinction between two ways ofengaging with appearances. Sextus says that, while thingsappearX to the skeptic, the skeptic does not affirm thatthey areX. Questions that are traditionally discussed interms of whether the skeptic has beliefs thus might be addressed interms of whether the way things appear to the skeptic has ajudgment-component. Arguably, “A appearsX tome now” can be construed in different ways. Certain examples(say, the way in which it makes sense to say both that the moonappears small and that it appears large) may suggest that appearancecan but need not involve something like a judgment (Barney 1992). Someformulations in Sextus seem to insist on a significant differencebetween the mental activity of something appearing to a cognizer onthe one hand, and on the other hand the mental activity that, on thelevel of language, is represented by assertion. This suggests that,for Sextus,A appearingF to me now does not entailthat I hold it to be true thatA isF (Vogt2012b).

(vi) Language. Another approach to the question of whether the skeptichas beliefs looks at skeptical language. Sextus insists that theskeptic does not accept or reject any impression, and associates theabsence of these mental acts with the fact that the skeptic does notaffirm or deny anything (e.g.,PH 1.4, 7, 10). Arguably, wecan infer from Sextus’ account of the skeptic’s utteranceswhat Sextus wants to say about the skeptic’s mental states andacts. That is, the question of language immediately bears on thequestion of belief. The report on Pyrrhonian skepticism in DiogenesLaertius is particularly instructive in this respect. Contrary toscholarly focus on belief in reconstructing skepticism, it barelymentions belief. Discussion of the skeptics’ attitudes is almostentirely conducted in terms of skeptical language and the skepticalformulae (for detailed discussion of the relevant paragraphs, DL9.74–7, cf. Corti 2015). If Sextus is read in light of thereport in DL IX,PH 1.13 may appear to be more of an isolatedpassage than scholarly debates imply. Sextus too devotes much space toaccounts of skeptical expressions and language. It may thus be askedwhether scholars should reframe discussions, and pay more attention toskeptical language then they previously have. As of now, there is onemonograph on skeptical language, Corti (2009), and one approach tobelief that focuses on language, Vogt (1998). Both scholars pursuefurther ideas put forward by Barnes (1982 [1997]), who compares theskeptic’s utterances to avowals. The skeptic lays open theirstate of mind, they announce or record (apangellein) it (Fine2003a). In order to do this, the skeptic must misuse language(Burnyeat 1984 [1997]). Some strategies to avoid assertion are givenin the context of the skeptical formulae (“non-assertion,”“I determine nothing,” and so on). (i) Skepticalexpressions can be used as signs, which reveal a state of mind(PH 1.187). (ii) Expressions like “oumallon” (no more) and “ouden mallon”(nowise more) can be used indifferently (in the sense ofinterchangeably) (PH 1.188). (iii) As is the practice inordinary language, the skeptic can use expressions elliptically; forexample “no more” for “no more this-than-that”(PH 1.188). (iv) People often use questions instead ofassertions and the other way around. Similarly, “no more”can be construed as a question: “Why more this-than-that?”(v) The skeptic misuses language and uses it in a loose way(PH 1.191).

