why nozick is a sceptic

8/7/2019 Why Nozick is a Sceptic

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Why Nozick is a Sceptic

Anthony L. Brueckner

Mind, New Series, Vol. 93, No. 370. (Apr., 1984), pp. 259-264.

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Mind 1984) Vol. xcrrr, 259-264

Why Nozick is a ScepticA N T H O N Y L . B R U E C K N E R

In Philosophical Explanations, Robert Nozick presents an analysis ofepistemological scepticism which, he thinks, helps reveal the source of itspersuasiveness. He then attempts to refute the sceptic.' In this paper, Iwish to show that his refutat ion is seriously flawed. This is because we canuse Nozick's own epistemological views to demonstrate scepticalconclusions.

The context of Nozick's discussion of scepticism is his analysis ofknowledge in terms of subjunctives. 'S knows that P' is true, according toNozick, if and only if

( I ) P is true( 2 ) S believes that P(3 ) If P weren't true, then S wouldn't believe that P(4) If P were true, then S would believe that P.(There is a major refinement on the core of Nozick's analysis (I)-(4) which

need not concern us here.) Nozick maintains tha t one consequence of hisanalysis is that one does not know that certain 'counterpossibilities' toordinary knowledge-claims do not obtain. For example, I do not know thatI am not floating in a tank near Alpha Centauri with my brain undergoingstimulation (stimulation which causes the rich stream of sense-impressionsI have in fact had and am now continuing to have). I claim to know that I amnow in the United States, and that proposition is incompatible with theproposition that I am now in a tank near Alpha Centauri. The latterproposition is sometimes said to be a counterpossibility to the former-acounterpossibility to what I claim to know. According to Nozick, I do notknow that I am not in an Alpha Centauri tank because the third conditionfor knowing that I am not in such a tank is violated. Let 'SK' stand for theproposition that I am in such a tank. The n I do not know that - S K becausein order to know this, the following would have to be true:

(a) If -S K weren't true , then I wouldn't believe that - SKSee ch. 3 of Philosophical Explanations (Cambridge: Harvard University Press,1981 ) .Although Nozick says that he does not 'seek to convince the skeptic', hedoes seek to 'formulate hypotheses about knowledge and our connection tofacts that show how knowledge can exist even given the skeptic's possibilities'(p . 1 6 7 ) Since such hypotheses, if correct, would refute the sceptic, I cannotsee any objection to attributing to Nozick an attempted refutation ofscepticism.

259

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260 A N T H O N Y 1, . RKUECKNER:Thi s subjunctive can be restated as

(a') If S K were true, then I wouldn't believe that -SKThe subjunctive (a') is false, because the fo llou~ingubjunctive is true and isincompatible with (a'):

(b) If S K were true, then I would believe that -SK.The subjunctive (b) is true , says Nozick, because even if I were floating inan Alpha Centauri tank undergoing the said stimulation, I would stillbelieve that I am not in such a tank.

Given that I do not know that -SK , the sceptic can show that I do notknow that I am now- in the United States by arguing that 'knowledge isclosed under known logical implication'. Nozick states the closureprinciple as follows:

'+ ' stands for the subjunctive conditional. Nozick uses 'K(P)' to mean'knowledge that P', where 'P' stands in for some particular proposition.Using 'K(s, P)' to mean 's knows that P', we could presumably restateNozick's closure principle as follows:

The sceptical argument would now proceed in the following way. Let 'P'stand for the proposition that I am now in the Uni ted States. We then havethese premises stated with regard to some arbitrary knower s:

( I ) [K(s, (P - 3- SK))& K(s, P)]+K(s, -SK )(2) K(s, (P - SKI)(3) -K(s, -SK ).

From (I)-(3) it follows (b y Modus Tollens and Disjunctive Syllogism) that

Here is Nozick's informal statement of the sceptical argument:You know that you; being in a tank on Alpha Centauri entails your notbeing in a place X where you are. . . . And you also know thecontrapositive. . . . If you knew you were at X you would know you'renot in a tank (of a specified sort) at Alpha Centauri. Rut you don 't knowthis last fact (the sceptic has argued and we have agreed) and so (heargues) you don't know the first (Nozick, p. 205).

Nozick wants to refute the foregoing sceptical argument by denying thatknowledge is closed under known logical implication. His strategy is toshow that since condition (3) for knowing is not closed under known logicalimplication, it follows that the closure principle (C) is false. T o show thatI (C) would presumably be intended to state a conceptual truth about knowledge

and hence would be intended to apply to all possible knowers.


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W H Y N O Z I C K I S -4 S C E P T I C 26 Icondition (3) is not closed under known logical implication, Nozick willshow that in the particular case of the propositions P and -S K consideredin the above sceptical argument, condition (3) is not closed under knownlogical implication. Hence the general closure principle for condition (3)will be refuted. Hence (C) will be refuted. Hence there will be nojustification for premise ( I ) of the sceptical argument (and hence nojustification for any parallel premise in any parallel sceptical argument).Furth er, the particular instance ( I ) of (C) will itself be shown to be false.

