Two Kinds of Skeptical Argument

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Two Kinds of Skeptical ArgumentAuthor(s): Stewart CohenSource: Philosophy and Phenomenological Research, Vol. 58, No. 1 (Mar., 1998), pp. 143-159Published by: International Phenomenological SocietyStable URL: http://www.jstor.org/stable/2653634 .

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Philosophy and Phenomenological Research Vol. LVIII, No. 1, March 1998

Two Kinds of Skeptical Argument

STEWART COHEN

Arizona State University

This paper compares two kinds of epistemic principles-an underdetermination principle and a deductive closure principle. It argues that each principle provides the basis for an independently motivated skeptical argument. It examines the logical relations between the premises of the two kinds of skeptical argument and concludes that the deductive closure argument cannot be refuted without refuting the underdetermination argument. The underdetermination argument, however, can be refuted without refuting the deductive closure argument. In this respect, the deductive closure argument is the stronger of the two.

Skepticism can be viewed as a problem that derives from the plausibility of certain epistemic deductive closure principles. But skepticism can also seen as a problem that stems from the plausibility of certain epistemic underdetermination principles. What is the relative importance of these two kinds of principles for formulating skeptical arguments?

According to one view, skeptical arguments must appeal to a deductive closure principle and do not require any appeal to an underdetermination principle.1 At the other extreme, some have held that skeptical arguments require an appeal to an underdetermination principle and that any appeal to a deductive closure principle is superfluous.2

I propose to defend a more moderate view, viz., that each principle provides the basis for an independently motivated skeptical argument. Because of this, neither principle is required to make the case for skepticism, nor is either principle superfluous. I will also argue that the deductive closure argument is, by one measure, the stronger argument.

I See Robert Nozick, Philosophical Explanations (Cambridge, Massachusetts: Harvard University Press, 1981), chapter 3, and Fred Dretske, "Epistemic Operators", Journal of Philosophy (December 1970).

2 See Anthony Brueckner, "The Structure of the Skeptical Argument", Philosophy and Phenomenological Research (December 1994) and Omit D. Yeltsin, "Skeptical Argu- ments from Underdetermination", 68 Philosophical Studies (1992), pp. 106-22

TWO KINDS OF SKEPTICAL ARGUMENTS 143



I

We can begin by considering a recent argument put forth by Anthony Brueckner.3 Brueckner argues that once we get clear on the structure of the "canonical Cartesian skeptical argument", we will see that the appeal to deductive closure principles is superfluous. He begins by considering skeptical argument A-a first statement of the canonical argument. Let P be some arbitrary proposition about the external world, such as I see a table, and let SK be a proposition about a skeptical hypothesis incompatible with P, such as I am a brain-in-a-vat with sense experience that is qualitatively indistin- guishable from my actual experience:

(1A) If I know P, then I know not-SK.

(2A) I do not know not-SK

(3A) I do not know P

In defense of (1A), the skeptic appeals to a deductive closure principle. Brueckner suggests that the principle be formulated in this way:

(CK) For all S, A, if S knows 0 and S knows that 0 entails A, then S knows A.

Assuming I know that P entails not-SK, we can derive (1A) from (CK). According to Brueckner, argument A ultimately relies on an underdetermi-

nation principle, as well. In defense of (2A), the skeptic appeals to

(UP) For all S, 0, W, if S's evidence does not favor 0 over some incompatible hypothesis W, then S's evidence does not justify o.4

In particular the skeptic appeals to this instance of (UP):

(*) If my evidence does not favor not-SK over SK, my evidence does not justify not-SK.

Brueckner, ibid. The relation between a closure principle and an underdetermination principle was discussed by Jonathan Vogel, Cartesian Skepticism and Epistemic Princi- ples (Doctoral dissertation, Yale University, 1986). Vogel argued that certain formula- tions of the principles are interderivable, as Brueckner does. In more recent work, Vogel explores a weaker version of an underdetermination principle, and arrives at a position that is somewhat similar to the one I defend here (see his "Skepticism and Knowledge of the External World, in preparation). In Brueckner's statement of the principle, the consequent of the conditional is "S lacks justification for believing that O." I state it the way I do for ease of exposition. The difference does not matter to the argument.

In my statement, "S's evidence does not justify 4" means that S's evidence does not provide justification for believing 4.

144 STEWART COHEN



This requires, of course, the further claim

(**) My evidence does not favor not-SK over SK.

