Content Externalism and Skepticism

First published Tue Mar 18, 2025

[Editor’s Note: The following new entry by Gary Ebbsreplaces theformer entry on this topic by the previous author.]

Content externalism (CE) is roughly the thesis that the contents of aperson’s thoughts are partly determined by facts about theperson’s social or physical environment. Although the centralarguments for CE presuppose our ordinary, non-skeptical beliefs, CEitself implies that we can each describe subjectively equivalentworlds in which we would receive the same sensory stimulations that wereceive in the actual world, but the contents of our thoughts differfrom the contents of our thoughts in the actual world. If we accept CEand ask how we know the contents of our own thoughts, we thereforeface the following dilemma. Either:

First horn. We cannot know the contents of our own thoughtsin the actual world without empirical investigation, since (a) we donot know without empirical investigation which of our subjectivelyequivalent worlds we are in, and (b) the contents of our thoughts arenot the same in all our subjectively equivalent worlds; or
Second horn. Wecan know the contents of our ownthoughts in the actual world without empirical investigation and fromthis knowledge we can deduce that we are not in any subjectivelyequivalent world in which the contents of our thoughts differ from thecontents of our thoughts in the actual world.

The first horn conflicts with our ordinary assumption that we can knowthe contents of our own thoughts without empirical investigation. Thesecond horn conflicts with the Cartesian skeptical intuition thatwe may actually be in any one of our subjectively equivalentworlds. Philosophers who accept CE typically reject the firsthorn by applying one of the strategies explained in the entry onexternalism and self-knowledge. The present entry grants that one of these strategies is successfuland focuses on the second horn. This horn forces us to confront thecentral question about CE and skepticism, namely,

(Q): Is the Cartesian skeptical intuition that we may actually be inany one of our subjectively equivalent worlds compatible with CEdespite appearances to the contrary, or, if not, are therereformulations of the intuition that are compatible with CE and enableus to formulate challenging Cartesian skeptical arguments?

There are two main streams of literature about the second horn thataim to clarify or answer (Q), one that begins with MichaelMcKinsey’s paper “Anti-Individualism and PrivilegedAccess” (McKinsey 1991) and another that begins with HilaryPutnam’s chapter “Brains in a Vat” (Putnam 1981).This entry surveys the literature in these two streams in sections 6and 7, after explaining key concepts, definitions, and arguments thatone needs to know to understand this literature, namely, Cartesianarguments and hypotheses (section 1), Cartesian assumptions aboutepistemic possibility and thought content (section 2), CE and itsvarieties (section 3), CE and epistemic possibility (section 4), andthe central dilemma for CE in skeptical contexts (section 5).

1. Cartesian skeptical arguments and hypotheses

It’s sunny today, or so I believe, as I look out the window. Butcould I be dreaming? And if so, do I know that it’s sunny today?According to a standard formulation of an argument in Descartes,Meditation One (paragraph 5), for any statement \(p\) aboutthe world around me that I believe, such asit’s sunnytoday, I should reason as follows:

(1): If I know that \(p\), then I know that I am not dreaming that\(p\).
(2): I do not know that I am not dreaming that \(p\).
(3): Therefore, I do not know that \(p\).

This is a paradigm example of aCartesian skeptical argument.If it is sound, it shows that we do not know any of the things weordinarily take ourselves to know about the world around us.

Argument (1)–(3) is valid and most of us find premise (1)compelling. Premise (2) appears compelling if one also acceptsDream:

Dream. Anything that can be going on or that one canexperience in one’s waking life can also be dreamt about.(Stroud 1984: 18)

ForDream appears to imply that no matter how vivid myexperience as of being awake may be, I cannot know that I am notdreaming. Argument (1)–(3) therefore appears to be sound.

AgainstDream one might argue that dreams are not as vivid orcoherent as waking experiences (Austin 1946, Leite 2011, Maddy 2017)or that “an interpretation of dreams presupposes thoughts in awakeful state” (Burge 1986a, p. 120, Bouswma 1949). To avoidsuch concerns, one might replace “I am dreaming” inargument (1)–(3) with “an evil demon is deceivingme” (Descartes,Meditation One, last paragraph). Butthe hypothesis thatan evil demon is deceiving me attributesmysterious, unexplained powers to the supposed evil demon. Perhaps forthese reasons, the skeptical hypotheses most widely used today arevariations on the idea that I am a bodiless brain in a vat whoseneuro-chemical receptors are stimulated by a super-computer in thesame way that the neuro-chemical receptors of an ordinary embodiedbrain are stimulated.

Suppose \(p\) is a statement about the world around me that I believe.Then a skeptical hypothesisH, such asI amdreaming,an evil demon is deceiving me, andI am abodiless brain in vat, is aCartesian skepticalhypothesis only if

(C1): The fact that I believe that \(p\) is compatible withH.
(C2): IfH then I do not know that \(p\).

For any statement \(p\) about the world around me that I believe andevery hypothesisH that satisfies (C1) and (C2) for my beliefthat \(p\), there is a corresponding Cartesian skeptical argument ofthe form:

If I know that \(p\), then I know that notH.
I do not know that notH.
Therefore, I do not know that \(p\).

Arguments of this form are designed “to convince us, on thebasis of assumptions we ourselves hold, that all or a large part ofour claims about the empirical world cannot amount to knowledge”(Putnam 1994: 284).

2. Cartesian assumptions: epistemic possibility and thought content

To construct a Cartesian skeptical argument one needs to formulate askeptical hypothesisH that satisfies (C1) and (C2) for allor a very large number of statements about the empirical world thatone believes. A hypothesisH satisfies constraint (C1) for anempirical statement \(p\) that I believe only if

(M): The content of my belief that \(p\) would be the same whether ornotH.

It may seem easy to construct Cartesian hypotheses that satisfy (M)for any statement about the empirical world that one believes. Forwhen one imagines a situation in which one’s sensory surfaceswould be affected in the same way they are in the actual world butone’s actual beliefs would be radically mistaken, as in astandard Cartesian brain-in-a-vat hypothesis, it is tempting to inferthat in the imagined situation one’s belief contents would bethe same as one’s belief contents in the actual world (Blackburn1984: 312; Searle 1983: 230). CE implies that such inferences may leadto false conclusions: our belief contents are not guaranteed to be thesame in every environment in which our sensory surfaces are affectedin the same as they are in the actual world (Burge 1986a: 123). CEtherefore implies that some traditional efforts to formulate skepticalhypotheses fail to satisfy (M) and hence also fail to satisfy (C1)(Falvey & Owens 1994: 121).

This observation sets part of the framework for debates in theliterature about whether CE is compatible with the Cartesian skepticalintuition that we may actually be in any one of our subjectivelyequivalent worlds. The debates take for granted that“conceptualization on any considerable scale is inseparable fromlanguage” (Quine 1960: 3). They also assume that arguments forsemantic externalism (SE)—roughly the thesis that thereferents and satisfaction conditions of a person’s words andthe associated truth conditions of the person’s utterances arepartly determined by facts about the person’s social or physicalenvironments—are a basis for, though distinct from, argumentsfor CE. The details of the debates depend on which varieties of SE andCE are under discussion. The next section presents the standarddefinitions of SE and CE and summarizes the varieties of SE and CEwhose consequences for skepticism are the topic of sections 6 and 7 ofthis entry.