At this point we need to bring in the possible worlds framework in orderto present IVozick's justification for denying the closure of condition (3)under known logical implication. Nozick maintains that the possibleworlds framework is merely an heuristic device and that his subjunctiveanalysis does not commit him to a possible worlds account of the semanticsof subjunctives. Yet he does use the framework to explain the failure ofclosure. So I will follow his practice. T o show non-closure of condition (3)we begin by supposing that condition (3) for my knowing that P is satisfied.In the possible worlds semantics, the subjunctive which is condition (3) formy knowing that P , viz.

(A) -P + -(I believe that P )is true if and only if in every sufficiently close -P-world, I do not believethat P.' On the o ther hand, the subjunctive which is condition (3) for myknowing that - SK , viz.

(B) S K + -( I believe that -SK)is true if and only if in every sufficiently close SK-wor ld , I do not believethat -SK. (B ) does no t follow from (A) taken together with the assumptionthat I know that (P+ -SK), and the (heuristic) possible worlds semanticsfor (A) and (B) shows why this is so. From the fact that in every sufficientlyclose -P-world, I do not believe that P, nothing follows about what Ibelieve or fail to believe in every sufficiently close SK-world , even given theassumption that I know that (P+ -SK) . Th is is because the close -P-worlds are not SK-worlds. T h e close -P-worlds are worlds in which I amnot in the United States but rather am vacationing in another country onEarth. T he close -P-worlds are not worlds in which the reason why P isfalse is that I am in an Alpha Centauri deception-device (an SK-wor ld). Sothe present case shows that condition (3) for knowledge is not closed underknown logical implication. Further, the sceptic's claim that I do not knowthat -SK is, according to Nozick, only justified by the fact that condition(3) is violated with respect to my belief that -SK. So Nozick's denial ofclosure with respect to knowledge does depend upon the correctness of hissubjunctive analysis of knowledge, but the feature of this analysis whichjustifies the denial, he argues, is the same feature which justifies the

If the reader directs-his attention to Nozick's n. 8 , pp. 680-681, he will be ableto compare the details of Nozick's possible worlds semantics for subjunctiveswith the various semantics familiar from the current literature (e.g., DavidLewis' semantics in Coun t~ r f a c tua l s(Cambridge: Harvard University Press,1973)).

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262 A N T H O N Y L . B R U E C K N E R :sceptic's claim that I do not know that -SK . Th is feature is the necessarycondition (3) for knowing. If a skeptic were to reject this condition andthereby sustain closure, he would then be forced to relinquish his claim thatI do not know that -S K, according to Nozick. But both this claim andclosure are required for the sceptical argument to go through.

Consideration of Nozick's argument against the closure principle raisesthe question: are there any propositions P' which I surely clairri to knowsuch tha t in the close worlds in which P' is false, I continue to believe that P'is true? Th is situation will arise if the close -P'-worlds are SK' worlds inwhich I am somehow deceived into believing that P' is true. If P' not onlyentails -SK' (as in Nozick's example) but, further, is entailed by - SK' (ifP' and -SK' are logically equivalent), then the close -P'-worlds will alsobe SKI-worlds. This is because if P' and -SK' are logically equivalent,then every -Pf-world will be an SK'-wor1d.l A proposition P' which doessatisfy the conditions I have laid down would be a proposition which,according to iVozick, I do not know, since condition (3) of his analysiswould be violated.

There are in fact such propositions P': propositions which one surelyclaims to know and which on Nozick's analysis one does not know. Theexistence of such o ro ~os it io ns hows that for one who holds Nozick'sepistemological views, a refutation of closure of knowledge under knownlogical implication is not sufficient to refute scepticism. We can grantNozick all the points so far discussed and still show that he is committed tostrongly sceptical conclusions. I t appears, then, that Nozick's condition ( 3 )is too strong, since on Nozick's analysis, one does not know the pro-positions P' because condition (3) is violated for these propositions. Thepropositions P' I shall discuss are ones which are considered in the classicalsceptical arguments. For example, I claim to know that my currentconscious state is a waking state (and hence a state in which I perceive myphysical surroundings). The Cartesian counterpossibility to what I claimto know in this case is the proposition that my current conscious state is adream state (and hence a state in which I do not perceive my physicalsurroundings). Let 'Fx ' stand for 'x is a waking state', 'Gx' for 'x is a dreamstate', and let 'A' rigidly designate my current conscious state. Th en what Iclaim to know is that FA. We can see that (FA+ -GA ). Do I know thatFA? I do not, according to Nozick's analysis. Thi s is because condition (3)for knowing that FA is violated: if my present conscious state were not awaking state, I would still believe that it is a waking state. Using Nozick'spossible worlds talk, the sufficiently close -FA-worlds are GA-worlds,worlds in which my present conscious state is a dream state. And in theseworlds, I still believe that A is a waking state. So what is true of me is

-FA -+ (I believe that FA).Th is true subjunctive is incompatible with condition (3) for my knowingthat FA, viz.

-FA -+ -( I believe that FA).I If our -SK' does not entail our P' but is only entailed by P', then, as in Noz~c li's

example, the close -Pi-worlds will not be SK' worlds.