InM 1 andM 2, Sextus says that the skeptic goesalong with ordinary ways of using language (M 1.172, 193, 206, 218,229, 233; M 2.52–3, 58–9). This seems to be a key resourcein construing skeptical ways of speaking: the skeptic exploits theways in which ordinary speakers can diverge from grammatically correctspeech, and still be understood. Apart from using their skepticalformulae, and apart from conducting philosophical investigations,which they can do in a dialectical mode, referring to theses,arguments, and inferences, the skeptic also has to talk in everydaycontexts. It is here where we see best how skeptical utterances aretailored to reveal a state of mind in which nothing is accepted orrejected. Sextus takes great pains to construe his examples ofskeptical utterances according to the following schema:“X appearsF to me now.” This willgenerally be understood as an elliptical version of “Xappearsto beF to me now.” However, Sextusconsistently avoids “to be” (Vogt 1998; for the view that“X appears F” avoids reference to externalobjects, see Everson 1991). The peculiar form of skeptical utterancessuggests that Sextus sees a relevant difference between“X appears to beF to me now” and“X appearsF to me now.” The formermight imply reference to a state of affairs, and an epistemic usage of“to appear” that could be rendered as “It appears tome, that p,” or, “I take it that p.” This, however,would be assertoric: the skeptic would state that it appears to themthat such-and-such is the case. But the skeptic’s ellipticalutterances about what appears to them aim to be purelyphenomenological. They aim to report a condition of theskeptic’s mind, without expressing a judgment of any kind(Burnyeat 1984 [1997]; Annas-Barnes 1985, 23–4; for anassessment of these strategies in terms of modern philosophy oflanguage, cf. Pagin 2020). As part of the skeptics’ way of life,language can also be seen as an activity. That is, how the skepticscan speak can be considered a sub–question of how the skepticscan act. Skeptical utterances have been compared to Wittgensteinianconfessions, arguably a kind of speech act that is consistent with theskeptics’ avoidance of belief (Corti 2009, Parts I and II).Moreover, the skeptics not only speak. They presumably also understandwhat others say. A persuasive account of skeptical language mustexplain both speaking and understanding (Corti 2009, Part III).

(vii) Action. Sextus says that appearances (phainomena) arethe practical criterion of the skeptic (PH 1.23–24). Byadhering to appearances, the skeptic is prevented from inactivity(anenergêsia). Note that Sextus does not describe theskeptic as performing actions in the sense of dogmatic theory ofaction, which involves belief and choice (cf.M11.162–166). Contrary to the Academic skeptic, a Sextan skepticdoes not view themselves as a rational agent, who chooses one courseof action over another. Sextus claims an active life for the skeptic,but not the life of a rational agent, as conceived by dogmaticphilosophers (Vogt 2010; Schwab 2020). This attitude has beencritically discussed for a long time, for example, with respect to itsethical (Bett 2019) and political upshots (Marchand 2015) and withrespect to the skeptics’ relation to the law (Marchand2021).

The skeptic’s forced assent is situated in the domain of action(PH 1.13, 19, 29–30, 193, 237–8). Thirst, forexample, necessitates assent, and that means, it moves the skeptic todrink. This kind of assent may be genuinely unrelated tobelief-formation of any kind. Rather, forced assent generates themovement of action. But what about more complex kinds of activities,such as applying a medication, or attending a festival? Sextus arguesthat the skeptic adheres to custom, convention, and tradition, and towhat they have been trained to do. In explaining how adherence toappearances in these domains generates activity, Sextus does notmention assent. However, he might have to concede that, like drinkingwhen thirsty, more complex actions also involve some kind of assent.InPH 2.102, Sextus says that the sceptic assentsnon-doxastically (adoxastôs) to the things relied on inordinary life. InPH 1.19, he mentions involuntary assent.Accordingly, non-doxastic and involuntary assent may figure in thosedomains of skeptical action that do not involve necessitation bybodily affections. Non-doxastic assent is, from the point of view ofthe Stoics, a contradiction in terms, just like forced and involuntaryassent. Assent is defined as in our power, and as that by whichbeliefs are formed. If Sextus intends skeptical assent to be genuinelynon-doxastic and involuntary, then it does not have the core featuresof assent as defined by the dogmatists.

(viii) Logic, Physics, Ethics, and the “disciplines.” Thespecial arguments of the skeptic are directed against particulartheories in the three disciplines of Hellenistic philosophy: logic,physics, and ethics. In addition, they address the so-calleddisciplines, namely grammar, rhetoric, geometry, arithmetic,astrology, and music-theory. Sextus’ treatments of logic divideup into two main topics: sign and criterion (cf. Bett 2005 on signs).This structure reflects central concerns of Hellenistic epistemologyas well as of ancient skepticism. Skepticism looks for a‘decider’ between conflicting appearances and thoughts. Adecider could be something evident. Dogmatic philosophers associatethe evident with the criterion of truth. For something to serve therole of criterion, it cannot be equally disputed as the matters ithelps to decide. Or something non-evident could take on the role ofdecider. For that to be the case, the skeptics argue, it would have tobe conclusively revealed by a sign or proof. If there is no compellingtheory of the criterion and no compelling account of sign and proof,then there is nothing that can decide between several conflictingviews. Sextus’ treatises on logic thus are not simply acollection of individual arguments against various dogmatic theories.Their main line of thought sketches a route intoskepticism. Along these lines, Vlasits (2020b) argues thatSextus’ treatment of logic in PH 2 is unified. Sextus’argument is structured such as to lead to suspension of judgment onthe methodologies of the dogmatists. The final section of M 8 (463–81)formulates a challenge for the skeptic that scholars interested inself-refutational arguments have examined (Castagnoli 2010): does theskeptic aim to prove that proof does not exist (Sienkiewicz2022)?

