Overarching surveys of the history of philosophy often leave theimpression that philosophical skepticism—roughly, the positionthat nothing can be known—had many adherents in the Ancient andHellenistic Periods, disappeared completely as a topic of intellectualinterest during the Middle Ages, and returned as a viable position inthe Renaissance and Early Modern Periods.

As a survey, this is quite understandable, since no thinker from theMiddle Ages professed an active allegiance to a systematicphilosophical skepticism. But a closer examination of MedievalPhilosophy shows that despite skepticism’s disappearance as anovert philosophical movement, it continued to swirl in the thoughts ofmany of the best philosophers of the period. A very few, includingmost prominently Augustine and Al-Ghazali, claimed to have beensystematic skeptics at some points in their pasts. Many others heldskeptical views about localized issues such as one’s ability toknow an efficient cause. And even more discussed and attempted torefute commonplace skeptical arguments in defense of their own,anti-skeptical positions.

Chronologically speaking, skeptical issues were most prominentlyconsidered in works from both the leading and tail ends of the MiddleAges. Augustine’s 4th and 5th century attacks against theAcademic Skeptics mark the beginning of such discussions, and asmattering of treatments of skeptical issues appears periodicallythroughout the next 800 years. From the late 13th century onwards,however, skeptical issues began to exert a dominant and wide influenceon epistemological discussions, as seen in the works of such importantfigures as Henry of Ghent, John Duns Scotus, William of Ockham, PeterAuriol, John Buridan, and Nicholas of Autrecourt.

Though medieval discussions of skepticism are often found buriedwithin larger, formulaic discussions of theological topics, thesetreatments had influence beyond the academic circles within which theywere originally created and considered. Among Early Modernphilosophers, Descartes in particular owes a debt to these earlieraccounts of skepticism: versions of both hiscogito and EvilDemon arguments may be found in the works of medievalphilosophers.

In what follows we will briefly examine the relevant views of a fewrepresentative figures from each tradition and era. Though none claimsto be inclusive of the entire Middle Ages, the best scholarlyoverviews of important aspects of the medieval epistemologicaltradition are Tachau (1988), Pasnau (1997), Perler (2006), andLagerlund (2010a).

• 1. Ancient and Hellenistic Sources
  • 1.1 Pyrrhonian Skepticism
  • 1.2 Academic Skepticism
• 2. Skepticism in Pre-Scholastic Christian Philosophy
  • 2.1 Augustine
  • 2.2 Other Pre-Scholastics
• 3. Skepticism in Islamic and Jewish Philosophy
  • 3.1 Islamic Philosophy
    • 3.1.1 Al-Haytham
    • 3.1.2 Al-Ghazali
  • 3.2 Jewish Philosophy
• 4. Scholasticism and Skepticism
  • 4.1 Thirteenth Century
    • 4.1.1 Thomas Aquinas and Siger of Brabant
    • 4.1.2 Henry of Ghent and the Condemnation of 1277
  • 4.2 Fourteenth Century
    • 4.2.1 John Duns Scotus
    • 4.2.2 Peter Auriol, William of Ockham, and Adam Wodeham
    • 4.2.3 William Crathorn
    • 4.2.4 Nicholas of Autrecourt and John Buridan
• 5. Concluding Remarks
• Bibliography
  • Primary Texts and Translations
  • Secondary Sources
• Academic Tools
• Other Internet Resources
• Related Entries

1. Ancient and Hellenistic Sources

There were many varieties of skepticism extant during the Ancient andHellenistic periods, but two were particularly important to the laterhistory of topic: Pyrrhonian Skepticism, especially as presented bySextus Empiricus, and the Academic Skepticism of Cicero.

Pre-medieval adherents of both types of skepticism not only heldparticular skeptical positions, but also participated in a skepticalway of life, taking their arguments and positions as part of anoverarching ethical worldview. Skeptics took their radical views ofknowledge as means to the end of reaching the state of tranquility. Byusing common argumentative moves called tropes, skeptics sought toelevate themselves and others to a state of suspension of belief(epochê). And once this was reached, they held,one’s worries about philosophical matters would dissolve intranquility.

Because of these ethical excellences, skeptics held themselves up aswise men. The more radical Pyrrhonian Skeptics, who doubted the truthof all claims, quickly ran afoul of the following objection, which hasbeen given in various forms throughout the history of philosophy: athoroughgoing skeptic, it seems, cannot live his or her skepticism. Ifone doubts (and thus fails to act) on the truth of such claims as“Food is necessary for human life”, it would follow thatone could not live at all. Academic Skeptics attempted to avoid thisobjection by arguing that though skepticism precluded living by thetruth, since the truth could not be known, nevertheless one could liveby the truthlike or plausible. Hence theirs was a more practicalversion of skepticism.

1.1 Pyrrhonian Skepticism

Pyrrhonian Skepticism, which was to play such an central role in theRenaissance and Early Modern Philosophy, had no significant, directinfluence on later medieval thinkers, since texts exploring theposition (primarily the works of Sextus Empiricus, and to a muchlesser extent, Diogenes Laertius) were not in wide circulation.Floridi (2002) and Wittwer (2016) explore the textual transmission ofSextus’s works; Floridi notes that there are only seven knownLatin manuscripts from the period, though Wittwer has found furtherevidence to supplement this.

A few scattered references to Pyrrhonian skepticism have been found inthe Latin West, in the works of Bede (early 8th century), RabanusMaurus (9th century), and Peter of Auvergne (late 13th century). Morewas known of it to Byzantine and Islamic philosophers, since knowledgeof Greek was preserved in their intellectual communities, and sincethey had access to a greater range of ancient texts that addressed thetopic.