To articulate further the structure of the skeptical argument, Brueckner notes that there is a justification analog for (CK):

(CJ) For all S, 0, A, if S's evidence justifies 0, and 0 entails A, then S's evidence justifies W. I

This enables us to formulate Brueckner's skeptical argument B:

(1B) If my evidence justifies P, then my evidence justifies not-SK. [from (CJ)]

(2B) If my evidence does not favor not-SK over SK, then my evidence does not justify not-SK. [from (UP)]

(3B) My evidence does not favor not-SK over SK. [premise]

(4B) My evidence does not justify not-SK. [from 2B, 3B]

(5B) My evidence does not justify P. [from 1B, 4B]

(6B) I do not know P. [from 5B]

As it stands, the skeptical argument appeals to two epistemic principles- (CJ) and (UP). But Brueckner observes "that the skeptical reasoning we are trying to capture can be simplified in such a way that it only makes appeal to one epistemic principle. We need not appeal to the closure principle."6 Here is argument C-his final version of the skeptical argument:

(iC) If my evidence does not favor P over SK, then my evidence does not justify P. [from UP]

Brueckner mentions two principles:

(CIJ) For all S, 4, A, if S has justification for believing that 4 and S knows that 4 entails A, then S has justification for believing that A.

(CIV) For all S, 4, A, if S has justification for believing that 4, and 4 entails A, that S has justification for believing A.

(CIJ) is stronger though Brueckner argues that it is as plausible as (CIJ) since S can have justification for believing 4 without actually believing it. Brueckner mentions a worry about (CIJ) concerning necessary truths and I think there are additional worries as well. In the end, I think they can be handled with minor modifications. (Brueckner suggest restricting (CIJ') to contingent W's.) None of these worries affect the substantive issue that I will discuss so I will stick to Brueckner's (CIJ') for ease of exposition. I will simply call it "(CJ)" however.

One final point: As I did in the case of (UP), I have, in stating (CJ) substituted "S's evidence justifies P" for "S has justification for believing P" Brueckner, op. cit., p. 833.




(2C) My evidence does not favor P over SK. [premise]

(3C) My evidence does not justify P. [from 1C, 2C]

(4C) I do not know P. [from 3C]

So, if Brueckner is correct about the structure of the skeptical argument, then a deductive closure principle like (CJ) turns out to be superfluous to the skeptical argument.

II

The thrust of Brueckner's remarks is that the Cartesian skeptical problem, although traditionally conceived as a problem stemming from deductive closure, is in fact a problem stemming from underdetermination. Let us examine more closely how he arrives at this conclusion.

His final reconstruction of what he calls "the canonical Cartesian skeptical argument" is given in argument C. We were led to argument C in trying to find support for (2A) in the original version of the skeptical argument. That argument appeals to the deductive closure principle (CK). Given (CJ), however, we can display more of the structure of the deductive closure argument:

(iD) If my evidence justifies P, then my evidence justifies not-SK. [from CJ]

(2D) My evidence does not justify not-SK. [premise]

(3D) My evidence does not justify P. [from 1E, 2D]

(4D) I do not know P. [from 3E]

As we can see from argument B, Brueckner holds that, in the end, we must appeal to (3B) (My evidence does not favor SK over not-SK) in defense of the premise (2D) (My evidence does not justify not-SK). But (3B) works in conjunction with (UP). And we can see from argument C that once we appeal to (UP) we can eliminate any appeal to (CJ).

But Brueckner has given no argument that we must appeal to (3B) in support of (2D). And I can see no reason to suppose that we must. (2D) says that my evidence does not justify not-SK. Suppose we conceive of my evidence as being represented by the proposition E.7 Now one thing that is particularly salient about the skeptical hypothesis SK is that if SK were true, it would explain the truth of E. But having noted this, it becomes extremely puzzling, to say the least, how that very evidence could justify not-SK. Thus, it becomes very hard to resist accepting (2D).8

7 This is a harmless simplifying assumption. We could express the point without it though it would be more cumbersome.

8 We can think of this reasoning as appealing to the principle:

146 STEWART COHEN



My claim is not that this reasoning will move everyone to skeptical doubt. But I do think it represents a very natural motivation for premise (2D). As an example, consider Robert Nozick's discussion of the origins of skeptical doubt. After introducing the possibility that some skeptical hypothesis like SK is true, he writes:

If one of these other things was happening, your experience would be exactly the same as it now is. So how can you know none of them is happening?9

The important thing about this reasoning is that it does not rely on the underdetermination premise (3B). In fact, it looks as if one who reasons in this way could reject (3B) altogether. Suppose I think E favors (to some degree) P over SK, and so favors not-SK over SK. I need not hold that E favors P, and not-SK, enough to justify either P or not-SK. But then I see no reason why one who believes E favors not-SK over SK, i.e., one who rejects (3B), cannot reason that because the truth of SK would explain the truth of E, E cannot justify not-SK.10

This shows that we can view skeptical argument D as a straight deductive closure argument-we need not view it as appealing to the underdetermination principle (UP). So there is no reason to think that (UP) is central to the

(Z) For all S, 4, if the truth of 4 would explain S's evidence, then S's evidence does not justify not-?