3. Semantic externalism and content externalism

3.1 Preliminary definitions

Semantic externalism (SE) and content externalism (CE) are defined interms of “narrow” psychological states, namely, thephysiological and functional states and processes of a person’sbrain and/or body that can be described without using any semanticalterms, without specifying the contents expressed by the states, andwithout mentioning any conditions beyond the person’s bodilysurfaces (Putnam 1975a; Burge 1979, 1986a; Farkas 2006; Haukioja2017):

Semantic Internalism (aka semantic individualism):The referents and satisfaction conditions of a person’s wordsand the associated truth conditions of the person’s utterancesare entirely determined by (supervene on) the person’s narrowpsychological states.

Content Internalism (aka content individualism): Thecontents of a person’s beliefs and other mentalattitudes—the mental states we describe by using sentencescontaining “that”-clauses, such as “Alex believesthat water is wet”— are entirely determined by (superveneon) the person’s narrow psychological states.

Semantic Externalism (aka semanticanti-individualism) is the negation of semantic internalism: Thereferents and satisfaction conditions of a person’s words andthe associated truth conditions of the person’s utterances arenot entirely determined by (do not supervene on) the person’snarrow psychological states.

Content Externalism (aka content anti-individualism)is the negation of content internalism: The contents of aperson’s beliefs and other mental attitudes are not entirelydetermined by (do not supervene on) the person’s narrowpsychological states.

Theories of SE and CE also imply that the semantics of our words andthe contents of our thoughts depend on our environment. The severalvarieties of SE and CE that have been influential in the literaturefall into two main groups:singular SE and CE, andgeneral SE and CE.

3.2 Singular semantic and content externalism

Singular semantic externalism: The referents of aperson’s singular terms, including proper names, such as“Aristotle,” demonstratives, such as “this”and “that,” and indexicals, such as “now” and“I,” are partly determined by causal relations that theperson bears to particular objects in the person’s environmentor to other speakers who are causally related to particular objects inthe person’s environment.

According to direct reference theories, the semantical role of aproper name in a sentence, such as “Aristotle was fond ofdogs,” is to supply an object, such as Aristotle, as aconstituent of the content expressed by the sentence (Kripke 1980),and the semantic role of a demonstrative, such as “that,”as it occurs in a speaker’s utterance of a sentence, such as“That is a cat,” when the utterance is supplemented by thespeaker’s appropriate and successful demonstration of a nearbyobjecto, such as a cat, and is to supplyo as aconstituent of the content expressed by the utterance (Kaplan 1989:500).

Singular content externalism: A person’ssingular thought contents are partly individuated by objects in theactual environment beyond the person’s narrow mental states.

Kripke 1979 presents a puzzle about how direct reference theories arerelated to singular CE. Perry 1979 and Salmon 1986 posit psychologicalanalogs of Fregean senses to try to solve the puzzle.

Singular CE implies that a speaker who utters a sentence containing asingular term on a given occasion may be confident that the singularterm, as the speaker uses it on that occasion, has a referent, when infact it does not. Such cases engenderillusions of singularexternal thought content.

3.3 General semantic and content externalisms

3.3.1 General semantic externalisms

General semantic externalism: The referents andsatisfaction conditions of a person’s non-singular terms,especially the person’s general terms, such as“elm,” are partly determined by (or supervene on) causalrelations that the person bears to objects and/or other speakers inthe person’s physical and social environments.

Singular SE and general SE are often combined, but they are logicallyindependent.

Arguments for general semantic externalism

Kripke supports general SE by proposing that we view natural kindterms, such as “gold,” as singular terms that refer tosubstances. He theorizes that the referents of such singular terms ofsubstances and the referents of ordinary proper names are fixed insimilar ways:

In the case of proper names, the reference can be fixed in variousways. In an initial baptism it is typically fixed by an ostension or adescription. Otherwise, the reference is usually determined by achain, passing the name from link to link. The same observations holdfor such a general term as “gold”. If we imagine ahypothetical (admittedly somewhat artificial) baptism of thesubstance, we must imagine it picked out by some such“definition” as, “Gold is the substance instantiatedby the items over there, or at any rate, by almost all of them”.(Kripke 1980: 135)

On this version of general SE, satisfaction conditions for thepredicate “is gold” must be defined in terms of thesubstancegold, roughly as follows: “is gold” issatisfied by an objectx if and only ifx is aninstance ofgold (Soames 2005: 58).

Putnam describes ostensive definitions of kind terms such as“water” without referring to a substance, as follows:

Suppose I point to a glass of water and say “this liquid iscalled water”…. My “ostensive definition” ofwater has the following empirical presupposition: that the body ofliquid I am pointing to bears a certain sameness relation (say,x is the same liquid asy, orx is thesame_L asy) to most of the stuff I andother speakers in my linguistic community have on other occasionscalled “water”. (Putnam 1975a: 225)

On this version of general SE, “‘water’ refers towater” is short for “x satisfies ‘iswater’ if and only ifx is water” (Putnam 1976b:190). The sense in which the kind term “water”rigidly refers to water, is that

(For every worldW) (for everyx inW)(x is water if and only ifx bearssame_L to the entity referred to as‘this’in the actual world W₁) (Putnam1975a: 231)

Kripke and Putnam argue that if water is H₂O, as webelieve, then in any possible world in which there is water, it isH₂O. (As Putnam later notes, “Normal water isactually a quantum mechanical superposition of H₂O,H₄O₂, H₆O₃ … Verylittle (if any) water is simply H₂O” (Putnam 2013:198n10). To save words below, I’ll use“H₂O” to denote these quantum states.)

Kripke and Putnam both emphasize, in effect, that speakers of the samenatural language typically take one another’s words at facevalue and evaluate their utterances accordingly, even when they knowvery little about the topics their utterances concern. Putnam devisedhis method of constructing Twin Earth thought experiments to highlightthe semantic consequences of accepting this observation about ourordinary linguistic practices. In his best-known Twin Earth thoughtexperiment (Putnam 1975a: 223–225), Putnam imagines Oscar, who(a) lives on Earth and (b) does not know that water is H₂Obut (c) is competent enough in the use of the English word“water” that his fellow English speakers take his uses of“water” at face value, and (d) would defer to a chemist onEarth who tells him that water is H₂O. Putnam judges andinvites us to agree, that Oscar’s word “water” istrue ofx if and only ifx is water, and that whenOscar says, while pointing at a liquid in a glass, “That iswater,” his utterance is true if and only if the liquid in theglass is water. Putnam then describes a narrow psychological twin ofOscar’s, Toscar, who lives on Twin Earth, which is exactly likeEarth except that wherever there is H₂O on Earth there is adifferent liquid, whose molecular structure, XYZ, is very differentfrom the molecular structure of H₂O, but whose superficialqualifies are similar to those of water at normal pressures andtemperatures. Putnam judges and invites us to agree that since XYZdoes not bear the appropriate same_L relation tothe portions of liquid that we call “water” on Earth, XYZis not water, but another liquid that we may call twater. He infersthat Toscar’s word “water” is true ofx ifand only ifx is twater and that when Toscar says, whilepointing at a liquid in a glass, “That is water,” hisutterance is true if and only if the liquid in the glass is twater.Putnam concludes that the truth conditions of Oscar’s andToscar’s utterances of sentences containing the term“water” are not determined by their narrow psychologicalstates. In a final step he argues that this was true even in 1750,before chemists on Earth or Twin Earth discovered the chemicalstructures of water and twater.

Illusions of reference for general terms

When we use a proper name, we presuppose that it has a referent.Similarly, when we use a natural kind term, we presuppose that thereare structural properties that are common to all the things to whichwe and others in our community apply the term. This similarity has ledmany philosophers (Schwartz 1977; Devitt & Sterelny 1987; Brown2004; Hacking 2007; Wikforss 2013) to conclude that just as a singularterm may fail to refer to anything due to a false presupposition, so anatural kind term may fail to have satisfaction conditions due to afalse presupposition. A corollary is that just as a speaker may beunder an illusion that a given proper name or a given use of ademonstrative refers to an object, so a speaker may be under anillusion that a natural kind term has satisfaction conditions. Thisconclusion and corollary result in apresuppositional theory ofgeneral SE.