1.2 Academic Skepticism

Academic Skepticism, so-called because of its birth among scholarsworking in Plato’s Academy, was the type most known to themedievals. Academic skepticism was presented through the sympatheticworks of Cicero (De Natura Deorum andAcademica,primarily), and especially through many of Augustine’santi-skeptical arguments, such as those found in hisContraAcademicos. In fact, for most of the Middle Ages—at leastup through the 1430s—the termscepticus wasn’tused in the Latin tradition;academicus was the most commonterm for the skeptic. Further complicating matters, the medievalsfailed to recognize the distinction between Academic and PyrrhonianSkepticism. See Floridi (2002) and Schmitt (1972).

Schmitt’s (1972) study of the textual transmission ofCicero’s skeptical works brings out many interesting aspects ofits history. As was the case with Sextus, there were few manuscriptsof Cicero’sAcademica extant in the Middle Ages. Inaddition, there were two versions of it in circulation, and themedievals had only parts of each. And yet another problem for thosewho had access to the texts was that Cicero’s position was oftenconfused with that of one of his interlocutors in the work,Lucullus.

Henry of Ghent (late 13th C.) is the first medieval philosopher bothto have obvious knowledge of theAcademica, and to have madea serious philosophical attempt to come to grips with the viewsexpressed there. When John Duns Scotus critiques Henry’sepistemology, he shows no evidence of knowing Cicero’s text. Andfor the most part, later medievals were equally ignorant of it. Theirdiscussions of skepticism seem not to have been based on anexamination of or engagement with skepticism as presented by ancientauthors; it was a skepticism of its own sort, as will be detailedbelow.

2. Skepticism in Pre-Scholastic Christian Philosophy

2.1 Augustine

Augustine of Hippo (354–430) was a classically trainedrhetorician who explored many different schools of thought (Platonism,Manicheanism, and Skepticism) before converting to Christianity. Afterhis conversion, he began to write philosophical and theological worksaddressing some of the views from these schools. The most importantanti-skeptical work was hisContra Academicos (Againstthe Academicians), which has been discussed by Matthews (1977 and1992), Burnyeat (1982), King (1995), Curley (1997), O’Daly(2001), Bolyard (2006), and Dutton (2016).

InContra Academicos, Augustine targets a few key Academicclaims: (a) that appealing to truthlikeness or plausibility iscoherent; (b) that skeptics are wise; (c) that nothing can be known;and finally (d) that skepticism leads to tranquility.

According to Augustine, three of the four claims can be relativelyquickly dispatched. The first claim, concerning truthlikeness, cannotfunction alone as a standard, since one cannot know that something islike the truth without also knowing the truth itself. Second,skeptics cannot be wise, since wisdom requires knowledge of some sort.Third, skepticism leads away from tranquility, rather than towards it,since it puts one at odds with the morals of the rest of society,which in turn is likely to lead to strife.

The most important claim for the epistemological history of theproblem is the third: that nothing whatsoever can be known. Augustinetreats of it in some detail.

He casts the issue as follows. The skeptic argues that a wise man mustretreat to skepticism since nothing can be known. This inability isdue to the fact that knowledge of a truth—at least as understoodby certain Stoics—is only possible if that truth could notpossibly be caused to appear mentally by something different than whatit is in fact caused by. For example, if an internal mental image orconcept of a tree’s being beside a housecould becaused by a dream, then the tree’s being beside the house cannotbe known, even if the tree isin fact beside the house. Withthese stringent causal requirements, it is unsurprising to find thatAcademic Skeptics take the line they do: since no appearance meetsthis strict standard, they argue, it follows that nothing at all canbe known.

Augustine thinks this standard can be met, however, at least in somecases. Augustine aims to uncover propositions about which doubt is anutter impossibility. He soon finds the following four disjunctivestatements:

I still know something about physics. For I am certain that (1) thereis either one world or not. And (2) if there is not just one, thenumber of them is either finite or infinite… In the same way, Iknow that (3) our world is disposed as it is either by the nature ofbodies or by some plan. And I know that (4) (a) either it always didexist and always will, or (b) it started to exist and will never stop,or (c) it did not start in time but will have an end, or (d) itstarted and will not last forever…These truths are [logical]disjunctions, and no one can confuse a likeness of something falsewith them. (Contra Academicos 3.10.23)

In short, Augustine challenges the skeptic to convince him that suchexhaustive, disjunctive propositions can be confused with, or have alikeness of, what is false.

At this point that the skeptic counters with external worldskepticism: “How do you know this world exists…if thesenses are fallible?” In other words, the skeptic argues, thesedisjunctive statements about the external, physical world all assumethe existence of an external world, and thus they cannot be known tobe true if the external world itself cannot be known to exist. Ifexternal world skepticism can be maintained, it follows thatAugustine’s disjunctionscan be mistaken for what isfalse, and thus this particular argument against global skepticismwill fail.

Augustine’s primary response to the external-world skeptic isAugustine’s claim that things “seem” to him, andthat these seemings constitute the world. He supports this view byarguing that seemings are required in order for error tooccur—otherwise, what would we be mistaken about? And since thepossibility of error is the main impetus for skeptical doubt,skepticism requires the admission that things seem. In other words,for Augustine, one cannot doubt that one has mental content, even ifone might have doubt about whether this content corresponds toanything external to the mind.

Augustine gives further, more central arguments against globalskepticism asContra Academicos continues, claimingmathematical truths (e.g., “2 + 3 = 5”) and logical truths(e.g., “nothing both is and is not”) to be undoubtedlytrue. As with the physical disjunctions, such truths can be knownwithout knowing external objects with any determinacy.