The order of justification is not clear here. It may be that the general principle (Z) has no more intuitive plausibility than the particular claim (2D). One possibility (suggested by an anonymous referee) is that the particular claim and the general principle provide mutual support through a process of reflective equilibrium.

I do not think, however, that the initial plausibility of (2D) depends on our being able to precisely articulate a general principle from which it can be derived. One way to think of this is that the plausibility of (2D) stems from an intuition about the concept of evidential support or justification. The intuition persists even if we are unable to come up with a general account that explains the intuition. On this view, the plausibility of (2D) is very much like the plausibility of the denial of the (sub)conclusion of the skeptical argument- My evidence does justify P. The claim that my evidence does justify P has enormous intuitive plausibility. And its initial plausibility does not depend on our being able to articulate a general principle of justification from which it follows. This is what explains our resistance to skeptical arguments. For this reason, the skeptical problem is best viewed as a paradox, a set of incompatible propositions, each of which has considerable intuitive plausibility.

Also (an anonymous referee noted that) the case where P is a contradiction looks worrisome for principle (Z), given standard semantics for subjunctive conditionals. Because of this we can restrict the principle to non-contradictions.

9 Op. cit., p.167. For further examples of philosophers who see the skeptical problem as being motivated in roughly this way, see John Pollock, Contemporary Theories of Knowl- edge (Rowan and Littlefield, 1986), p. 3, and Barry Stroud, The Significance of Philosoph- ical Skepticism (Oxford, 1984) pp. 20-21.

10 Brueckner seemingly makes this same point though his statement of it is puzzling. He says, "...S's evidence might favor one of two incompatible hypotheses without being strong enough to justify a belief of either". See footnote 7, Ibid. Presumably he means to say "...to justify a belief in the favored hypothesis or the denial of the unfavored hypothesis."




"canonical skeptical argument" while (CJ) is superfluous. In fact we have two skeptical arguments, argument C based on (UP) and argument D based on (CJ).

My point is not that we must, in the end, endorse this reasoning in support of (2D) (or 2D itself). Neither must we accept the underdetermination based reasoning to which Brueckner appeals. (Brueckner himself raises what he takes to be a serious objection to the underdetermination based reasoning.") Most philosophers presuppose that there has to be something wrong with any argument for skepticism. Nonetheless, for many, the premises of these arguments have enough plausibility to make skepticism a philosophical problem worth taking seriously. My point is only that insofar as there is a philosophical problem of skepticism, deductive closure and underdetermination appear to be distinct sources.

III

The underdetermination principle (UP) and the deductive closure principle (CJ) each leads to skeptical problems via a different route. (UP) proceeds through (2C) (My evidence does not favor P over SK) while (CJ) proceeds through (2D) (My evidence does not favor not-SK). This makes it look as if the issue of whether one principle generates skeptical problems is logically independent of the issue of whether the other principle generates skeptical problems. But this turns out not to be true.

Brueckner, op. cit. p. 835. Brueckner says the motivation for (2C) (my evidence does not favor P over SK) is that E (my evidence) would be true, regardless of whether P or SK is true. This is similar to what I state as the motivation for (2D). I appeal to the fact that if SK were true, it would explain the truth of E. This is because the mere fact that E would be true if SK were true does not seem to be enough. Let SK be the bare hypothesis (not-P & E). The mere fact that if SK were true, E would be true, does not seem to count against saying that E justifies not-SK. We need some skeptical hypothesis that would explain the truth of E. That is the reason for specifying the hypothesis that I am a brain-in-a-vat...

For this reason, the motivation for (2C) is better conceived as the fact that the truth of E would be explained, regardless of whether P or SK is true.

Brueckner argues that this reasoning in support of (2C) "ultimately derives from" the controversial view that the evidence required to justify P must entail P. (p. 835) I find Brueckner's argument for this conclusion somewhat unclear. I do think this much is true. The reasoning in support of (2C) may, in conjunction with (UP), provide an argument for this controversial "entailment" view. For E's not entailing P would seem to be all that is needed for the skeptic to argue that there is some Q incompatible with P, such that E does not favor P over Q. (Similar consideration apply to the motivation I have suggested for (2D.)

Is this a problem for the reasoning in support of (2C)? Arguably, a large part of what makes the entailment view controversial is that it leads to skepticism. It is not that the view has no intuitive appeal. (Recall that we are concerned here with justification to the degree necessary for knowledge.) It is just that most philosophers have opted for reject- ing this view over accepting skepticism. Insofar as the reasoning for (2C) in conjunction with (UP) leads to the entailment view, we have an additional argument for the entailment view.