An equally influential view is that natural kind terms are notanalogous to proper names in this way. Putnam 1962b argues, forinstance, that we can make sense of discovering that cats are notanimals, but automata, and Kripke (1980: 122) agrees. If we can makesense of discovering that cats are automata, then our word“cat” may be true of (satisfied by) the verythings—automata as it turns out—that we have called catsall along, even if our presupposition that “cat” is anatural kind term is false. Putnam thinks we can also make sense ofdiscovering that a term such as “pencil,” which we believeto be an artifact term, is in fact a natural kind term. In short, wemay discover that a term we take to be a natural kind term is anartifact term, and we may discover that a term we take to be anartifact term is a natural kind term, without concluding that theterms do not refer (Putnam 1975a: 240–242). Thisholistictheory of general SE is shaped by a commitment to epistemologicalholism, according to which any (or almost any) statement may bechallenged and perhaps revised without changing the satisfactionconditions of the general terms that occur in it (Quine 1951; Putnam1962a; Burge 1986b; Williamson 2007).

3.3.2 General content externalisms

General content externalism: The contents of aperson’s general (non-singular) thoughts, such asElms aredeciduous trees, especially (but not exclusively) the parts of aperson’s general contents that are expressed by theperson’s uses of general terms, such as “elm,” arepartly determined by causal relations that the person bears to objectsand/or other speakers in the person’s physical or socialenvironments.

Singular CE and general CE are often combined, but they are logicallyindependent.

Arguments for general content externalism

The above arguments for general SE leave it open whether they can beextended to support general CE. McGinn 1977 suggests this extension.Burge 1979 argues in detail that our default practices of takingothers’ words at face value is of a piece with our ordinary waysof attributing beliefs and thoughts to other speakers. Thisclarification of the consequences of SE seems inevitable inretrospect. If my local arborist says, for instance, “There areelm trees in Bloomington,” in taking his utterance of thesewords at face value I thereby also take him to have said and to haveexpressed the thought thatthere are elm trees inBloomington. One can accept this observation without supposingthat language is prior to thought, or vice versa. The notions ofreference, linguistic meaning, and thought may be “interwoven incomplex ways which render it impossible fully to analyze one in termsof the other” (Burge 1986b: 718; see also Williamson 2007:Ch.1).

Illusions of content expressed by general terms

On the presuppositional theory of general SE, if there are nostructural properties that are common to all the things to which weapply a term T that we take to be a natural kind term, then the termfails to be true of anything. In these circumstances, a person who isunaware of the reference failure and tries to express a thoughtcontent by using a sentence that contains T fails to express anycontent at all. Such situations generateillusions of contentexpressed by general terms. Illusions of this sort, and theresultingpresuppositional theory of general CE, are centralto some of the debates discussed in section 6 below.

An equally influential variety of general CE implies that“Nearly anything can be topic of nonstandard theorizing”(Burge 1986b: 709). We may theorize, for instance, that cats areautomata while knowing that others believe that cats are animals. Thediscovery that cats are automata would not, on thisholistictheory of general CE, engender illusions of content expressed bythe general term “cat.”

4. Content externalism and epistemic possibility

Both presuppositional and holistic theories of general CE make itdifficult to formulate some of our commonsense judgments about what isepistemically possible. For instance, suppose that, as we believe,elms and beeches are different species of tree. Then according togeneral CE, in a way that is mirrored in SE by the rigidity of thesatisfaction conditions for “elm” and “beech,”elms can’t be beeches: there is no possible world in which elmstrees are beech trees. Before I learned that elm trees are not beechtrees, however, it was in some sense “epistemicallypossible” for me that elm trees are beech trees. I cannotanalyze this sense of epistemic possibility in terms of a possibleworld in which elm trees are beech trees, since I have now learnedelms are not beech trees and I therefore believe there is no suchpossible world.

In non-skeptical contexts the standard solution to this problem isroughly to say that given our sensory evidence, there is a“qualitatively identical epistemic situation” in which thereferences of some of our words differ in such a way that the contentwe express by our utterances of “elm trees are beechtrees” is true. More precisely,

Suppose that in the actual world \(w_1\), at a given time \(t\),
speaker \(A\) uses a sentence \(S\) to express content \(p\), onthe basis of \(A\)’s sensory evidence \(E,\)
the negation of \(S\) is true in \(w_1\), and
speaker A lacks sufficient evidence to judge responsibly that thenegation of \(S\) is true in \(w_1\).
Then it is “epistemically possible_K” that \(p\)for speaker \(A\) with sensory evidence \(E\) at time \(t\) if andonly if there exists a counterfactual possible world \(w_2\) inwhich
There exists an epistemic counterpart \(A^*\) of \(A\) whosesensory evidence \(E\) and linguistic behavior up to and includingtime \(t\) are the same as \(A\)’s,
\(A^*\)’s utterances of sentence \(S\) express a truecontent \(q\).
(The subscript “K” in “epistemicallypossible_K” refers to Kripke, who first sketched thisanalysis in Kripke 1980 [103–104]. For applications of the samesort of analysis, see Block & Stalnaker 1999: 6–7; Putnam1990: 62; and Yablo 2008: 185–188.)

To apply this analysis to the commonsense judgment that it was onceepistemically possible for me that elm trees are beech trees, thecontent \(q\) cannot be thatelm trees are beech trees. Inmany other cases, however, the content \(q\) need not differ from thecontent \(p\) for it to be “epistemicallypossible_K” that \(p\) for speaker \(A\) with sensoryevidence E at time \(t\). In 2018 it was “epistemicallypossible_K” for me thatthere are no Asian Carp inLake Michigan. I now know that there are Asian Carp in LakeMichigan and that they have been there since 2010. If, as I believe,it is a contingent fact that there are Asian Carp in Lake Michigan,then there is a counterfactual possible world in which my evidence in2018 was the same as my evidence in 2018 in the actual world, but myutterances of “There are no Asian Carp in Lake Michigan”express thetrue content thatthere are no Asian Carp inLake Michigan. Cases of this second type are also important tothe debates discussed in sections 6 and 7 below.

5. The central dilemma for content externalism in skeptical contexts

When we solve the problem explained in section 4 by analyzingepistemic possibility as epistemically possible_K, we applya method that is structurally similar to the method we use toconstruct Twin Earth cases that support CE: we hold constant some of aperson’s physical and mental states, including theperson’s “qualitative epistemic situation,” whilestipulating that his environment differs from his actual environmentin ways that imply that his thought contents differ from his thoughtcontents in the actual world. In contexts in which skepticism is notat issue, this method has convinced many philosophers to accept bothCE and the thesis that one’s sensory experiences are determinedby one’s narrow psychological states. When one considers themethod in skeptical contexts, however, this combination of views ispuzzling.

The puzzle arises because the method enables us to describe, for eachof us, any number of subjectively equivalent worlds in which oursensory surfaces are affected in the same way, we express our beliefsand thoughts by using sentence tokens with the same syntactic shapes,but the beliefs and thoughts we thereby express differ from our actualbeliefs and thoughts (Ebbs 1996: 510; see also Blackburn 1984: 312, on“spinning the possible worlds”). Our descriptions of theseworlds appear to imply that from our subjective, first-person point ofview, we cannot discriminate between them. We therefore seem toface

The Central Dilemma:

Either:

First horn. We do not know without empirical investigationthe contents of the thoughts we express by using our sentences, sincewe do not know without empirical investigation which of oursubjectively equivalent worlds we are in, and the contents of thethoughts we express by using our sentences are not the same in all oursubjectively equivalent worlds.