Beyond his discussions in theContra Academicos, Augustinefrequently tackles epistemological topics in other works. Mostfamously, Augustine makes proto-Cartesian moves frequently, arguingthat the mere fact that he doubts and has various other mentalhappenings proves his own existence:

…who would doubt that he lives, remembers, understands, wills,thinks, knows, and judges? For even if he doubts, he lives; if hedoubts, he remembers why he doubts; if he doubts, he understands thathe doubts; if he doubts, he wishes to be certain; if he doubts, hethinks; if he doubts, he knows that he does not know; if he doubts, hejudges that he ought not to consent rashly. Whoever then doubts aboutanything else ought never to doubt about all of these… (Onthe Trinity 10.10.14)

Later, Augustine will draw on his theory of illumination to providethe grounds for certainty. According to this theory, God’sDivine Ideas serve as the guarantors of certainty, and they functionin much the way that Plato’s Forms do. Augustine first presentsthis view inDe Magistro (On the Teacher), and hemakes other references to it in later works. Augustinian Illuminationhas been widely discussed in the secondary literature, and Nash (1969)still remains one of the best introductions to the position.

2.2 Other Pre-Scholastics

There is little interest in skepticism exhibited in Christianphilosophy until the rise of the Universities in the 13th century.Hadoardus (9th C.) includes many quotations from theAcademica in his compilation of Cicero’s viewsgenerally, but he did no philosophical work with these quotations.John of Salisbury (12th C.) discusses Academic Skepticism to somedegree in hisPolicraticus, but there’s no evidencethat he had direct access to Cicero’s text; he most likely gotthe information either from Augustine or from some other secondarysource.

3. Skepticism in Islamic and Jewish Philosophy

3.1 Islamic Philosophy

Two Islamic thinkers are particularly important to the history ofmedieval skepticism. Al-Ghazali (Algazel to the Latin-speaking world)(ca. 1058–1111) travelled throughout the Middle East, but spentmost of his time in what are now Iran and Iraq. Al-Haytham (= Alhazen)(965–1039), who was born in what is now Basra, Iraq, wrotewidely on various scientific and mathematical subjects. In addition,while the Persian philosophers Rāzī (1149–1210) andṬūsī (1201–1274) are not skeptics, theirconcerns with global skepticism and the knowledge of first principleslead them to have extended discussions of skeptical arguments. Formore on Rāzī and Ṭūsī, see Fatoorchi (2013).See also the special issue ofTheoria (Lagerlund (ed.)2022); most of the contributors focus on medieval skepticism in theIslamic tradition.

3.1.1 Al-Haytham

Al-Haytham’sKitab al-Manazir (Book of Optics)was of particular importance for the later history of skepticism.Beyond his Arabic-speaking audience, it widely read in the Latin Westunder the titlePerspectiva orDe aspectibus,beginning with such philosophers as Roger Bacon (ca. 1214–1294).His views about the perceptual process had a wide influence throughoutthe later Middle Ages.

Al-Haytham held that many perceptions are inferential, and he explainshis views in II.3 of theOptics. Rather than always graspingsensed things in an unmediated way, he argues, we sometimes grasp themthrough sudden, “imperceptible” inferences. Theseinferences proceed so rapidly as to seem immediate, and thus weusually don’t notice that they are occurring at all. Al-Haythameven argues that seemingly self-evident propositions such as“the whole is greater than its [proper] part” areinferential. Given this inferential process, cognitive error becomes amore reasonable possibility.

He catalogued a number of optical illusions as well (OpticsIII.7), examining such problems as the way the moon when low on thehorizon appears larger than it does when higher in the sky, and theway that when one is in a boat floating down a river, the trees on theshore appear to be moving. Though Al-Haytham was not a skeptichimself, these illusory experiences provided fertile material forlater thinkers to consider. Tachau (1988) discusses his wide influenceon the scholastic tradition.

3.1.2 Al-Ghazali

Al-Ghazali sounds surprisingly Cartesian in an important section ofhisMunkidh min al-Dalal (Deliverance from Error).He begins by declaring his desire to reach certain knowledge, which heexplains as “that in which what is known is laid bare in such away as to leave no room for doubt, and is unaccompanied by thepossibility of error or illusion, to the point that the mind cannoteven conceive it.” (Deliverance 82)

He gives a (by now) familiar list of reasons for doubting thecertainty of things. First, disagreement among competing theoriesgives some initial doubt. Second, a few cases of sensory skepticism(e.g., a shadow cast by the sun appearing to remain still, when infact it is slowly moving as the day passes; the apparently small sizeof celestial bodies) lead him to lose confidence in all of his sensorybeliefs. This distrust of his senses also suggests, third, thatanother of his faculties—reason itself—may be faulty, andhe wonders whether even apparent logical truths might be false. Andfinally, he concludes by invoking dream skepticism. After setting upthese doubts, he says the following:

When these notions occurred to me and made an impression on my mind, Isought a cure but found none. For they could only be rebutted with aproof, and a proof can only be constructed by combining the first[principles of] knowledge. If these are not given, then it isimpossible to arrange a proof. This disease defied all cure and lastedfor almost two months, during which I embraced the [skeptical] creedin actual fact, though not in speech or expression. Eventually, Godcured me of this disease and my mind was restored to health andbalance. The rational necessary beliefs were once again accepted andtrusted, both securely and certainly. This did not come about bycomposing a proof or by an arrangement of words, but rather by a lightthat God almighty cast into my breast, which is the key to the greaterpart of cognizance. Whoever supposes that enlightenment depends uponexplicit proofs has narrowed the expanse of God’s mercy.(Deliverance 86)

Beyond this, Ghazali also questions the nature of causation in hisIncoherence of the Philosophers (Tahafutal-falasifa). Though he ultimately holds that all causation canbe traced to God, he argues that our observations of so-called naturalcauses are not sufficient for proving a direct causal link between theapparent cause and that which is caused. This Humean-leaning positionhas been discussed widely in the secondary literature. See, e.g.,Halevi (2002) for a recent treatment. For another recent account thatdoes much to situate Ghazali’s discussion of skepticism within abroader Islamic intellectual conversation about the subject, anddownplays the supposed connections between Ghazali and the EarlyModerns, see Kukkonen (2010).