148 STEWART COHEN



(UP) and (CJ) will have skeptical significance only for those cases where S's evidence does not justify not-?. For if S's evidence does justify not-?, it is uncontroversially true, independently of either (UP) or (CJ), that S fails to know 0.12

So let's suppose my evidence does not justify not-P. We can then show that, although (2D) can be true without (2C) being true, (2C) cannot be true without (2D) being true.

Suppose E (my evidence) favors P over SK. Still, as we just noted in section II, E may not favor P enough to justify either P or not-SK. If, for example, Pr(P/E)=.5 while Pr(SK/E)=.4, then E favors P over SK, while E fails to justify not-SK. But then (2D) (My evidence does not justify not-SK) is true, while (2C) (My evidence does not favor P over SK) is false.13

Now suppose E does justify not-SK (the denial of 2D). Then E must favor P over SK (the denial of 2C). Where E justifies not-SK, E fails to favor P over SK only if E justifies not-P. But we are assuming E does not justify not-P. Thus the denial of (2D) entails the denial of (2C) and so (relative to our assumption) (2C) entails (2D).14

Here is the derivation of (2D) from (2C):

(1) E does not favor P over SK. [(2C)]

(2) E justifies not-SK. [denial of (2D), assume for reductio]

(3) E does not justify not-P. [premise]

(4) E justifies not-P. [from 1, 2]

So although (relative to our assumption) (2D) can be true without (2C) being true, (2C) cannot be true without (2D) being true. So, in principle, the deductive closure principle (CJ) can have skeptical consequences without the underdetermination principle (UP) having any such consequences. But if the

12 I am assuming that S's evidence cannot justify both 4 and not-?. 13 I mean to be making a logical point here-that where 4 and v are incompatible proposi-

tions, my evidence could favor 4 over A, without favoring 4 enough to justify 4 or not-v. Now one might think that in the case of P and not-SK, the evidence could not favor P over not-SK or that there could be no reason for thinking that the evidence favors P over SK that was not also a reason for thinking the evidence justifies not-SK. But it would take an argument to show this. In section V, I discuss one kind of antiskeptical strategy which if correct, would make defensible the view that (2D) is true but (2C) is false.

14 If (2C) and (2D) are true, they are necessarily true. So, if they are true, each will entail the other. When I say (2C) entails (2D), but not conversely, I mean that anyone who accepts (2C) is committed to accepting (2D), but not conversely.

Moreover, as is clear from the derivation in the text that follows, one is so committed only if one accepts this principle:

For all S, 4, A, if S's evidence does not favor 4 over v and S's evidence justifies not- y, then S's evidence justifies not-?.

But that principle looks to be a necessary truth.




underdetermination principle (UP) has skeptical consequences, then so does the deductive closure principle (CJ).

IV

Given that (UP) has skeptical consequences only if (CJ) does one might think that a skeptic who accepts (CJ) has little to gain by appealing to (UP). We will take up this issue in section VI. For now we can see that regardless of whether there is anything to be gained by it, one who accepts (CJ) is committed to accepting (UP). For it is a consequence of my argument in section III, that (CJ) entails (UP).

To see how, it will be useful to refer to an observation of Brueckner's. He notes that (UP) is equivalent to

(UP') For all S, 0, A, if S's evidence justifies 0, and 0 and v are incompatible, then S's evidence favors 0 over A.

and that (CJ) is equivalent to

(CY) For all S, 0, W, if S's evidence justifies 0, and 0 and v are incompatible, then S's evidence justifies not-W.

In section III, I argued that (2C) (My evidence does not favor P over SK) entails (2D) (My evidence does not justify not-SK). If so, the denial of the consequent of the conditional embedded in (UP') entails the denial of the consequent of the conditional embedded in (CY). Since the antecedents of the conditionals embedded in the two principles are identical, it follows that (CY) entails (UP'). Thus (CJ) entails (UP).'"

We can show this more straightforwardly by deriving the consequent of the conditional embedded in (UP') from the conditional embedded in (CY) and the antecedent of conditional embedded in (UP,):16

(1) If S's evidence justifies 0, and 0 and v are incompatible, then S's evidence justifies not-W. [(CJ')]

(2) S's evidence justifies 0, and 0 and v are incompatible. [antecedent of (UP')]

(3) S's evidence justifies not-W. [from 1, 2]

(4) S's evidence justifies 0 and S's evidence justifies not-iv. [from 2, 3]

15 As we noted, (2C) entails (2D) only given the restriction that S's evidence does not justify not-?. But this restriction is entailed by the antecedent of the conditionals. So my conclusion that (CJ) entails (UP) is unaffected.