Second horn. We know without empirical investigation thecontents of the thoughts we express by using our sentences, and fromthis knowledge we can infer that we are not in any of our subjectivelyequivalent worlds in which the contents of the thoughts we express byusing our sentences differ from what we know them to be.

The first horn is incompatible with our ordinary assumption that weeach haveminimal self-knowledge—we each know withoutempirical investigation what thoughts our own utterances express. Thesecond horn conflicts with the Cartesian skeptical intuition that wemay actually be in any one of our subjectively equivalent worlds.

Content externalists typically reject the first horn. They note, forinstance, that

[W]hatever fixes the content of the first-order belief that I expressby saying “There is water in the glass” also fixes in thesame way the embedded content in the second-order belief I express bysaying “I believe there is water in the glass”. (Shoemaker1994: 260n7; see also Davidson 1988: 664)

Some content externalists (Gibbons 1996; Brown 2004) argue that suchsecond-order beliefs are knowledge if they are true in a contextuallyrelevant range of circumstances. Other content externalists (Burge1988; Wright 1994; Ebbs 1996; Goldberg 2002) argue that the key toreconciling our ordinary assumption that we have minimalself-knowledge with CE lies not in the reliability of our second-orderbeliefs about the contents of our thoughts, phenomenological signs, orempirical investigation, but in our activity of thinking andself-ascribing our own thoughts. See the entry onexternalism and self-knowledge for details.

The rest of the present entry grants that minimal self-knowledge iscompatible with CE. The entry focuses on the second horn of thecentral dilemma, revisiting the first horn only as needed to assessdebates about the second horn. It is this horn that forces us toconfront (Q), namely,

Is the Cartesian skeptical intuition that we may actually be in anyone of our subjectively equivalent worlds compatible with CE despiteappearances to the contrary, or, if not, are there reformulations ofthe intuition that are compatible with CE and enable us to formulatechallenging Cartesian skeptical arguments?

6. Content externalism and knowledge of one’s external environment

McKinsey 1991 challenges content externalists to explain how they canconsistently accept three claims to which they are apparentlycommitted, namely,

(1): Oscar knows independently of empirical investigation that he isthinking that water is wet.
(2): The proposition that Oscar is thinking that water is wetconceptually implies E.
(3): The proposition E cannot be known independently of empiricalinvestigation.

where E is a proposition describing “the relations that Oscarbears to other speakers or objects in his external environment”(McKinsey 1991: 10). To say that the proposition thatOscar isthinking that water is wet “conceptually implies” aproposition E, according to McKinsey, is to say that

there is a correct deduction of [E] from [the proposition thatOscar is thinking that water is wet], a deduction whose onlypremisses other than [the proposition thatOscar is thinking thatwater is wet] are necessary or conceptual truths that areknowable a priori, and each of whose steps follows from previous linesby a self-evident inference rule of some adequate system of naturaldeduction. (McKinsey 1991: 14, with the bracketed substitutions for\(p\) and \(q\))

McKinsey reasons that if (1) and (2) are true, then contrary to (3)one can know a proposition E describing “the relations thatOscar bears to other speakers or objects in his externalenvironment” without empirical investigation.

To support (3) McKinsey points out that CE is defined in terms ofnarrow and wide psychological states. He calls these notions Cartesianbecause

a narrow state should be (roughly) a state from which the existence ofexternal objects cannot bededuced, and a wide state would beone from which the existence of external objectscan bededuced. (McKinsey 1991: 14)

From this he infers (on page 16)

(*): One cannot know a priori that there are external objects in eachof one’s subjectively equivalent worlds (worlds in whichone’s narrow psychological states are the same as they are inthe actual world).

McKinsey 1991 concludes that a content externalist who accepts (1) and(2) is thereby also committed to accepting (3), which is incompatiblewith (*). This is a sharpened and strengthened version of the secondhorn of the Central Dilemma explained in section 5.

6.1 General content externalism and conceptual implication

One way to evaluate McKinsey’s challenge is to ask whether acontent externalist is committed to accepting a proposition of theform (2), where E is a proposition describing relations that Oscarbears to other speakers or objects in his external environment(Brueckner 1992a: 113). As a candidate for E, McKinsey may have hadsomething like the following proposition in mind (from Brueckner1992a: 112):

(E1): Oscar inhabits an environment containing H₂O and notXYZ.

The problem with this candidate for E is that on a holistic theory ofCE, as Burge explains,

An individual or community might (logically speaking) have been wrongin thinking that there was such a thing as water. It is epistemicallypossible—it might have turned out—that contrary to anindividual’s belief, water did not exist. (Burge 1982: 97)

We may read “epistemically possible” here as“epistemically possible_K,” as the continuationof the passage makes clear:

Part of what we do when we conceive of such cases is to rely on actualcircumstances in which these illusions do not hold—rely on theactual existence of water—in order to individuate the notions wecite in specifying the propositional attitudes. We utilize—mustutilize, I think— the actual existence of physical stuffs andthings, or of other speakers or thinkers, in making sense ofcounter-factual projections in which we think at least some of thesesurroundings away. (Burge 1982: 97)

Applying this method we may describe counterfactual worlds in which(a) Oscar’s sensory experiences are the same as they are in theactual world, and (b) there is no water, and (c) either Oscar’slinguistic community theorizes that water is H₂O or Oscarhimself does so, and (d) Oscar’s uses, whether in thought orspeech, of “water is wet” express the same contents thatthey actually express (Burge 1982: 98). (Clause (d) would be true, forinstance, if Oscar or some members of his linguistic community arecausally related to instances of hydrogen and oxygen, so that thecompound expression H₂O has the same content for Oscar asit has for us.)

The reasoning just sketched supports the conclusion that Oscar cannotknow (E1) without empirical investigation. It also supports (N):

(N): It is necessary that if Oscar is thinking that water is wet, thenone of the following must be the case: (i) water exists, or (ii) Oscartheorizes that H₂O exists, or (iii) Oscar is part of acommunity of speakers some of whom theorize that H₂Oexists. (Brueckner 1992a: 116)

The above argument for (N) relies on “the actual existence ofwater…in order to individuate the notions we cite in specifyingthe propositional attitudes” (Burge 1982: 97), however, andtherefore does not establish or presuppose that CE conceptuallyimplies (N) (Brueckner 1992a: 116).

If the holistic theory of CE that leads to these conclusions iscorrect, there may be a sound argument for the conclusion that in eachof our subjectively equivalent worlds there exists some physicalobjects (Brueckner 1992a: 118). Although such an argument wouldchallenge (*), it would be compatible with the Cartesian intuitionthat we may actually be in any of our subjectively equivalent worldsand would therefore evade the second horn.

6.2 Atomic natural kind concepts and a priori knowledge

Some content externalists (e.g. McGinn 1989) accept (C)

(C): If the concept of X is an atomic, natural kind concept, then it ismetaphysically impossible to possess it unless one has causallyinteracted with instances of X.

Suppose that (C) and the presuppositional theory of general contentexternalism (section 3.3.2) are true solely in virtue of conceptualimplications (section 6.1) and that one can know without empiricalinvestigation that one has the conceptwater. Then one willbe tempted to suppose that a person who has no views about thechemical structure of water may infer, on purely a priori grounds,i.e. independently of empirical investigation, that water exists, asfollows (from Boghossian 1997 [1998: 275], with numerals changed toletters and the order of the premises switched):

(a): I have the concept water.
(b): If I have the concept water, then water exists.
(c): Water exists.