3.2 Jewish Philosophy

There is no strong evidence of any significant skeptical tendencies orinterests among medieval Jewish philosophers. Judah Halevi (ca.1075–1141) discuses skepticism briefly in hisKuzariI.4–8; in this passage, a character in the poem professesskepticism about religious truths, and presents his requirements forwhat would count as knowledge. See Kogan (2003).

There has also been limited discussion of Maimonides as a skeptic.Some of it focuses, e.g., on his claims in theGuide for thePerplexed 2.24 that humans cannot have knowledge of heavenlythings. To take this to imply either a thoroughgoing skepticism or athoroughgoing concern with skepticism, however, is probably too strongan inference. For more on this issue, see Ivry (2008) and Haliva(2018).

4. Scholasticism and Skepticism

4.1 Thirteenth Century

The thirteenth century saw the birth of Scholasticism in the LatinWest. As Universities began to develop in such important centers oflearning as Paris and Oxford, so too did highly formalized andargumentative styles of debate and writing. At the same time, some ofthe intellectual consequences of the Crusades came to play animportant role in the history of skepticism: Muslim and Jewishscholars and writings came to the attention of Christians working onsimilar topics. Of particular importance was the translation of all ofAristotle’s works into Latin, along with many commentaries onthem (as well as original works) by Ibn-Rushd (Averroes) and Ibn-Sina(Avicenna).

With these texts came others (such as Al-Haytham’sOptics), and Christian scholars such as Roger Bacon began toinvestigate the cognitive process more thoroughly in their ownwritings. The dominant Augustinian theory of knowledge began to comeunder attack as the wealth of new accounts were contrasted, rejected,or synthesized. And as Augustine was reinterpreted, so too was hisrejection of skepticism.

4.1.1 Thomas Aquinas and Siger of Brabant

Thomas Aquinas (ca. 1225–1274) and Siger of Brabant (ca.1240–ca. 1282) were philosophers of vastly different reputations(the first was declared a saint and holds a preeminent place inCatholic theology; the second was accused of heresy and died undermysterious circumstances). Yet they both shared a deep commitment tosynthesizing the new Aristotelian texts into their respectiveviews.

Aquinas, as with Aristotle, exhibits no serious concerns withskepticism or with skeptical arguments. He occasionally makesreferences to sensory illusions, e.g., but he sees them as noepistemological threat. Baertschi (1986) and Pasnau (1997) treat ofthis issue briefly. Indeed, most of the secondary literature onAquinas focuses on the question ofwhy he has no suchinterest in skepticism. Varying accounts are given, and among them isAquinas’s Aristotelian belief that the cognitive process isfundamentally a reliable one. For the most part, Aquinas and mostlater medievals aim to explain the processes by which knowledge isacquired, rather than aiming to justify knowledge.

Furthermore, many scholars argue that the Aristotelian doctrine of theformal identity of knower and known plays a significant role forAquinas in particular. If (on this interpretation) the knower quiteliterally takes on the form of the known object, and thus becomesidentical to the known object in this formal way, then there is nochance for error. The knower is not at a remove from the known objectat all, on this account. There is considerable disagreement aboutAquinas’s motivations here; for a few representative views, seeGilson (1986), MacDonald (1993), Pasnau (1997), Jenkins (1997), andHibbs (1999).

Siger of Brabant, on the other hand, dealt directly with skepticismand skeptical arguments in hisImpossibile 2 and hisQuestions on the Metaphysics. Though, as Côté(2006) argues, he also declines to take skepticism to be a seriousthreat, he does take the time to address it. Most notably, Sigerraises the following question for consideration inImpossibile 2: “everything that appears to us areillusions and similar to dreams, so that we are not certain of theexistence of anything.” Siger has various responses in hisdiscussions, but his most important claims are (a) that a failure ofthe senses in some cases does not automatically imply failure in allcases; and (b) that if a sense report is not contradicted by another,more reliable sense report, then it itself is reliable. Furthermore,Siger gives a rather unconvincingreductio, arguing that ifthe senses are unreliable, no knowledge at all is possible. Takingthis as areductio of skepticism obviously would do little toassuage the worries of the committed skeptic.

Siger’s responses, though somewhat unsatisfying, do indicate thebeginning of a growing interest in skeptical problems. Henry of Ghentshows this interest even more starkly.

4.1.2 Henry of Ghent and the Condemnation of 1277

Henry of Ghent (ca. 1217–1293) was one of the most importanttheological masters of his day, and he was a contemporary of bothAquinas and Siger. Beyond his own philosophical work, Henry was acentral figure in one of the crucial events in medieval intellectualhistory: the Condemnation of 1277, which will be discussed at the endof this section. Brown (1973), Marrone (1985), Pasnau (1995), andAdams (1987) discuss Henry’s views in some detail.

Henry’s most concentrated attention to skeptical issues occursin the first two questions of hisSumma QuaestionumOrdinariarum (Ordinary Questions). Henry’sdiscussion of skepticism stands out when placed alongside other worksfrom the same period. Though Augustine’sContraAcademicos was extant, and though Augustine’sDeTrinitate echoed many of the anti-skeptical arguments from hisown earlier work, the vast majority of Henry’s scholasticcontemporaries (including Aquinas) took no serious interest inskepticism. Various explanations of this general attitude can begiven. Perhaps Augustine’s self-proclaimed refutation ofAcademic skepticism was taken to be the final word on the subject;Aristotle’s dismissive attitude towards skepticism would havereinforced this idea. But for whatever reason, Henry thought the issueof skepticism important enough to raise it in the opening question ofhis own most important theological work.