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W H Y N O Z I C K I S A S C E P T I C 263

So according to Nozick's analysis, I do not know that my curren t consciousstate is a waking state. Nozick recognizes that just as he does not know thathe is not on Alpha Centauri, he does not know that he is not dreaming (sincecondition (3) is also violated in the latter case). Nozick of course recognizesalso that given non-closure of knowledge under known logical impl ication,there is no valid inference from his failure to know that he is not dreamingto his failure to know that he is awake. But what Nozick apparently does notrecognize is that apart from any considerations concerning closure, hisanalysis of knowledge commits him to the claim that he does not know thathe is awake.

Consider next my judgement that my sense-impressions are caused by aworld of physical objects. The Cartesian Evil Genius Argument is in partintended to undermine my claim to know that m y sense-impressions arephysically caused. T h e sceptical counterpossibility to what I claim to knowin this case is the proposition that there is an Evil Genius who causes mysense-impressions, thereby deceiving me into falsely believing that there isa world of physical objects which causes my sense-impressions. Let us state aslightly more general counterpossibility as follows. Let 'Hx' stand for thepredicate 'x is physical', 'Ix' for 'x is non-physical ', 'Cxy' for 'x is a state ofaffairs which causes y', and let 'B' rigidly designate the series of my actualsense-impressions (past, present, and futu re). The n what I claim to know isthat H I X C X B that the state of affairs which causes the series of my(actual) sense-impressions is physical). We can see that (Hlx Cx BH- I IxCXB).Do I know that HixCxB? I do not, according to Nozick'sanalysis. This is because the third condition for knowing is violated. T heformal representation of this third condition in the present case is

- H I xCxB + -( I believe that Ht xCxB).This subjunctive is false. What is true is a subjunctive which is incom-patible with it:-HtxCxB + (I believe that H 1 xCxB).

That is, if it were not the case that my sense-impressions are physicallycaused, I would still believe that they are physically caused.

Consider next the judgement that my current visual sense-impression iscaused by a white wall before me. If my sense-impression were not socaused, I would still believe that it is so caused. Hence condition (3) isviolated. So I do not , according to Nozick's analysis, know the propositionunder consideration concerning the causation of a particular one of mysense-impressions: I do not know that my current visual sense-impressionis caused by a wall standing before me.

T h e propositions P' I have discussed differ from the sort which Nozickconsiders (such as that I am in the United States, or in a certain room) in acrucial respect which has emerged in the foregoing discussion. In the closeworlds in which my propositions P' are false, my sense-experience remainsthe same and hence my beliefs about my physical surroundings do notI Seep. 2 1 9 .


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264 A N T H O N Y L . B R U E C K N E R : N O Z I C Kchange. In the close worlds in which I am not in the United States, on theother hand, I have different sense-experience from my actual sense-experience and hence my beliefs about my location are different from myactual beliefs. My beliefs that the propositions P' are true are not sensitiveto counterfactual changes in the causation of my experiences, and since thepropositions' truth-values are sensitive to such changes, it follows thatNozick must deny that I know the propositions. I claim to know that mycurrent conscious state is a waking state in which I am perceiving myphysical surroundings: I claim to know that my physical surroundings arecausally responsible for my current conscious state's having the character ithas. According to Nozick's analysis, I do not know this. I claim to knowthat my sense-impressions are in general physically caused. According toNozick's analysis, I do not know this. I claim to know that my currentvisual sense-impression is caused by a white wall standing before me: Iclaim to know that I am seeing a white wall. According to Nozick's analysis,I do not know this. Hence Nozick's analysis of knowledge commits him tosome strongly sceptical conclusions of a traditional sort familiar fromclassical discussions of scepticism.'

If one's inclination is to say that one surely does know the propositions P'to be true, then one will want to reject Nozick's condition (3) as being toostrong. What is in any case clear is that one can retain condition (3) only ifone holds that the propositions P' are not objects of knowledge. What canbe said in favour of condition (3)? f one thinks that it is not possible to ruleout the various sceptical counterpossibilities, then condition (3) will affordone a way of (i) rejecting closure of knowledge under known logicalimplication and thereby (ii) sustaining some knowledge-claims which areseemingly threatened by the counterpossibilities (such as the claim that Iam now in the United States). However, since the closure principle seemsso plausible, one may well want to attack the sceptic by attacking thepremise which Nozick's condition (3) grants to the sceptic: that one doesnot know that the sceptical counterpossibilities do not obtain. If one doesknow that these possibilities do not obtain, then the sceptic's argument is

D E P A R T M E N T O F P H IL O SO P HYY A L E U N I V E R S I T Y ,P . O . B O X 3 6 5 0 , Y A LE S T A T I O N , N E W H A V E N , C O N N E C T I C U T 0 65 20 , U . S . A .

I Nozick can consistently hold that his failure to know the propositions P' in thetext does not invalidate his claims to know a number of propositions aboutphysical objects. For example, he could maintain that even though he does notknow the propositions P', he nevertheless does know that he is in a room in theUnited States facing a white wall.

2 I would like to thank Christopher Hill for a great many valuable criticisms andsuggestions which improved this paper.

why nozick is a sceptic

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