16 Here I follow Brueckner's reasoning in ft. 7, op. cit., p. 7.

150 STEWART COHEN



(5) S's evidence favors 0 over iy. [from 4]17

Brueckner makes the stronger claim that (CJ) and (UP) are equivalent.'8 Since I claim in section III that (CJ) can have skeptical consequences without (UP) having skeptical consequences, I must deny that (CJ) and (UP) are equivalent. So I must deny that (UP) entails (CJ).

Brueckner sets forth a proof of his claim that (UP') and (CY) (and so (UP) and (CJ)) are equivalent. The proof consists of a derivation of each principle from the other. We have already seen how to derive (UP) from (CIJ). If I am correct, Brueckner's attempt to derive (CY) from (UP') should fail. Here is Brueckner's account of the derivation:

Assume (UP') and (CJ)'s antecedent. Then it follows that S's evidence for believing that 4 favors 4 over the incompatible V. This by itself is not sufficient to show (CY)'s consequent [S's evidence justifies not-v]. This is because S's evidence might favor one of two incompatible hypotheses without being strong enough to justify a belief of either. However, we have assumed the antecedent of (CJ) in the course of the present attempted derivation, and thus we have it that S's evidence is sufficient to justify a belief that 4. Now it seems that evidence justifies not-V].19

As Brueckner shows, from (UP') and the antecedent of (CY), we can derive:

(Q) S's evidence favors 0 over V.

This, he argues, is not by itself sufficient to derive the consequent of (CJ):

(R) S's evidence justifies not-vi.

His reason is that "...S's evidence can favor one of two incompatible hypotheses without being strong enough to justify a belief of either". Pre- sumably, the point here is that we cannot get either (R) or

(T) S's evidence justifies 0.

from (Q). Brueckner goes on to say that what does enable us to derive (R) is that we

are assuming (T) (part of the antecedent of (CJ)) as part of the derivation. But this is puzzling because we cannot get from (T) to (R) unless we assume (CJ). We can get from (T) to

17 This derivation assumes the following principle:

For all S, 4, A, if S's evidence justifies 4, and S's evidence justifies not-c, then S's evidence favors 4 over V.

But presumably, this is a necessary truth. 18 In the text he says they are "virtually equivalent" and sets aside the question of whether

there is any significant difference. But in ft. 7, he sets out a proof that they are equivalent. 19 Op cit., pp. 832-34.




(U) S's evidence does not justify xg.

given that 0 and vg are incompatible.20 But of course (U) does not entail (R). Perhaps Brueckner thinks that (R) follows from (T) and (Q) together. But

(Q) follows from (T) and (U). And since (U) follows from (T) (given that 0 and vg are incompatible), (Q) follows from (T) alone (given that 0 and vg are incompatible). So (Q) can be of no help in deriving (R). If we cannot derive (R) from (T), unless we assume (CJ), then we cannot derive (R) from (T) and (Q), unless we assume (CJ). So the derivation of (CJ) from (UP) fails and we can reject the claim that (CJ) and (UP) are equivalent.

V

In section III, I argued that (2C) (My evidence does not favor P over SK) entails (2D) (My evidence does not justify not-SK) -but not conversely. As a result, if (UP) has skeptical consequences, so must (CJ). But (CJ) can have skeptical consequences without (UP) having skeptical consequences.

In section IV, I argued that (CJ) entails (UP)-but not conversely. So if (2D) has skeptical consequences, so must (2C). But (2C) can have skeptical consequences without (2D) having skeptical consequences.

What can we conclude about the relative strength of the deductive closure argument and the underdetermination argument? Consider the two arguments:

(UND)

(1C) If my evidence does not favor P over SK, then my evidence does not justify P. [from UP]

(2C) My evidence does not favor P over SK. [premise]

(3C) My evidence does not justify P.

(DC)

(1D) If my evidence justifies P, then my evidence justifies not-SK. [from CJ]

(2D) My evidence does not justify not-SK. [premise]

(3D) My evidence does not justify P.

Each argument has a premise that states an alleged epistemic particular fact and a premise that states an instance of an alleged epistemic principle.

20 I am assuming that S's evidence cannot justify incompatible propositions. This principle is of course distinct from the closure principle. I discuss it in section VII in connection with the issue of deriving (UP). Thanks to Tony Brueckner for pointing out to me that, in an earlier draft, my argument in this section conflicted with this principle.

152 STEWART COHEN



Whereas the principle appealed to in (DC) entails the principle appealed to in (UND), the factual claim in (UND) entails the factual claim in (DC) (given our restriction). So (UND) appeals to the weaker principle, while making the stronger factual claim. And (DC) appeals to the stronger principle while making the weaker factual claim. So it looks as if each argument has a respect in which it is more vulnerable relative to the other-(UND) in its reliance on (2C), the stronger factual claim, and (DC) in its reliance on (ID), an instance of the stronger principle.