Henry lists a number of different skeptical arguments, drawing fromthe critical accounts of Aristotle, Cicero, Augustine, and Averroes,and mentioning the support skepticism garners from the views ofHeraclitus, Zeno, Protagoras, and Democritus, and Plato. He gives noevidence here of having direct access to any of the texts of thelatter five thinkers, though he knows of their views through the worksof others.

He begins by listing preliminary arguments both for and against thepossibility of knowledge. On the skeptic’s side, Henry discussescases of sensory relativism (what seems sweet to one person does notseem sweet to another, e.g.); the changeable nature of the sensoryworld; and the Learner’s Paradox from theMeno. Amongthe anti-skeptical arguments is Aristotle’s view(Metaphysics IV) that in denying knowledge, one is therebyclaiming certainty that one does not know, and thus the skeptic mustadmit to knowing something. He also pulls from Augustine’soft-repeated claim that in doubting, one knows that one doubts (Devera religione xxxix.73).

Henry then argues in a number of different ways that knowledge is infact possible. First, he draws from Augustine and Cicero. His weakestclaim here is that we can rely upon the testimony of others;otherwise, he says, knowledge of the distant past, or of places thatone has never visited, would be impossible. He also explains that onecan trust the veracity of a given sense experience provided ithasn’t been contradicted by a more reliable sense experience. Inaddition, he says that even if one is dreaming, one still knows thatone lives. As with many who follow him, Henry cites the certainty ofthe law of non-contradiction as well.

In the final section of the question, Henry replies directly to theskeptical arguments he outlined in the beginning. Though he gives toomany responses to detail here, Henry’s core idea is that thoughthe senses grasp only changeable things, one has the ability toabstract what he calls the “created exemplar” from theobjects of the senses; from this created exemplar, we can obtain alow-level knowledge of external objects (he calls this knowledge ofthe “true” or of the “truth”). Knowledge inthe full sense—that is, knowledge of the “puretruth”—requires knowledge of the “uncreatedexemplar”, or Divine Idea. Because the created exemplar ismutable in itself, it is only by seeing how it accords with theuncreated exemplar in God’s mind that full and certain knowledgeis possible. In short, Henry follows Augustine in spirit, even if notin detail: for both philosophers, knowledge is impossible withoutDivine Illumination.

In the second question of hisSumma, Henry exploresIllumination in more detail. As he begins to explain things, it soundsas if God’s general background influence is sufficient toexplain human knowledge. Later, however, Henry limits his optimisticoutlook. First, he argues that God illuminates each person

according to his condition and capacity, unless someone by displayinggreat malice merits that it be taken away from him altogether. Such aperson, as a result, would not see any truth at all…but woulddissipate into the error that he deserves. (Summa I.2.134)

Echoing some of Augustine’s remarks in theDe Magistro,Henry here seems to restrict epistemic certainty to those who aremorally worthy. Second, Henry diverges even further from his initialargument, saying that God offers the “rules of the eternallight”—that is, the Divine Ideas—

to whomever he wants and takes them away from whomever hewants… Thus God sometimes bestows the eternal rules on badpeople, with the result that in these rules they see many truths thatthe good cannot see… Sometimes, too, God takes these same rulesaway from such people and allows them to fall into error… [God]bestows [pure truth] through free will, on whomever he wants.(Summa I.2.131–132)

In short, according to this second argument, our ability to know withcertainty is entirely dependent upon God’s whim. We will knowonly in cases in which God wants us to. This emphasis on God’srole in the knowing process is of a piece with the emphasis on DivineOmnipotence one finds in the Condemnation of 1277, with which Henrywas intimately involved.

As the newly rediscovered Aristotelian texts began to find their wayinto university curricula in the thirteenth century, more conservativefaculty reacted. Bonaventure and Henry were among the latter, and eachargued against those who sought to replace the reigning Augustinianismwith too many new Aristotelian elements. Aquinas, Siger of Brabant,and others sought to synthesize Aristotle and Christianity in a muchmore thoroughgoing way than Henry thought acceptable. And as part ofthe commission organized at the Pope’s request, Henry helpedcreate a list of 219 propositions—some held by Aquinashimself—that were condemned as heretical to the Catholic faithin 1277 by Bishop Etienne Tempier.

If there were ever an instance of philosophical irony in the MiddleAges, this would be it. Despite Henry’s strong aversion toskepticism, and despite his arguments against it, the most importantpractical effect of the Condemnation of 1277 was to introduce anentirely new level of skeptical doubt. The Condemnation emphasizedGod’s omnipotence, and declared views that denied this to beheretical. As a result, the realm of the possible was expandeddramatically in medieval discussions. This concern quickly spreadsthroughout most Christian epistemological discussions, up through theend of the Middle Ages. If God is omnipotent, according to thisconcern, couldn’t he be deceiving us either in particular cases,or perhaps even globally? For a fascinating discussion of the varietyof responses one finds in the 13th and 14th century treatments of thisproblem, see Perler (2010).

4.2 Fourteenth Century

After the Condemnation of 1277, Christian philosophers became evenmore focused on epistemology. Debates often centered on the medievaldistinction between intuitive cognition and abstractivecognition—roughly, the distinction between knowing something aspresent and existent, and knowing something from a remove (e.g.,through memory, or through an inference). In addition, manyphilosophers began to explore the nature of sensory illusions in moredetail. And of course, the Evil Demon hypothesis loomed ever larger asthe notion of Divine Omnipotence was explored more fully.