The key question is whether one can know that (b) a priori. (C)implies that if water is an atomic natural kind concept, then (b) istrue. But this does not show that one can know that (b) a priori. Aholistic general content externalist who accepts (C) may reject theclaim that one can know that (b) a priori if (as McLaughlin & Tye1998: 311, argue) they can make sense of accepting

i.: We can know a priori that we have the concept of water, even ifthe concept of water is an atomic, natural kind concept,

while rejecting

ii.: We can know a priori that the concept of water is an atomicnatural kind concept.

The key point is that on some holistic theories of general CE, whethera concept is atomic depends on external factors and therefore cannotbe known a priori (McLaughlin & Tye 1998: 311). To see why,suppose Oscar is minimally competent in the use of the linguisticexpression “H₂O” in English, while notrealizing that in English “H₂O” is semanticallycomplex and expresses a complex (non-atomic) concept. By the standardsof CE, Oscar’s uses of “H₂O” express acomplex concept, even though Oscar is unaware of this fact. Nowsuppose that on Twin Earth, “H₂O” expresses anatomic concept, and Toscar, whose narrow states are the same asOscar’s, is minimally competent in the use of“H₂O” in Twin-English. By the standards of CE,Toscar’s uses of “H₂O” express an atomicconcept. This thought experiment (from Korman 2006: 514–515) andothers like it imply that one cannot know a priori whether a givenconcept that one has is atomic.

6.3 Transmission of warrant

Another line of inquiry grants that by reflecting on the arguments forCE one can know that (a) and (b) (of section 6.2) “withoutempirical investigation” but denies that I may infer, withoutrelying on any additional premises, that (c), and thereby come to knowthat (c) without empirical investigation. This line of inquiryaccepts

Closure for (a)–(c). If I know that (a) and I know that(b) [i.e. if (a) then (c)], then I know that (c).

The focus of the inquiry is not on Closure for (a)–(c), but on acorresponding principle regarding justifiable inferences, understoodaschanges in view (Harman 1986):

Warrant Transmission for (a)–(c). If I know that (a)and (b) without empirical investigation, then I may infer, withoutrelying on any additional premises, that (c), and thereby come to knowthat (c) without empirical investigation.

Against Warrant Transmission for (a)–(c) one might reason asfollows. Suppose I know that (a) “without empiricalinvestigation” yet also without ruling out the empiricalpossibility that my word “water” fails to express aconcept due to the falsity of my presupposition that there arestructural properties that are common to all the things to which I andothers apply “water” (see section 3.3.2 above on thepresupposition theory of general CE). Suppose also that I know that(b) without empirical investigation. Since (a) and (b) logically imply(c), given Closure for (a)–(c), I know that (c). I neverthelessdo not know that (c) solely based on my knowledge that (a) and (b) andmy knowledge that (a) and (b) logically imply (c). The reason is thatmy knowledge that (a)—my knowledge that I have the concept ofwater—was acquired by my being in a state which is subjectivelyindistinguishable from being in a different state in a different worldin which, due to facts about my external environment in that worldabout which I am ignorant, my word “water” does notexpress a concept at all. This argument against Warrant Transmissionfor (a)–(c) is due in essentials to M. Davies 1998 and Wright2000. It implies that a necessary condition of my coming to know that(a) without empirical investigation is that Ialready, andindependently, know that (c). (Wright 1985 applies an earlierversion of the same basic strategy to explain “the intuitiveinadequacy of G. E. Moore’s ‘proof of the existence of theexternal world’” [1985: 434].)

One might object that this type of criticism of Warrant Transmissionfor (a)–(c) “rests on an internalist epistemology thatwould be rejected by epistemological externalism” (Brown 2004:251). On a reliabilist account of knowledge (Goldman 1976), I may knowthat (a) and (b), infer, without relying on any additional premises,that (c), and thereby come to know that (c), if the sorts ofsubjectively equivalent situations that are incompatible with myknowing these propositions are not relevant to assessing whether myacceptance of them is reliable in the sense required forknowledge.

This objection is not decisive if one accepts a contextualistexplanation of the failure of Warrant Transmission for (a)–(c)with the following two parts. First, in ordinary contexts theepistemic possibility that there is no water is not relevant towhether or not I know that (a), so I can know “without empiricalinquiry” that (a) in such contexts. Second, in a context inwhich I claim to know that (c) without empirical inquiry, thepossibility that (c) is false is relevant and must be ruled out for meto know (c) without empirical inquiry (M. Davies 1998, 358–359).This explanation of why Warrant Transmission for (a)–(c) failsis compatible with a reliabilist account of my knowledge that (a) andis therefore not vulnerable to the criticism in the previousparagraph. It also suggests that the initial appearance that there isa transmission of warrant for (a)–(c) is based on anequivocation on the meaning of “know without empiricalinvestigation.” I may “know that (a) without empiricalinvestigation” if I presuppose (c) and I am in fact able toexpress the thought that, say, water is wet, simply by using mysentence “water is wet” without engaging in any empiricalinvestigation beyond those I have already completed. But I cannot know(c) without empirical investigation in this way.

One might also reject the supposition that one can derive knowledgethat (b) solely from one’s a priori knowledge of CE. If oneaccepts a holistic theory of CE, one might argue that the sort ofknowledge we have of our own thoughts without empirical investigationdoes not yield any substantive knowledge of the “individuationconditions” of those thoughts, hence does not yield knowledgethat (b) (Raffman 1998).

6.4 On knowing one’s environment without empirical investigation

Despite the arguments reviewed in sections 6.1–6.3, one mightargue that one can know a truth about one’s environment“via introspection and conceptual analysis” (Sawyer 1998:525, with Arabic numerals changed to letters) by reasoning asfollows:

(d): I am thinking awater-thought.
(e): If I am thinking awater-thought, then I’m in awater-world.
(f): Therefore, I am in a water-world.

where a “water-thought” is a thought, such as the thoughtthatwater is wet, that contains or involves the conceptwater, and a subject is in a “water-world” if andonly if “either the subject’s environment contains waterand water is a natural kind, or the subject is part of a communitywhich has the concept of water, whether or not water is a naturalkind” (Sawyer 1998: 525). The argument (d)–(e) is clearlyvalid. Premise (d) follows from our having minimal self-knowledge on agiven occasion when we are thinking that water is wet. And somephilosophers (Sawyer 1998; Warfield 1998) argue that we can know (e)without empirical investigation, hence by conceptual analysis alone.These philosophers in effect claim that the antecedent of (e), namely(d), conceptually implies the consequent of (e), namely (f), in thesense of “conceptually implies” that McKinsey 1991defines. They conclude that CE is incompatible with (3), the standardview that one cannot know about one’s external environmentwithout empirical investigation. As Sawyer later observes, however,“the heart of the so-called ‘McKinseyparadox’” is that an argument of the sort developed bySawyer 1998 “seems to provide an implausibly quick response tothe external world skeptic” (Sawyer 2015: 75).