But (ID) cannot really be a weakness of (DC) relative to (UND). For the premises of (UND) entail (iD). The denial of (ID) says that my evidence justifies me in believing P but does not justify me in believing not-SK. This, of course, entails the denial of (3C). So the denial of (ID) entails the denial of (3C) and so (3C) entails (ID). But the conjunction of (IC) and (2C) entails (3C). So the conjunction of (iC) and (2C) entails (ID).

Here is the derivation of (ID) from (iC) and (2C):

(1) My evidence does not favor P over SK. [2C]

(2) If my evidence does not favor P over SK, then my evidence does not justify me in believing P. [1C]

(3) My evidence justifies me in believing P but my evidence does not justify me in believing not-SK. [denial of (ID), assume for reduction]

(4) My evidence justifies me in believing P. [from 3]

(5) My evidence does not justify me in believing P. [from 1, 2]

We have just seen that the conjunction of the premises of (UND) entail premise (ID) of (DC). In section III, we saw that premise (2C) of (UND) entails (given our restriction) premise (2D) of (DC). So we can now see that the premises of (UND) entail both premises of (DC). But though the premises of (DC) entail premise (iC) of (UND) (since (ID) entails (iC)), they do not entail premise (2C) of (UND). Since both arguments are valid, it follows that (UND) is a sound argument only if (DC) is-but not conversely. Thus, (UND) can be refuted without refuting (DC), but (DC) cannot be refuted without refuting (UND).2' In this way, (DC) is the stronger argument.

How significant is the greater strength of (DC)? This will depend, in part, on whether there actually are any serious antiskeptical arguments against (UND) that do not count against (DC). Though a thorough discussion of this issue would take us far beyond the scope of this paper, there are two kinds of arguments worth considering briefly.

21 I am assuming that refuting a valid argument entails showing that one of its premises is false.




According to the first kind of antiskeptical strategy, pragmatic criteria like simplicity or conservatism play a role in determining what hypotheses our evidence supports. These arguments maintain that our ordinary claims (or hypotheses) about the external world fare better than skeptical hypotheses when measured against these pragmatic criteria. If this is correct, then insofar as we are counting these criteria as relevant to what our evidence supports, our evidence, in general, favors external world hypotheses over skeptical hypotheses. So suppose P is simpler than SK. Then given our assumption that considerations of simplicity play a role in determining what my evidence supports, it follows that all things considered, my evidence does favor P over SK. That is, (2C) (My evidence does not favor P over SK) is false.

Should we go on to say that because P is in some way simpler than SK, my evidence justifies me in believing not-SK? It is not at all clear that we should. Our concern is whether we are justified in believing not-SK to a degree sufficient for knowing not-SK. And it strikes me as a defensible view to hold that considerations of simplicity tip the evidential scales in favor of P without tipping them to the extent that my evidence justifies not-SK to that degree. So even if we suppose that in virtue of pragmatic criteria like simplicity my evidence favors P over SK (the denial of (2C)), it may still be that (2D) (My evidence does not justify not-SK) is not thereby threatened. The upshot is that antiskeptical arguments that appeal to pragmatic criteria pose a greater threat to the underdetermination argument (UND) than to the closure argument (DC).22

The second kind of antiskeptical argument I want to consider focuses on the role of global skeptical alternatives in classic skeptical arguments. Con- sider the hypothesis that I am a brain-in-a-vat or the hypothesis that I am deceived by a Cartesian demon. These alternatives are global in that they compete with any proposition I claim to know about the external world.23 Many philosophers-Carnap, Wittgenstein, Davidson, Putnam, to name a few-have objected to skeptical arguments on the grounds that to suppose

22 We might stipulate that pragmatic criteria like simplicity are not relevant, strictly speaking, to evidential justification. On this way of viewing matters, neither (2C) nor (2D) is threatened by an appeal to pragmatic criteria, as they concern only evidential justification.

If this is how we view matters then I could still make my point with regard to skeptical arguments analogous to (UND) and (DC) that proceed in terms of justification all things considered, i.e., both evidential and non-evidential justification. The appeal to pragmatic criteria would pose a greater threat to the analogue of (2C) than the analogue of (2D).

23 This depends slightly on how we formulate SK and the propositions about the external world. The way I have formulated SK, it competes with proposition about the external world like I see a table but not with propositions like There is a table. We could build into SK that I am a deceived brain-in-a-vat to make it conflict with proposition of the latter kind as well.

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that I am deceived in the way global skeptical alternatives entail is somehow incoherent or self-refuting.