4.2.1 John Duns Scotus

John Duns Scotus (1265–1308) worked in Oxford, Paris, andCologne. Living roughly a generation before Ockham, Scotus was afollower of Aristotle, and as with many of his time, Avicenna too hada profound impact on the development of his thought. As far asskepticism is concerned, Scotus is unconvinced by Henry’santi-skeptical arguments, but he thinks the threat of skepticismdangerous enough that he devotes considerable attention to arguingagainst the problem. Adams (1987) and Pickavé (2010) discusshis position in connection with skepticism.

In hisOrdinatio I.3.1.4, Scotus finds Henry’s createdexemplar/uncreated exemplar distinction insufficient for defeatingskepticism. Scotus’ critique of Henry has two main foci:Henry’s appeal to mutability, and Henry’s need for anuncreated exemplar. First, Scotus finds numerous problems withHenry’s worries about change, and he argues that change as suchdoes not prevent knowledge, and that even if it did, much of what weknow is sufficiently stable to support our knowledge claims. Indefense of his initial claim he argues, e.g., that our own mutabilitywould make knowledge utterly impossible, if Henry’s views arecorrect. His second claim about change also receives support invarious ways, most notably by his appeal to what he calls a nature(natura), which is (roughly) the essence of a thing. Here, heargues that since natures in themselves are immutable, and since eachcan have what Scotus calls an immutable relation to something else, wehave sufficient grounds for stability-based certainty.

Henry’s appeal to an uncreated exemplar to ground knowledge andcertainty is also problematic, according to Scotus. If we understandthe created exemplar as a species (roughly, an image or intentionalobject) formed in the soul during an act of cognition, then we areoften unsure whether that created exemplar existing in the soul trulycorresponds to an extramental object. Thus,

…if it cannot be judged when such a species represents itselfas such and when it represents itself as an object, then [no matter]what else concurs with such a species, one cannot have [any] certitudeby which the true may be distinguished from the truthlike.(Ordinatio I.3.1.4.104)

In other words, showing that the species in the soul corresponds to anuncreated exemplar—that is, a Divine Idea—doesnothing to help us determine whether that species corresponds tosomething in the sensory world.

According to Scotus, God has created the world in such a way thatknowledge is possible by means of his general, backgroundillumination, which amounts, in Scotus’ view, to a naturalprocess. With this in mind, we may now turn to an examination ofScotus’ positive view and its relation to skepticism.

Scotus holds that we have “necessary certitude” about fourkinds of knowledge. The first type is knowledge of self-evidentpropositions (propositionsper se notae)—such as‘a whole is greater than its parts’—as well asknowledge of propositions derived syllogistically from them. This typeof knowledge amounts to necessary, analytic truths, in his view: onceone knows the terms that enter into such a proposition, and once thoseterms are combined into the proposition, one cannot help but assent.Scotus’ second type of knowledge is knowledge of our owncontingent acts, including such propositions as ‘I amawake’ and ‘I am alive’. Scotus follows Augustine inholding that such knowledge is immune to skeptical attack because evenif the senses are deceived, once these terms are grasped, we can knowthe truth about them in such propositional contexts.

Though much can be said about these types of knowledge, the mostrelevant discussions for our purposes deal with the remaining types.Our certitude here depends crucially on the following claim:

Whatever happens frequently through something that is not free, hasthis something as its naturalper se cause.(Ordinatio I.3.1.4.106)

In other words, Scotus suggests a general inductive principle:whenever something occurs frequently over time, such repeatabilitycannot be due to chance. God has ordained that such regularities willoccur, and thus we can reach a general principle based on thoseinitial cases. Such regularities amount to natural occurrences, andthus require no appeal to special illumination.

Given this, his third type of certainty is discussed: what Scotuscalls things knowable “through experience”—e.g.,that “a certain species of herb is hot”. Such generalclaims, derived through our experience of numerous instances of thehotness of such herbs, are certain in virtue of the “non-freecause” principle above. Recognizing, however, that inductionsdon’t hold the same level of assurance that he is claiming forfirst two types of knowledge, Scotus backs off of his claim a bitlater, calling it “the lowest degree of scientificknowledge”, and admitting that such inductions may only tell usthat such regularities are “aptitudes”, not certainties(Ordinatio I.3.1.4.110–111).

When Scotus begins discussing his fourth type ofcertainty—particular knowledge claims about the external world,known through the senses—he ignores this weakened conception ofour senses’ reliability. Though later thinkers will be clearerin their indebtedness to the Condemnation of 1277 here, Scotus givesminimal notice of this. Instead, appealing again to his non-free causeclaim, he gives explanations of two main types of such experience.

First, because it is often the case that different sense modalitiesagree in their judgment about an external object—e.g., when wecan both touch and see the size of a ball—we have an inductionof sorts running here, and thus we can infer that this regularity isenough to give us certainty regarding the object underconsideration.

Second, in cases in which the sense modalities arenot inagreement—either because one modality yields a different resultthan another modality, or because a single modality yields differentresults at different times—we can appeal to the intellect toadjudicate among them. Using his example, we know that a stick inwater that appears broken cannot really be broken, because ourintellect knows the truth of the claim ‘the harder object is notbroken by the touch of something soft that gives way before it’(Ordinatio I.3.1.4.114–115). Thus, in such a case, wecan discount the testimony of sight. Scotus makes a similar moveregarding the apparent deception that occurs in dreams. In his view,“a person can tell when his faculty is disposed and when it isnot”, and thus he can tell whether he is asleep or dreaming(Ordinatio I.3.1.4.118–120).