Sextus’ discussions of ethics also focus on issues thatplausibly lead into skepticism. Again, there are two centralquestions: whether there is anything good and bad by nature; andwhether there is an art of life (Bett 2010, 2011 and 2019),as the Epicureans and Stoics claim there is. If we could settle whatis good and what is bad, some of the most disturbing anomalies wouldbe resolved. If there were an art of life, there would be a teachablebody of knowledge about the good and the bad. In both cases, questionsthat can cause a great deal of puzzlement would be resolved.Sextus’ discussions of ethics are in part famous because Sextusascribes outlandish and shocking views to the Stoics. As Sextusconstrues his arguments, the contrast between ‘ordinarylife’ and philosophical views leads to suspension of judgment(Vogt 2008a, ch. 1). In the modern tradition, a number of philosophersincluding Hegel and Nietzsche have engaged with aspects ofSextus’ outlook and in particular with the skeptical adherenceto ordinary life (Berry 2010 and 2020; Bett 2020a). Recently,scholars have asked how ethical a skeptic can be, to use a phrasedemployed by Bett (2019), given that it may seem that suspension ofjudgment on grave ethical challenges can seem facile. At the sametime, scholars point out that some dimensions of ethical frameworkswent unquestioned a long time, for example, with respect to gender;here suspension of judgment may appear innovative (Olfertforthcoming).

The books on physics discuss god, cause, matter, bodies, mixture,motion, increase and decrease, subtraction and addition, whole andpart, change, becoming and perishing, rest, place, time, and number.Notably, god is one of the topics explored in physics. This stands instark contrast to medieval and early modern discussions, where thequest for knowledge of God often frames and motivates engagement withskepticism. In PH 3, Sextus prefaces his discussion of argumentsfor and against the existence of gods by saying that theskeptics’ ordinary life without opinions(adoxastôs) includes the following: the skeptics saythat there are gods, are pious toward the gods, and say that the godsare provident. Scholars discuss how this relates to Sextus’discussions of theology qua topic in physics on the one hand (PH3.2–12, M 9.11–194), and to his portrayal of the skeptics’ordinary life in PH 1 on the other hand (Annas 2011, Veres 2020a). Theskeptics come to suspend judgment on all central questions in ancientphysics (Bett 2012). This means, they come to suspend judgment onwhether, for example, there are causes, time, place, and bodies (cf.Bobzien 2015 and Warren 2015). Their suspension does not merely meanthat they have not yet found a satisfying theory of, say, body. Itmeans that they find themselves unable to say whether there is body(Burnyeat 1997). (On the cumulative force of these arguments, seesection 5.4.)

The six books entitledAgainst those in the Disciplines(M 1–6) have traditionally received less attention.Only in the last few years have scholars begun to explore them withthe kind of philosophical subtlety that has been brought to bear inthe study of ancient skepticism in recent decades.M1–6 skeptically examine six fields of study, namely grammar,rhetoric, geometry, arithmetic, astronomy, and music-theory. Sextusbegins with an astonishing move. Contrary to his usual strategy ofemphasizing the distance between skeptics and dogmatists, he admitsthat Pyrrhonians and Epicureans share much in viewing standarddisciplines as useless (M 1.1–7; cf. Thorsrud 2019).Generally speaking, increased attention toM 1–6 mayprovide additional occasion to modify the long-standing assumptionthat the Stoics are Sextus’ most important dogmaticinterlocutors. ThroughoutM 1–6, resonances betweenEpicurean and Pyrrhonian philosophy are remarkably visible (Bett 2018,“Introduction”). Engagement with Epicurean philosophyshapes Sextus’ approach deeply, to the extent that both schools,Stoics and Epicureans, should be considered fundamental points ofreference.