6.5 Singular content externalism and a priori knowledge of the world

Sections 6.2–6.4 focused on the consequences of general CE.Singular CE raises different but related puzzles about whether one canhave a priori knowledge of one’s environment. To see why,suppose Suzy utters the sentence, “That is poisonous,”while pointing at a snake named Fred, thereby expressing her thoughtthatthat [i.e. Fred] is poisonous. According to the directreference theory of demonstratives (due to Kaplan 1989, see section3.1 above), the content of the thought Suzy thereby expresses is asingular proposition that contains Fred, the snake, and the propertyof being poisonous. Suzy might have been subject to anillusion of demonstrating a particular snake when in factthere was no snake to refer to. It therefore seems that Suzy cannotknow a priori that Suzy is thinkingthat [Fred] is poisonous,or thatthat [Fred] exists. But consider the followingproposition:

(**): If Suzy is thinking thatthat [Fred] is poisonous thenthat [Fred] exists.

(**) has the form“\(Fa \supset \existsx(x=a)\)”. Assuming the rule EG (existentialgeneralization) and standard axioms of identity, Suzy can thereforeinfer (**) a priori. Moreover, (**) itself implies thatthat[Fred] exists, since the propositionthat [Fred] ispoisonous is “embedded” in it (Lasonen-Aarnio 2006:441). These points imply that Suzy can know (**) a priori and that shecan infer a priori from (**) thatthat [Fred] exists. Acorollary is that privileged access to the contents of one’ssingular thoughts is not necessary for deriving contingent existenceclaims without empirical investigation (2006: 442).

These inferences depend on the assumption that the logic of singularCE is classical. In a non-classical free logic, by contrast, the form“\(Fa \supset \exists x(x=a)\)” isnot logically valid: in cases in which “\(a\)” is notassigned a reference, “both ‘\(Fa\)’ andconditionals containing ‘\(Fa\)’ have no truth value andso are not true on those interpretations” (McKinsey 2006: 449).If one adopts a free logic, then if singular CE is true, privilegedaccess to the contents of one’s singular thoughts is necessaryfor deriving contingent existence claims without empiricalinvestigation.

7. Is the hypothesis that we are always brains in vats self-refuting?

In “Brains in a Vat” (Putnam 1981: Ch. 1) Putnam arguesthat the supposition that we are and always have been brains in vatsis “self-refuting.” His argument for this surprisingconclusion started a stream of illuminating literature about how toreconstruct and evaluate it.

7.1 The first formulation of the BIV argument

Suppose my brain was transferred to a vat last night without myknowledge. Then by the standards of general CE, the contents of thethoughts I express by using my sentences would remain the same, atleast for a while. In the first several days after the transfer, andperhaps for much longer, my utterances of “I have hands,”for instance, would express the false content thatI havehands. (Section 7.4 discusses the epistemological consequences ofthis sort of possibility.) Putnam’s argument takes this pointfor granted and considers the more extreme situation in which amolecule-for-molecule twin of my brain exists from the beginning tothe end of its “life” in a vat of nutrients stimulated byautomatic machinery that “came into existence by some kind ofcosmic chance or coincidence” (Putnam 1981: 12). Let“BIV” abbreviate this extreme situation.

In an ordinary non-skeptical context, it seems obvious that we can usethe English sentence “Are we BIVs?” to ask ourselves,Are we BIVs? and that the answer is “No.” Thiscomplacent answer seems precarious, however, if we suppose, as manyphilosophers do, that our sensory experiences would be the samewhether or not we are BIVs. For this supposition appears to imply thatour ordinary non-skeptical belief that we are not BIVs receives nosupport from our sensory experiences and therefore lacks the kind ofepistemological justification we ordinarily take it to have.

A BIV has never had and never will have any causal relations withthings to which English speakers apply their words, or with members ofthe English-speaking community, on which the references of Englishwords and the contents express by utterances of English sentencesdepend, according to SE and CE. If we accept CE in any of its standardforms (see section 3 above), we are thereby committed to acceptingPutnam’scausal constraint on reference, according towhich, “One cannot refer to certain kinds of things, e.g. trees,if one has no causal interaction at all with them, or with things interms of which they can be described” (Putnam 1981: 12). Thecausal constraint on reference implies that since BIVs have no causalrelations with trees, brains, or vats, or with anything in terms ofwhich trees, brains, or vats can be described, such brains are unableto refer to or think about trees, brains, or vats.

The causal constraint does not tell us what thoughts a BIV expresseswhen it uses sentences of Vat-English, the language it speaks. Putnam1981 suggests three possible ways of interpreting a BIV’s basicobservational vocabulary, such as “tree”:“trees-in-the-image,” “electrical impulses thatcause tree experiences,” and “features of the program thatare responsible for those electronic impulses.” (Putnam 1981:14). Suppose we grant that for each of us there is a subjectivelyequivalent world in which we are BIVs, and that in those subjectivelyequivalent worlds our utterances may be interpreted in one of the waysPutnam suggests. It then appears that for all we know on the basis ofour sensory experiences, we may actually in the world in which we areBIVs and our words “brain” and “vat” are notsatisfied by brains or vats, respectively, but, instead, by (as it maybe) brains in the image and vats in the image, respectively.Putnam’s BIV argument is meant to show that that this appearanceis self-refuting, as follows:

If we are …brains in a vat, then what we now mean by “weare brains in a vat” is that we arebrains in a vat in theimage or something of that kind (if we mean anything at all). Butpart of the hypothesis that we are brains in a vat is that we arearen’t brains in a vat in the image (i.e. what we are“hallucinating” isn’t that we are brains in a vat).So, if we are brains in a vat, then the sentence, “We are brainsin a vat” says something false (if it says anything). In short,if we are brains in a vat, then “We are brains in a vat”is false. (Putnam 1981: 15)

This reasoning is “is neither ‘empirical’ nor‘a priori’, but has elements of both ways ofinvestigating”; it is based on reflections “about what isreasonably possibleassuming certain generalpremisses, or making certain broad theoretical assumptions”(Putnam 1981: 16). Its central premises, including the causalconstraint on reference, rest on empirical generalizations about howordinary English speakers interpret each other, what their words referto, and what thoughts they express by uttering them.

7.2 A priori reconstructions of the BIV argument

Standard reconstructions of Putnam’s BIV argument neverthelesstake for granted that the goal of Putnam’s BIV argument is, orshould be, to show by strictly a priori methods that we are not BIVs(Brueckner 1986: 160; Wright 1994: 222).

Brueckner’s reconstruction begins by stipulating that“brain*” refers to “the computer program featurethat causes experiences in the BIV that are qualitativelyindistinguishable from normal experiences that representbrains,” and “vat*” refers to “the computerprogram feature that causes experiences in the BIV that arequalitatively indistinguishable from normal experiences that representvats” (Brueckner 2011: 503). These stipulated interpretations ofthe BIV’s terms “brain” and “vat” implythat a BIV is not a brain* in a vat* (Brueckner 2011: 503). Thereconstructed argument runs as follows:

Either I am a BIV (speaking vat-English) or I am a non-BIV(speaking English).
If I am a BIV (speaking vat-English), then my utterances of“I am a BIV” are true iff I am a brain* in a vat*.
If I am a BIV (speaking vat-English), then I am not a brain* in avat*.
If I am a BIV (speaking vat-English), then my utterances of“I am a BIV” are not true. [b,c]
If I am a non-BIV (speaking English), then my utterances of“I am a BIV” are true iff I am a BIV.
If I am a non-BIV (speaking English), then my utterances of“I am a BIV” are not true. [e]
My utterances of “I am a BIV” are not true.
My utterances of “I am a BIV” are not true iff myutterances of “I am not a BIV” are true.
My utterances of “I am not a BIV” are true. [g, h](Brueckner 2011: 503–504, with “false” replaced by“not true”)

Brueckner notes that if I am agnostic about whether I am a BIV, thenthis argument does not establish that I am not a BIV. To get to theconclusion that I am not a BIV from (i) I need the followingpremise:

(t): My utterances of “I am not a BIV” are true iff I amnot a BIV.