We could however sidestep this kind of antiskeptical strategy by construct- ing a skeptical argument that proceeds solely in terms of restricted skeptical alternatives. Alternatives of this kind compete only with a restricted class of knowledge claims. For example, the alternative that my car has been stolen competes only with propositions I claim to know concerning the location of my car, who has been in it recently, etc. Now, for almost any proposition I claim to know, we could devise some restricted alternative to it. So we could make the case for skepticism (less elegantly) by a series of arguments, each of which appeals to some restricted alternative. This kind of skeptical argument, though perhaps not as far reaching as the classic skeptical argument, would still support the conclusion that we fail to know much of what we ordinarily take ourselves to know. Moreover, it would be immune to the kind of antiskeptical argument that attacks the coherence of the global skeptical alternatives.24

This bears directly on the issue of the relative strength of (UND) and (DC). Although plausible deductive closure arguments can be constructed us- ing either kind of skeptical alternative, underdetermination arguments require global skeptical alternatives. For though our evidence justifies us to some degree in believing restricted skeptical alternatives are false, it is very plausible to suppose it does not justify us to the degree necessary for knowing skeptical alternatives are false. But it is not very plausible to suppose that our evidence does not favor the mundane propositions we claim to know over restricted skeptical alternatives. For example, while it is very plausible to suppose that my evidence does not justify me to the degree necessary for knowing my car has not been stolen, it is not very plausible to suppose that my evidence does not favor the hypothesis that my car is parked in my garage over the hypothesis that my car has been stolen. The fact is that I have fairly good statistical evidence that favors the former hypothesis over the latter. Nonetheless, it is natural to balk at saying I know my car has not been stolen.25

What this shows is that premise (2C) of (UND) is plausible only for global alternatives, unlike premise (2D) of (DC) which is plausible for both global and restricted alternatives. Because underdetermination arguments must appeal to global alternatives, they cannot avoid the kind of antiskeptical response that specifically attacks the coherence of global skeptical alterna-

24 This kind of skeptical strategy is discussed by Jonathan Vogel in "Are There Counter- examples to the Closure Principle?" in Doubting: Contemporary Perspectives on Skepti- cism, Michael D. Roth and Glenn Ross, eds. (Dordrecht: Kluwer Academic Publishers, 1990), pp. 20-23.

25 Am I justified in believing that my car has been stolen? Of course, I am justified to some degree-but not, I am inclined to say, to the degree necessary for knowing.




tives. So if one of these responses succeeds, it would refute any underdetermination argument while leaving intact the deductive closure argument which proceeds in terms of restricted alternatives.

So how significant is the greater strength of (DC) over (UND)? Though the two kinds of antiskeptical strategies we have considered pose a greater threat to (UND) than to (DC), we have not taken up the question of how suc- cessful either of these strategies is. And to do so would take us far beyond the scope of this paper.26

VI

Given that any refutation of (DC) would also be a refutation of (UND), we might wonder whether any appeal to (UND) is dialectically superfluous. Here, however, we must distinguish between the refutability of an argument and the cogency of an argument. Though the soundness of (UND) is linked to the soundness of (DC), it does not follow that the cogency of (UND) is linked to the cogency of (DC). (DC) may fail to be cogent even though it has not, strictly speaking, been refuted. Suppose when I consider (DC) by itself, I find its premises to have not very much plausibility or, at least, to have not enough plausibility to withstand the conflict with our ordinary claims to know.27 It does not follow that I must conclude that the conjunction of those premises is false and therefore the conjunction of the premises of (UND) is false (since they entail the premises of (DC)). For suppose further that I find the premises of (UND) to have considerable plausibility or at least enough plausibility to withstand the conflict with our ordinary claims to know. Then I will have reason to accept the premises of (UND) and, because they are entailed by the premises of (UND), the premises of (DC) as well. In such a case, (UND) would be critical for establishing the skeptical threat.

Under this scenario, even though the soundness of (UND) requires the soundness of (DC), the cogency of (DC) requires the cogency of (UND). Thus (DC) would be the superfluous argument-a kind of dialectical epiphe- nomenon.

Even if I find the premises of (DC) to be at least as compelling as the premises of (UND), it does not follow that (UND) is superfluous. For (UND) would still provide an independent route to skepticism. Certainly an additional argument for skepticism exacerbates the skeptical problem. If either argument is undermined, the other might remain as a skeptical threat. Though, as we have noted, in order for (UND) to survive the failure of (DC),

26 My own view is that neither kind of antiskeptical argument is very convincing. 27 Sometimes we reject skeptical arguments because their premises are less plausible than

the claim that we know things about the external world. For example, that is the fallibilist response to the skeptical argument based on the premise that my evidence gives me knowledge of P only if my evidence entails P.

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the premises of (DC) would have to be undermined in a way that did not show them to be false.