After his remarks on how Pyrrhonians and Epicureans take issue withthe presumed usefulness of the disciplines, Sextus lays out generalarguments, suitable for skeptical examination of any field. He arguesthat, for there to be a discipline, there must be the matter beingtaught, the teacher, the learner, and the means of learning. If,however, neither of these things exists, then the disciplinedoesn’t exist. This is how Sextus proceeds. He argues, or seemsto argue, for the non-existence of the disciplines (M 1.9).Already in the very first sentence ofM 1, Sextus describeshis own approach as one of putting forward counterarguments, astrategy that he mentions repeatedly throughoutM1–6.

These moves give rise to the most contentious question regardingM 1–6. Are these books negatively dogmatic? Or do theyfit in with Sextus’s outlook inPH 1–3, whereskeptical arguments are described as leading up to suspension ofjudgment? Bett (2018, “Introduction”) argues that, withsome qualifications, Sextus’ approach is to be explained asfollows. Sextus lays out counterarguments based on the assumption thatthe arguments of the dogmatists have already been formulated. Forthere to be arguments of equal weight on both sides, only theanti-dogmatic arguments need to be adduced. The intended effect isthat jointly, these opposing sets of arguments lead us to suspendjudgment. In addition, Bett notes that the remarkable emphasis oncounterarguments, non-existence, and uselessness suggests that some ofthe material inM 1–6 goes back to an earlier phase ofPyrrhonism.

In four of the six fields, namely grammar, rhetoric, music, andastrology, Sextus admits non-technical versions into theskeptic’s life, while subjecting all theoretical claims toskeptical examination. For example, he distinguishes between theordinary ability to read and write on the one hand and grammar as atechnical discipline on the other, or the ability to play a musicalinstrument on the one hand and music theory on the other (cf. Corti2015b on why this kind of contrast does not come up in the books onarithmetic and geometry, Corti 2015c on Sextus’ attack onthe Platonic-Pythagorean notion of the Two, and Cortiforthcoming).

It is remarkable that, qua theoretical field, Sextus examinesastrology rather than astronomy. The latter would make for a moretypical sequence: apart from the fact that in Sextus, logic isconsidered part of philosophy rather than a “discipline,”the six fields otherwise correspond to the so-called trivium ofgrammar, logic, and rhetoric and quadrivium of arithmetic, geometry,astronomy, and music theory. Sextus’ attention to astrologyrather than astronomy highlights a deep feature of his philosophy(Corti 2015, Bett 2018). Astronomy, from Sextus’ point of view,is concerned with appearances; and hence there is a sense in which theskeptic does not object to it. Astronomy, then, is the“version” of astrology that skeptics can admit into theiradherence to ordinary life (M 5.1–3). Presumably,astronomy is concerned with predicting things like droughts, floods,earthquakes, and plagues based on appearances. Astrology, on the otherhand, is concerned with matters of great obscurity.

5. Ancient and Modern Skepticism: Transitions

5.1 Augustine: Re-Conceiving Skepticism in a Theological Framework

Historians of philosophy sometimes argue that Henri Etienne’srediscovery of Sextus in 1562 initiated an era of epistemology. Earlymodern engagement with skepticism is here seen as a turn to argumentsfound in Sextus (Annas-Barnes 1985, 5–7; Bailey 2002,1–20). Via Cicero, early modern and modern philosophers seem tohave engaged also with Academic arguments (for a recent analysis ofAcademic skepticism in Hume and Kant, cf. González Quintero2022). In particular the beginning of Descartes’Meditations may display a kind of Socratic spirit: acommitment to calling into question all one’s beliefs. However,early modern philosophers work within a theologically framed traditionthat importantly begins with St. Augustine (354–430) (cf. Menn1998 and Lagerlund 2009; on the history of medieval skepticism cf.Lagerlund ed. 2009; cf. Carriero 2009 on Descartes’ engagementwith Aquinas; for an analysis of the transformation of skepticism thatturns immediately to Descartes, cf. Williams 2010).