4.2.2 Peter Auriol, William of Ockham, and Adam Wodeham

Peter Auriol (1280–1322) and William of Ockham (1285–1347)were contemporaries, though they took different paths bothphilosophically and ecclesiastically. Auriol spent most of his time atthe University of Paris, and eventually became an Archbishop beforehis untimely death. Ockham studied and taught at Oxford before beingbrought up on charges of heresy by the papal court in Avignon; hespent the last years of his life excommunicated from the Church, afterhaving fled to Munich. Though there is no evidence of the two havingever met, Ockham often argues against Auriol’s views in somedetail. Adam Wodeham (ca. 1300–1358), who commented on both oftheir views, was the personal secretary of Ockham for a time, andworked at Oxford.

Auriol’s role in the history of skepticism is twofold, and hehas been discussed in this connection most recently by Tachau (1988),Perler (1994), and Denery (1998). First, he develops an account ofintuitive cognition that raises the possibility of sensory illusion;second, he discusses particular cases of sensory illusion in somedetail in hisScriptum (prologue, q. 2 and d. 3, q. 14).

He begins by diverging from Scotus’s account of cognition.Scotus suggests that cognition of God, and cognition generally, canoccur in one of two ways: either abstractively or intuitively.Intuitive cognition is meant to include a human’s more-or-lessdirect sensory experience of the external world. Abstractivecognition, on the other hand, is knowledge from a distance; itabstracts from the presence and existence of the thing, as when weremember a deceased acquaintance or perform astronomical calculationsin a windowless room.

Auriol agrees with much of Scotus’s account of intuitive andabstractive cognition. Yet he imbues it with a psychological characterthat is absent in the latter’s work. For Auriol an intuitivecognition is had when one has theexperience of somethingas if it is present and existent. It is even possible, inAuriol’s view, to have such a cognition when the thing itself isabsent or non-existent. Auriol’s abstractive cognition, on theother hand, does not involve this experience or feeling ofsomething’s presence and existence, even if the thingis both present and existent. For any given state of theextramental world, both abstractive and intuitive cognitions canoccur. As a result, his position leaves him open to skepticalattack.

He realizes this possibility, and discusses many illusory experiencesbefore developing a response. These illusory experiences include suchstock examples as dreams, hallucinations, mirror images, theafter-images of the sun, the bent appearance of a straight stick thatis immersed in water, and the apparent motion of trees experienced bythose traveling down a river. He also mentions such cases as thedouble image of a candle that appears when one’s eyes aredistorted, the shimmering, changing appearance of colors on adove’s neck, and most interestingly, the fiery circle thatappears when a burning stick is whirled rapidly through the air.Though Auriol’s discussion stresses some experiences more thanothers, his basic point is that failing to identify such events asintuitive cognitions amounts to the assertion that “all thingsthat appear, are” (Scriptum 3.14.697).

Auriol responds to these challenges by distinguishing between realbeing(esse reale) and apparent being (esseapparens). This distinction has perplexed most readers of Auriol,and there is considerable disagreement about how to interpret it. Evenso, it is generally agreed that real being is what the object hasindependently of any perceiver, and also that whatever it is that ismeant byesse apparens, it is to be identified with a mentalor sensory appearance of some sort. Some scholars (e.g., Tachau) readAuriol as a representationalist, which of course does little to solvethe skeptical problem; others (e.g., Perler) see him as a directrealist. Whatever the answer in this particular case, Auriol is noskeptic. Not only does he believe that we can know external objects;we also know many self-evident propositions with certainty (logicaltruths, e.g.). For more on this aspect of Auriol’s thought, seeBolyard (2000).

William of Ockham considers Auriol’s perceptual problems, but heconcludes that they are not a serious threat. On his view, ourperceptual process (which occurs by means of intuitive cognitions) issuch that it is infallible: for any such intuitive cognition, if it isof a thing that exists, we will know this fact, and if it is not, wewill know this as well. He holds this view even given the possibilitythat God is deceiving us about such perceptions (e.g., by destroyingthe object while maintaining the perception of it).

Adam Wodeham disagrees with Ockham on this point; for him, there is noclear mark by which we can distinguish a true perception from a falseone in the case of a deceptive God. Nevertheless, he holds that ourperceptual process is generally reliable despite these problems. Formore on Ockham and Wodeham, see especially Karger (2004), Panaccio andPiché (2010), and Wood (2003); Adams (1987) and Tachau (1988)also discuss their skeptical and anti-skeptical views.

4.2.3 William Crathorn

William Crathorn (fl. 1330) was not considered by his contemporariesor later medieval commentators to be of the stature of such thinkersas Aquinas, Henry of Ghent, Scotus, or Ockham; still, his views give awindow into some of the skeptical worries extant at the time. Heworked at Oxford, flourishing in the generation after Scotus, andduring the time of Ockham. Tachau (1988) and Pasnau (1997) discuss hisviews.

In hisQuestions on the First Book of Lombard’sSentences, q.1, the Condemnation-inspired acknowledgment ofGod’s omnipotence generates and reinforces many skepticalproblems for Crathorn. In response, Crathorn uses God to bring himselfback from the skeptical abyss. Though not as rhetorically compellingas Descartes’ analogous moves in theMeditations, thephilosophical similarities among the two are striking.

Crathorn also makes frequent appeal to God’s omnipotence andpower to deceive us—nearly every page makes reference, directlyor obliquely, to this possibility. A favorite non-epistemologicalexample he uses concerns heat and fire: God, he repeatedly says, hasthe power to separate the heat from the fire that normally producesit. He also extends such divine powers to sensory cases. God, itseems, could maintain the vision of something even after that thingceases to exist. And as he tells us later, God’s power to dothis is vast. Here, his example is that of the lighted, fiery circlewe see when a torch is rapidly twirled through the air at night:

…if God were to preserve in your head for a whole year thatcircular color or another like it while no color existed externally,it would appear to you seeing that circular shape that you were seeingfor the whole year a flaming circle and the color of a circular shapeexisting outside you—when nevertheless there was no such thing.(Questions I.98–99)

Similar examples are used to show that we can be deceived in otherways as well. Afterimages of colors can remain briefly afterwe’ve turned away from that which caused the initial colorsensation. And it is within God’s power both to preserve asensible species of color after destroying the thing, and even tocreate such a sensible species even when no extramental thing everexisted. And finally, he mentions dream skepticism. Unlike Scotus andmost others who discuss this problem, Crathorn explains a case inwhich one who is awake thinks he is dreaming.