This is an instance of the following schema for specifying theconditions under which the predicate “true” applies tosentences I can use:

(T): My utterances of “_____” are true iff _____.

Instances of (T), such as (t), employ what is called disquotation:they explain the conditions under which a quoted sentence is true bydropping the quotes and directly using the sentence. Instances of (T)are unproblematic in most ordinary, non-skeptical contexts.

If I don’t know whether or not I am a BIV, however, it appearsthat I do not know whether or not the truth conditions of myutterances of “I am not a BIV” are true iff I am not abrain* in a vat*, so I am not justified in asserting (t) (Brueckner1986: 160; see also Stephens & Russow 1985; Kinghan 1986). Thisargument focuses on (t) but it also implies that I am not justified inasserting instances of (Sat)

(Sat): My word “___” refers to (is satisfied by)xif and only ifx is a ____.

such as

(Sat_brain): My word “brain” refers to (is satisfied by)xif and only ifx is a brain.

in the context of the BIV argument. For like (t),(Sat_brain) employs disquotation: it explains the conditionsunder which a quoted predicate is true by dropping the quotes anddirectly using the predicate. And if I don’t know whether or notI am a BIV, it appears that I do not know whether or not the truthconditions of my word “brain” refers to (is satisfied by)x if and only ifx is a brain*, so I am notjustified in asserting (Sat_brain).

Against this, one might reason as follows:

Disquotation is, on both the standard and the skeptical hypotheses, avalid step within either English or vat-English. And surely wedon’t need to know which of these languages we are speaking inorder to know we are speaking the one we are speaking. But that seemsto be the only thing we do need to know, in order to applydisquotation to ourselves with confidence. (Christensen 1993: 305; seealso Hill 1990: 110n4; Wright 1994: 225)

The idea is that we do not suppose that we can toggle back and forth,when considering the BIV argument, between two distinct languages,English and Vat-English, but thatwe can use disquotationin the one language we actually speak, whether we suppose itis English or Vat-English. This reasoning motivates the a priorireconstructions of Putnam’s BIV argument that acceptdisquotation.

The simplest such reconstruction was first proposed independently byMcIntyre (1984: 59–60) and Williams (1984: 261) and has sincebeen modified and adopted by several others (Tymoczko 1989; Brueckner1992b and 2011; and Button 2013 and 2016). Here’s a typicalformulation of it (Brueckner 2011: 504, with “tree”replaced by “brain”):

(A): If I am a BIV, then my word “brain” does not refer tobrains.
(B): My word “brain” refers to brains.
(C): Therefore, I am not a BIV.

Premise (A) is a consequence of Putnam’s causal constraint onreference (Putnam 1981: 12). Premise (B) is an instance of (Sat). Ifone accepts the defense of disquotation quoted above, it appearsplausible to suppose that it is not question-begging to accept (Sat)when justifying premise (B) of the argument (A)–(C), but premise(A) becomes doubtful for reasons discussed below.

There are similar a priori reconstructions that do not explicitlyconcern reference, such as the following:

(A′): If I am a BIV, then I cannot consider the question whether I am aBIV.
(B′): I can consider the question whether I am a BIV.
(C): Therefore, I am not a BIV.

This argument assumes that we know (B′) a priori. This is not afundamentally new assumption, since part of the assumption that we canapply disquotation to our own words is that we know what thoughts wethereby express, and if this sort of knowledge is available to us apriori, then so is knowledge of what thought we express when we useand accept (B′).

One possible concern about reconstructions (A)–(C) and(A′)–(C) is that warrant for their premises does nottransmit to their conclusions (see 6.3 above). The problem abouttransmission of warrant does not seem to arise for Putnam’s BIVargument, however, because “there is no suggestion … thatenvathood would generate illusions of content” (Wright 2000:159; see also Thorpe & Wright 2022: 74).

Another possible concern about reconstructions (A)–(C) and(A′)–(C) is that if I suspend my ordinary beliefs to theextent of supposing I am a BIV, it is unclear why I am entitled toaccept (A), and (A′). It seems that if I am reasoning strictly apriori and accept disquotation then contrary to (A) I should accept“If I am a BIV, then my word ‘brain’ refers tobrains.” But this conditional is of no help in deriving (C)given (B). For parallel reasons, (A′) should be replaced by“If I am a BIV, then I can consider the question whether I am aBIV” (Ebbs 2016: 28–29; Kinghan 1986: 165).

One might suppose that I can know a priori that “Vat-English isalways something other than the language we are speaking”(Tymoczko 1989: 284–285). In the context of Putnam’s BIVargument it is not question-begging to suppose that Vat-English is thelanguage spoken by a brain that is always in a vat. I cannot infer apriori from this supposition, however, that Vat-English is not thelanguage I speak (Brueckner 1995: 92–93; Ebbs 1996: note32).

7.3 Contextually a priori reconstructions of the BIV argument

A different approach to reconstructing Putnam’s BIV argument isto accept Putnam’s methodological claim that his BIV argument“is neither ‘empirical’ nor ‘a priori’,but has elements of both ways of investigating” (Putnam 1981:16). There are several reasons for doing so.

First, CE implies that it takes empirical investigation to learn whatour words, including our words “brain” and“vat,” refer to (are satisfied by), and, by doing so, tolearn whether there is a possible subjectively equivalent world inwhich duplicates of our brains “live” in vats, tended byautomatic machinery. Aristotle believed that the function of the brainis to cool the blood. It was an empirical discovery that the brain isthe center of the human nervous system. If we were to suspend ourempirical beliefs, as radical skeptics invite us to do, we would haveno basis for concluding that there is a possible subjectivelyequivalent world in which duplicates of our brains “live”in vats, tended by automatic machinery (Putnam 1998 [2012: 527]).

Second, Putnam’s versions of SE and CE are rooted inobservations about our actual practices of taking speakers to beminimally competent in the use of English words. A speaker whosuspends all her empirical beliefs about what trees are and does notaffirm any demonstrative sentences that contain the word“tree”, such as, “That’s a tree,” cannotbe regarded as minimally competent in the use of the word“tree.” This is not to say that to be minimally competentin the use of “tree” one must be able to distinguish atree from every possible type of thing that looks and feels like atree. A speaker may count as minimally competent in the use of a wordeven if all the sentences containing the word that she accepts arefalse. But to be minimally competent in the use of a word one musthave some idea of what thoughts one expresses by uttering sentencesthat contain the word. Without some idea of what one can use the wordsto say, one is not able to use them at all (Putnam 1975a:247–257; Ebbs 2016: 29–30).

When we consider the BIV possibility we presuppose many empiricalbeliefs and theories. This point about where we begin when evaluatingthe argument fits well with Putnam’s more generalmethodological principle (MP) according to whichwe cando no better in our inquiries that to start in the middle, relying onalready established beliefs and inferences, and applying our bestmethods for re-evaluating beliefs and inferences and arriving at newones. (Putnam’s commitment to this principle is evident inPutnam 1962a, the conclusions of which, he later says, are“closely connected with what was later called‘externalism’” (Putnam 2013: 193).)

This methodological principle does not beg the question against BIVskepticism, since it allows for the possibility of areductioad absurdum of our starting assumption that our empiricalbeliefs are justified (Quine 1975: 68). Assuming MP, Putnam’sBIV argument can be interpreted an attempt to show that thesupposition that we are BIVs is self-refuting and therefore does notreduce our starting beliefs to absurdity.