A major part of Augustine’s early education consisted in thestudy of Cicero’s writings. He was thus closely acquainted withAcademic skepticism (Cicero was one kind of Academic skeptic).Augustine sees the force of ancient skeptical strategies. Even thoughhe does not become a skeptic, he integrates distinctively skepticalmoves into his thought. This has a long-standing effect on the historyof theology and science. For example, Galileo Galilei is able to citeAugustine when he defends himself against the charge that his physicsis in opposition to the Bible (Letter to the Grand Duchess,in Drake 1957). Augustine supplies arguments to the effect that weshould keep an open mind. Both our physical theories and ourinterpretations of the Bible are likely to evolve. This idea figuresimportantly in Pyrrhonism. Past experience tells us that, on everygiven issue, someone eventually came up with a new argument.Accordingly, even if the skeptics cannot find an objection to a givenclaim right now, they expect that in the future, a conflicting viewwill be formulated.

However, such traces of skepticism are integrated into an ultimatelynon-skeptical philosophy. InContra Academicos, Augustinerecognizes a core feature of ancient skepticism, namely that it is acommitment to ongoing inquiry (cf. Nawar 2019 on Augustine’sdefense of knowledge against the skeptics inContraAcademicos 3). The question that Augustine considers vital, then,is whether a life devoted to inquiry can be compelling, if seeminglythere is no prospect for ever attaining truth (cf. Lagerlund 2009). Itis as a philosophy of inquiry that skepticism makes a lastingcontribution to ethics, continuing, as it were, a Socratic legacy(Vogt 2017).

Augustine creates the framework that will become characteristic ofearly modern discussions. First, in his work skeptical arguments areexplored in order to be refuted. Second, the key issue is whether wehave knowledge, not whether we should hold anything to be true. InAugustine, the background for caring so much about knowledge is thepressing question of whether we can know God: whether we can know thathe exists and what his properties are. This might also be the reasonwhy knowledge of testimony gains importance (De Trinitate,15.12; in Schoedinger 1996). The Bible, or parts of it, might beconsidered testimony about God, and accordingly as one possible way ofattaining knowledge of God.

Third, in the process of asking whether we can have knowledge of Godit makes sense to distinguish between kinds of knowledge (sensory,rational, by testimony, etc.). If we know God, then we do so via oneof the kinds of knowledge. This becomes a standard feature ofdiscussions of skepticism. Philosophers go through the different kindsof knowledge that are conceivable, and examine them in turn. InAugustine as well as later authors, this includes mathematicalknowledge (Nawar 2022). Though Sextus wrote on geometry andarithmetic, questions about knowledge in mathematics do not shapeHellenistic debates between skeptics on the one hand and Epicureansand Stoics on the other hand. Fourth, Augustine conceives of what hecalls ‘inner knowledge.’ He envisages a skepticalscenario. Suppose we have no sensory knowledge, no rational knowledge,and no knowledge of testimony. We still know that we think, love,judge, live, and are (De Trinitate 15.12). In theCity ofGod 11.26, Augustine uses his well-known phrase “sienim fallor, sum” (even if I err, I am). That is, Augustinesuggests that we have knowledge of our mental acts (cf. Nawar 2022 onAugustine’s concern with self-knowledge). However, Augustinedoes not consider these pieces of knowledge foundational. While hepoints to them when he discusses the challenges of Academicskepticism, he does not systematically build upon them in refutingskepticism about sense perception, rational knowledge, and knowledgeof testimony. Rather, he refutes skepticism by stating that Godcreated us and the things that are known to us; God wanted thesethings to be known to us (De Trinitate 15.12). In laterepistemology, the idea of the turn into one’s mind and anintrospective access to one’s mental acts becomes a secularidea. Augustine is a transitional figure in the philosophy of mind,and thereby re-conceives skepticism. By focussing on a gap between‘what is in the mind’ and the world outside, he as it wereinvites external world skepticism (Vogt 2014a). But Augustine isconcerned with the path to God: the mind turns into itself andfrom there it moves further, toward God (e.g.,Confessions 7.17,23).