It is here that Crathorn begins to move us out of our skepticalposition, by putting limits on God’s power to deceive. First, heshows us cases in which God’s power cannot extend to thelogically contradictory: even God, Crathorn says, cannot make a stonethink. Second, he agrees with Scotus that seeming claims (e.g.,‘I feel hot’) and standard self-evident propositions(e.g., ‘the whole is greater than its part’) cannot bedoubted. Furthermore, he follows Augustine in arguing that thisinference cannot be doubted: ‘I doubt that I exist; therefore, Iexist’.

For more standard sensory skepticism, however, he combines theapproaches offered by Henry and Scotus. By appealing to a self-evidentproposition concerning God’s goodness, Crathorn tells us, we canshow that external world skepticism is incoherent. A benevolent Godwould not systematically deceive us in this way.

4.2.4 Nicholas of Autrecourt and John Buridan

Nicholas of Autrecourt (ca. 1300–ca. 1350) and John Buridan (ca.1295–1361) were contemporaries at the University of Paris. WhileBuridan maintained a good relationship with his ecclesiasticalsuperiors, Nicholas did not: the latter’s works were condemnedand publicly burned. Of particular interest in what survives are twoof hisLetters to Bernard of Arezzo. Recent discussions ofAutrecourt’s views may be found in Beuchot (2003), Zupko (2003),and Grellard (2007).

In hisFirst Letter, Autrecourt argues that Bernard’sviews lead to an extreme form of skepticism. As he interprets theview, it would follow that intuitive cognitions cannot guarantee theirown certitude: sensory illusions and the possibility of a deceptiveGod preclude this. But he goes further. As he explains, “youmust say that you are not certain of your own acts, for example, thatyou are seeing or hearing”. Furthermore, “you are notcertain whether anything appears to you at all” (FirstLetter 11). In short, one cannot be certain about any aspect ofthe external world, including even its very existence. And as he goeson to say, the existence of the past is equally uncertain, as is thevery existence of one’s own mind.

Autrecourt’sSecond Letter seeks to temper thisskepticism, but only to a degree. According to him, the only things ofwhich we can be certain are the principle of non-contradiction (i.e.,“nothing both is and is not”) and other propositions thatcan be derived from this principle. He maintains a causal,proto-Humean skepticism about existential inferences: “From thefact that some thing is known to be, it cannot be inferred evidently,by evidentness reduced to the first principle, or to the certitude ofthe first principle, that there is some other thing” (SecondLetter 11). As he continues, he says that the only substance ofwhich we can possess evident knowledge is his own soul

Nicholas of Autrecourt espoused the most radical form of skepticismfound at any point during the Middle Ages, and he was punished for it.Buridan, however, argues specifically against Autrecourt in his ownworks.

In hisQuestions on Aristotle’s Metaphysics II.1, forinstance, Buridan discusses various skeptical challenges, includingsensory illusion, dream skepticism, skepticism about induction, andAutrecourt’s causal skepticism. Again, the deceptivepossibilities of an omnipotent God play a large role in his worrieshere.

In response, Buridan takes a few different approaches. First, as withAutrecourt, Buridan holds the principle of non-contradiction to beundeniable, as is every proposition that can be derived from it. Buthe also says that there is a “virtual infinity of self-evidentprinciples through the senses, or through experience, or through theinclusion of terms without having to be proved by means of the firstprinciple [i.e., non-contradiction]” (QuestionsII.1.147, Klima trans.). In addition, Buridan drops hisepistemological standards for sensory knowledge in general: because ofthe possibility of God’s deceptiveness, at best we are capableof “conditional evidentness”. Similar reductions instandards occur in cases of induction, causation, etc. As he says,mathematical certainty is not expected in every subject. For more onBuridan and his broader intellectual context, see Zupko (2003),Grellard (2007), Lagerlund (2010b), and Karger (2010).

5. Concluding Remarks

Medieval Skepticism was not a movement. Rather, it was a series of(sometimes isolated) worries and responses to such skeptical problemsas those outlined above. While some impetus for later discussions wasgained from classical skeptical sources, for the most part medievalskepticism took its own path. Among the distinctly medieval additionsto the debate were an emphasis on the certainty of self-knowledge, andespecially on a widespread recognition across traditions thatGod’s omnipotence, and thus the possibility of Divine deceit onthese grounds, provides a special challenge to the epistemology ofanyone who holds a theistic worldview.

The fate of skepticism in the Renaissance and Early Modern Periods hasbeen discussed widely, but connections between these later versionsand those of their medieval antecedents have been less thoroughlystudied. Heider (2016) explores skeptical themes in the “SecondScholasticism” of the 16th and 17th centuries. Thinkers such asFrancisco Suárez, John Poinsot, and Francisco de Oviedocontinue to treat the Scotistic/Auriolian/Ockhamist issue of theintuitive cognition of non-existent objects. They do not considerglobal skepticism a live threat, as Descartes does, and their accountsare thus closer to those of 13th and 14th century philosophers.

For an overview of the later history of the skepticism, with a focuson canonical Early Modern philosophers, see Popkin (2003).

Bibliography

Primary Texts and Translations

Note: Texts in this section are alphabetized according to thefirst name of later medieval Latin authors, according toscholarly convention. Hence “William of Ockham”, e.g., islisted in the Ws, not the Os.

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