The a priori reconstructions suppose that we may be BIVs, andHoudini-like, try to show that we are not. If we allow ourselves torely on our starting beliefs, we need to rethink the role of thesupposition that we are brains in vats in Putnam’s BIV argument.Putnam compares the statement “I am a BIV” with thestatement thatall general statements are false (Putnam 1981:7). At first it appears that this second statement may be true orfalse, and that it would take some investigation to discover what itstruth-value is, but on reflection we see that it must be false, sincethe supposition that it is true yields a contradiction. Putnam alsonotes that “I am a BIV” is self-refuting in something likethe way that “I do not exist” is self-refuting: myutterances of “I do not exist” express contents that maybe evaluated as true only in contexts of evaluation that differ fromtheir contexts of utterance (Kaplan 1989: 495). When I utter “Ido not exist,” what I say is obviously false. The statement“I am a BIV” is similarly self-refuting, according toPutnam, but in a way that requires more reflection for me to see.

These comparisons suggest that we read Putnam’s BIV argument asfollows. The supposition “I am a BIV”appears tobe possibly true, just as, when one first considers it, the statementthatall general statements are false appears to be possiblytrue. On reflection, based on our background empirical beliefs aboutbrains and vats and the principles of SE and CE, however, we can seethat the supposition cannot be true if we are considering it, hence itmust be false. We can see that it is false not because we define theBIV possibility as one that we are not in, but because “theworld picture that [the skeptic] needs to give content to his talk ofcomputers, brains, electrical impulses, chemicals needed to keep thebrains a live, and so on is simply stolen from our worldpicture” (Putnam 1998 [2012: 527]). This version of the BIVargument iscontextually a priori in roughly the followingsense: if it is successful, it shows that the principles and premiseson which it relies, while not a priori in the traditionalphilosophical sense, are so deeply embedded in our current theory ofthe world that we cannot specify any minimally plausible way in whichthey may be false. (The term “contextually a priori” isfrom Putnam 1976a [1983: 95]. See Ebbs 2015 and Putnam 2015 for adiscussion of how to clarify it.)

Arguments (A)–(C) and (A′)–(C) are unproblematic ifwe do not require that their premises be justified a priori. We mayinstead support their premises with contextually a priori reasons thatpresuppose our empirically informed “world picture,” sothat our starting position when considering the BIV argument is notone of suspending all our empirical beliefs. When viewed in this way,the arguments are successful if they show us how and why our initialimpression that we might be a BIV is not compatible with ourcommitment to general CE. (See Ebbs 1992, 1996, and 2016 for adevelopment and defense of this interpretation of Putnam’s BIVargument.)

7.4 Consequences for epistemic possibility

Suppose one of the above reconstructions of Putnam’s BIVargument reconstructions is sound. Then it is not epistemicallypossible_K that I am a BIV: there is no counterfactual worldw in which (a) I exist, (b) my sensory evidence andlinguistic behavior are the same as it they are in the actual world,and (c) my sentence “I am a BIV” expresses a content thatis true inw. Putnam’s BIV argument focuses on clause(c). But the argument may also lead me to doubt that in Putnam’sBIV scenario my sensoryevidence is the same as it is in theactual world. For if I may rely on the argument to show that is not“epistemically possible_K” for me that I am aBIV, I may also characterize my sensory evidence in representationalterms. A further, more speculative step is to theorize that even mysensoryexperiences are not determined by my narrowpsychological states (Byrne & Tye 2006).

The BIV argument does not rule out that my brain was recentlytransferred to a vat without my knowledge. Some philosophers (e.g.Tymozcko 1989; Putnam 1998; Thorpe 2019; Thorpe & Wright 2022)argue that we may rely on our beliefs about the current state ofmedical technology, not on semantics, to rule it out. Others arguethat “the problem posed by the recent-envatment hypothesis isequivalent in power and plausibility to that posed by the Putnamianhypothesis” (Christensen 1993: 314–315).

7.5 Skepticism about objectivity

Thomas Nagel argues that Putnam’s BIV argument, if sound,immediately raises another skeptical problem:

If I accept the argument, I must conclude that a brain a vatcan’t think truly that it is a brain in a vat, even thoughothers can think this about it. What follows? Only that I can’texpress my skepticism by saying “Perhaps I am a brain in avat.” Instead, I must say, “Perhaps I cannot even thinkthe truth of what I am, because I lack the necessary concepts and mycircumstances make it impossible for me to acquire them!” Ifthis doesn’t qualify as skepticism, I don’t know whatdoes. (Nagel 1986: 73)

Nagel presupposes that our representation of the world is moreencompassing, less limited, and therefore more objective than that ofa BIV. He raises the possibility that (a) there are true thoughts weare unable to express or acquire and that (b) among these there may betrue thoughts that, if we could grasp them, would lead us to questionthe objectivity of our current thoughts, by revealing to us truthsabout our situation that are more encompassing, less limited, than theones we can now grasp. One might try to give some content to thisworry by reasoning by analogy: just as we grasp thoughts relative towhich the thoughts of a BIV are severely limited, so perhaps anotherbeing can grasp thoughts relative to which our thoughts areanalogously limited (Ebbs 1992: 253–255; see also Smith 1984;Wright 1994: 240; Forbes 1995: 220; Button 2013: 137; Sundell 2016:section 14.5; and Thorpe & Wright 2022: 86–87). Thorpe &Wright 2022 argue that

at least one, bare bones, characterization of [this skeptical]predicament needs onlylogical concepts, concepts ofourselves asthinking,epistemic agents,concepts ofmetasemantics and certain kinds ofevaluative concepts: it is the predicament in whichthereare states of affairs to whichwe are so related that wecannotrefer to their constituentobjects andproperties, nor therefore represent them intruethoughts, and whose obtaining would be ofenormoussignificance to us could we butknow of it. (Thorpe& Wright 2022, 86–87; for a similar strategy for respondingto Putnam’s BIV argument, see Chalmers 2018: 630)

This worry, if unanswered, generates skepticism about objectivity(Ebbs 1992: section IV).

A natural first question to ask is whether we can make sense ofskepticism about objectivity if we accept CE. The following argumentsuggests that we cannot. We have good reasons to believe that if wewere to learn a natural language we don’t already know, such asDanish or Chinese, we would thereby learn new concepts and come to beable to express true thoughts that we could not express before. We maybe surprised by the concepts and thoughts we acquire in this way. Butthe speakers of these languages live in the same spatiotemporal worldthat we live in and are causally related to many, if not all, of thesame types of things that we are causally related to. There aretherefore no good reasons to suppose that the new concepts andthoughts we come to grasp when we learn a natural language wedon’t now know would lead us to question the objectivity of allour current thoughts. To make sense of there being true thoughts thatare as alien to us as our thoughts are to the BIV while stillaccepting SE and CE, we would have to make sense of there being apractice of applying words and sentences to “objects” thatcannot be described or individuated in terms of anything we can locatein space and time. If we take the causal constraint on reference,meaning, and thought content seriously, however, we have no resourcesfor making sense of this. Skepticism about objectivity thereforeappears to be without any content for a content externalist. (Theargument sketched here is from Ebbs 1992: section XII. Differentarguments for similar conclusions are presented in Putnam 1994; D.Davies 1997; and Button 2013: section 13.4.)

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McKinsey, Michael, “Skepticism and ContentExternalism”,Stanford Encyclopedia of Philosophy(Winter 2024 Edition), Edward N. Zalta & Uri Nodelman (eds.), URL= <https://plato.stanford.edu/archives/win2024/entries/skepticism-content-externalism/>. [This was the previous entry on this topic in theStanfordEncyclopedia of Philosophy – see theversion history.]

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