Next to Augustine, Al-Ghazali (1085-1111) plays a major role inre-conceiving the questions relevant to skepticism (Menn 2003,Kukkonen 2009). InThe Rescuer From Error, Al-Ghazaliliterally describes God as the rescuer from error (in Khalidi 2005).Like Augustine before and Descartes after him, Al-Ghazali movesthrough different cognitive faculties. Do the senses or reason allowus to gain knowledge? These questions are framed by the quest forknowledge of God. While Augustine thinks that knowledge of God comesthrough a combination of seeking God on the one hand and God’sgrace on the other, Al-Ghazali thinks it comes through spiritualexercises. However, once confidence in God is secured, trust in themore familiar ways of gaining knowledge—sense perception,rational reasoning, and so on—is restored (for a detailedtreatment of skepticism in Classical Islam, cf. Heck 2014).

One key difference between ancient skepticism on the one hand, andmedieval as well as Cartesian skepticism on the other, is that ancientskepticism is not framed by theological concerns. Note that inCartesian skepticism, God is not only invoked when it comes torefuting skepticism. More importantly, the skeptical problems arise ina way that depends on God as creator. Our cognitive faculties are seenas created faculties, and the world as a created world. A kind of‘faculty-skepticism’ that asks whether our cognitivefaculties are built so as to be erroneous is formulated, and apotential gap between our minds and the world opens up. Perhaps Godmade us in such a way that we are fundamentally wrong about everything(or, as later secular versions have it, a mad scientist experiments ona “brain in a vat”).

These are important steps away from the non-theological ancientconstrual of skepticism. The theological premises of early modernskepticism are not only foreign to ancient debates; they would be seenas misguided. From the Hellenistic point of view, theology is part ofphysics. An account of god is part of an account of the natural world(as such, it is unrecognizable as ‘theology’ from thepoint of view of later theologies). Human beings and their cognitivefaculties are natural parts of a natural world. They are organic andfunctional parts, interconnected with the other parts of the largewhole which the universe is. A mind-world-gap (of the kind envisagedin the Cartesian tradition) is inconceivable. Each ‘mind,’and that is, rational soul, is an integrated physical part of thephysical world. Like a part of a complex organism, it would not existwere it not for the interrelations it has with the other parts. Aphysiological account of the mind makes the stark divide between mindand world that figures in early modern skepticism unimaginable.

5.2 Skepticism About Induction

Contemporary discussions inherit long-standing problems from earlymodern philosophy. Among them, external world skepticism, skepticismabout other minds, and skepticism about induction are particularlyprominent. In assessing ancient skepticism, we might ask whether theancients saw these problems.

Among the skeptical problems of modern philosophy, skepticism aboutinduction stands out. Its early formulation in Hume does not depend onthe idea that our faculties are created by God, who also created theworld. Hume takes himself to engage with Pyrrhonian skepticism(Ainslie 2003). Induction proceeds from particular observations to ageneral conclusion. Skepticism about induction points out that, nomatter how many particulars were observed, the general claimpronounces also on what has not been observed. In this respect, theinference seems unwarranted and so we should suspend judgement on itseffectiveness. Induction can concern the ascription of properties tosome kind of entity as well as causal claims. In the latter, theskeptic observes that what regularly precedes a type of event may notbe its cause. Perhaps we cannot infer anything from the fact thatcertain properties or events regularly occur together (Vlasits 2020a;for a related argument in Al-Ghazali, cf. Kukkonen 2009).

Relevant ideas can be traced in various aspects of Sextus’philosophy. First, Sextus sides with anti-rationalist tendencies inmedicine. According to these schools, a doctor remembers that, inearlier cases, symptomA was alleviated by medicationB. They do not infer that medicationB makes symptomA disappear, or that the illnessC is the cause ofsymptomA. Second, the Five Modes do not exclusively targetproof. They address everything that lends credibility to somethingelse. They may thus also call into question signs that are taken asindicative of their causes. Sextus’ skeptic does not accept suchindicative signs. Third, Sextus discusses the role of so-calledcommemorative signs in the skeptic’s life (PH2.100–102; Allen 2001). For example, a scar is a commemorativesign of a wound. Both were co-observed in the past. A skeptic willthink of a wound when seeing a scar. But they do not commit to causalor explanatory claims. Fourth, Sextus records a set of Causal Modes(PH 1.180–86), which are specifically targeted towardcausal explanations (Corti 2014; on the relation between the CausalModes and Hume cf. Garrett 2020). He does not ascribe the kind ofrelevance to them that the Ten Modes and the Five Modes have. Indeed,he thinks the Five Modes can do the work of Causal Modes (that is,call into question causal and explanatory theses and theories).However, the Causal Modes go into great detail on how the skepticinvestigates any kind of causal thesis or theory.

5.3 Subjectivity and Other Minds

The distinction between subjectivity and objectivity is central tomodern discussions of skepticism. It is not envisaged in ancientthought. However, this does not mean that ancient philosophers do notreflect on questions relevant to this distinction. Arguably,Pyrrhonism conceives of the affections of the mind in ways thatanticipate later thought about subjectivity (Fine 2003a and 2003b).Sextus describes the skeptic’s states of‘being-appeared-to’ as affections of the mind. A skepticcan report these states in their utterances. Illustrating this point,Sextus uses expressions associated with the Cyrenaics, a Socraticschool of thought. These expressions literally mean something like‘I am being heated’ or ‘I am being whitened.’They aim to record affections without claiming anything about theworld. Fine argues that the skeptic’s beliefs are beliefs aboutthese affections (2000). With this proposal, Fine turns against twoprominent positions in scholarly debate about skeptical belief (seesection 4.4), that skeptics have no beliefs whatsoever, and that theyhave beliefs that fall short of holding true. Fine envisagesreflective beliefs: beliefs about one’s states of mind (onrelated issues in contemporary epistemology, cf. Feeney andSchellenberg 2020 and Glüer 2020).

Whether or not Sextus’ account of the skeptic’s mentallife includes reflective beliefs about one’s mental states, weshould note an important difference to later proposals of that kind.Later philosophers focus on the particular kind of certainty attachedto reflective knowledge. Reflective knowledge is sometimes seen as astepping-stone towards greater confidence in our cognitive powers, andour ability to also attain other kinds of knowledge. But it may not beobvious that reflective knowledge can take on this important role. Inthis respect, Augustine is still closer to ancient than to modernintuitions. He says that such pieces of knowledge as “I knowthat I think” are not what we are looking for. Augustineenvisages that reflective knowledge-claims can be iterated, so that wewould have infinitely many pieces of knowledge (“I know that Iknow that I think…”). But from his point of view, thiskind of knowledge leads nowhere. When we ask whether we can haveknowledge, we are interested in knowledge of the world and of God(De Trinitate 15.12).

Modern philosophers also pay great attention to the privileged accesscognizers have to their own cognitive activities. Augustine introducesa distinction that paves the way for this idea. He argues that themind cannot know what kind of stuff it is (De Trinitate 10.10and 15.12). The mind does not know its substance, but it knows itsactivities. For Augustine, this means that the mind knows itself. Themind is precisely what it knows itself to be: in knowing that onethinks, judges, lives, and so on, one knows the mind (Vogt 2014a).Note that this argument indicates that knowing that one lives and isare not pieces of knowledge about one’s bodily existence, butabout the activities of the mind.

Finally, modern philosophers conceive of the special kind of access toone’s own mind not only in contrast with our access to theworld. They also compare it to our access to what goes on in otherminds. One of their core problems is skepticism about other minds. Ifour own minds are accessible to us in the way nothing else is, then wemight not be able to ascribe mental states to others. For example, wemight not know that someone who looks as if she is in pain really isin pain. The ancient skeptics envisage nothing of this kind (Warren2011). This suggests that, insofar as they draw a distinction betweenaffections of the mind and the world, this distinction is construeddifferently than in modern skepticism.

5.4 External World Skepticism

In its early modern versions, external world skepticism involves theidea that there is a creator—someone who made the world and ourfaculties, and the fit or misfit between them. If this assumption iscrucial to external world skepticism, then the ancients do notconceive of this skeptical problem. From the point of view of modernphilosophy, ancient skepticism may appear limited by not addressingsome of the most radical skeptical scenarios (Burnyeat 1980 [1997] and1982; Williams 1988; Fine 2003a and 2003b). From the point of view ofancient skepticism, early modern skepticism and the long life that itsproblems enjoy, however, would seem to originate in a flawedtheology.

Contemporary philosophers sometimes discuss external world skepticismin terms of a paradox: one thinker finds herself torn between thestrength of skeptical arguments and her ordinary convictions. Forexample, she thinks thatthis is her hand. But she concedesthat the skeptical hypothesis that a mad scientist might have setthings up so that she has such perceptions and thoughts (the so-calledbrain in a vat scenario) is hard to refute. This way of framingdiscussions of skepticism is foreign to antiquity. In antiquity,skeptics and their opponents are different thinkers, each of them withone set of intuitions, arguing against each other. It is also foreignto ancient skepticism insofar as it inherits the early modern ideathat some greatly powerful agent sets things up, and in a sensecreates the thinker’s faculties and the world as it appears tothem.

However, the ancient skeptics might conceive of their own kind ofexternal world skepticism. One way to explore this question is to turnto the way in which Sextus describes that which is outside of theskeptic’s mind (Fine 2003 and 2003(2)). Throughout his work,Sextus employs the distinction between appearances and what really isthe case. Consider some of the detail of how he characterizes thelatter. One phrase in the Ten Modes is “ta ektoshupokeimena,” the externally underlying things(PH 1.61, 127, 128, 134, 144). Other phrases, meant todemarcate roughly the same contrast with appearances, are how thingsare in their nature (phusei) (PH 1.78, 123, 140),“the underlying things” (hupokeimena)(PH 1.106), as well as a combination: “the nature ofthe underlying external things” (PH 1.117, 163). Theexpressions which contain the word “external” might readas if Sextus was talking about the external world in a sense familiarto us from early modern skepticism.

But what are the underlying or external objects, as Sextus conceivesof them? For example, Sextus speaks of the underlying reality ofwhether honey really is sweet (PH 1.19). In such a case, itis assumed that there are ordinary objects. But we do not have accessto the properties they really have (Fine calls this PropertySkepticism, 2003b). However, external objects in Sextus’ sensealso include objects that later philosophers would not obviously seeas part of the external word. For example, an answer to the questionof what kind of life really is good would count as a claim aboutexternal or underlying reality (cf. Pellegrin 2010).

Another approach to the question of whether Sextus envisages some kindof external world skepticism is to turn to his discussions of physics.Ordinarily we take ourselves to live in a world in which there arebodies, movement, place, time, and so on. But as Sextus argues, we donot have compelling accounts of any of these core conceptions ofphysics. This leads to suspension of judgment on whether there arebodies, movement, place, time, and so on. Sextus’ discussions ofphysics might add up to a rather far-reaching skepticism about thenatural world.

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Other Internet Resources

• Groarke, Leo, “Ancient Skepticism”,StanfordEncyclopedia of Philosophy (Spring 2009 Edition), Edward N. Zalta(ed.), URL = <https://plato.stanford.edu/archives/spr2009/entries/skepticism-ancient/>. [This was the previous entry on ancient skepticism in theStanford Encyclopedia of Philosophy — see theversion history.]
• Podcasts on ancient skepticism by Peter Adamson:
  • Beyond Belief: Pyrrho and Skepticism
  • The Know Nothing Party: the Skeptical Academy
  • Healthy Skepticism: Sextus Empiricus

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Acknowledgments

I am grateful to John Cooper, Jens Haas, Chloe Layman, Wolfgang Mann,Christiana Olfert, Jim Pryor, and Carol Rovane for feedback on the2010 version of this entry, and to Abigail Breuker for feedbackon the 2022 version.