VII

Let's return to the original question of this paper. What is the relative significance of the underdetermination principle (UP) and the deductive closure principle (CJ) for formulating skeptical arguments? Assuming that both principles have considerable intuitive plausibility, it now looks as if neither principle can be said to be superfluous in the case for skepticism. Each principle figures centrally in an independently motivated argument for skepticism. Measured by refutability, (DC) is the stronger argument. Measured by cogency, however, it need not be the stronger argument. Which (if either) argument is the more cogent will depend on nothing more than which (if either) has the more compelling premises.

One wrinkle remains. We have seen that (UP) can be derived from (CJ). Suppose it turned out that (UP) depends for its intuitive appeal on being so derivable. Then (UP) would not provide an independent motivation for the skeptical problem. If this were true, we would have to conclude that the appeal to (UP) is superfluous, and that the skeptical problem is, at root, a problem about deductive closure.

Does the intuitive appeal of (UP) depend on its being derivable from (CJ)? This is, to a certain extent, a psychological question. Certainly it does not follow from the fact that (UP) can be derived from other principles that its intuitive plausibility derives from those other principles. Perhaps the intuitive plausibility of (UP) does not depend on any other principles.

Suppose, however, that the plausibility of (UP) does derive from more basic principles. It still does not follow that its plausibility depends on (CJ). For there are other ways to derive (UP). Think about what (UP) says:

(UP) For all S, 0, Ag, if S's evidence does not favor 0 over some incompatible hypothesis Ag, then S's evidence does not justify 0.

The antecedent of the conditional (embedded in the principle) is a conjunction: S's evidence does not favor 0 over A, and 0 and v are incompatible. So perhaps we can see (UP) as deriving from a principle about evidential favoring and a principle about incompatible hypotheses. That is in fact how the derivation of (UP) in section III proceeds. (CJ) (or its equivalent formulation (CJ)) is a principle about incompatible hypotheses and the other principle we needed to assume was a principle about evidential favoring.28

But even supposing that (UND) is derived from a principle about evidential favoring and a principle about incompatible hypotheses, it need not be

28 See my note 17.




thought of as deriving from (CJ). Here is another way to think of it: Suppose S's evidence does not favor 0 over Ag. This does not by itself have the consequence that S's evidence does not justify 0. Perhaps S's evidence justifies both 0 and Ag. What follows from the assumption that S's evidence does not favor 0 over vg is that if S's evidence justifies 0, then it justifies w:

(FAV) For all S, 0, Ag, if S's evidence does not favor 0 over vg and S's evidence justifies 0, then S's evidence justifies Ay.

But the fact that 0 and vg are incompatible rules out the possibility that S's evidence justifies both 0 and A:

(INC) For all S, 0, A, if S's evidence justifies 0, and 0 entails not-ei, then S's evidence does not justify Ag.

Given (FAV) and (INC), if S's evidence does not favor 0 over vg and 0 and xv are incompatible, then S's evidence does not justify 0. That is, the conjunction of (INC) and (FAV) entails (UND).29 Moreover, neither (INC) nor (UND) follows from (CJ).30 Given the intuitive plausibility of (INC) and (FAV), we can conclude that the intuitive appeal of (UP) does not depend on its being derivable from (CJ).3'

29 Here's the proof:

(1) If my evidence justifies 4, and 4 entails not-e, then my evidence does not justify v [INC]

(2) If my evidence does not favor 4 over A, and my evidence justifies 4, then my evidence justifies V. [FAV]

(3) My evidence does not favor 4 over v and 4 entails not-c and my evidence justifies 4 [denial of UP, assume for reductio]

(4) My evidence does not favor 4 over v and my evidence justifies 4. [from 3]

(5) My evidence justifies V. [from 2, 4]

(6) My evidence does not justify 4 or 4 does not entail not-v. [from 1, 5]

(7) 0 entails not-e [from 3]

(8) My evidence does not justify Q. [from 6, 7]

(9) My evidence justifies 4. [from 3] 30 (CJ) entails (INC) given that if S's evidence justifies 4, then S's evidence does not justify

not-?. But that's just an instance of (INC). So it would be hard to argue that (INC) depends on (CJ) for its plausibility.

31 This is not to say that there could not be someone for whom the intuitive appeal of (UP) depends on its derivability from (CJ). It is just that such a person would have to not accept either (INC) or (FAV). And it is hard to see how one could fail to accept either.

Of course, even if one does not accept either (INC) or (FAV), it does not follow that one's acceptance of (UP) depends on its derivability from (CJ).

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If this is correct, then the underdetermination principle and the deductive closure principle do provide independent motivations for the skeptical problem.32, 33

32 Just as (ID) is entailed by (IC) and (2C), so (CJ) is entailed by (UP) and a generalization of (2C):

(2CG) For any (), my evidence does not favor 4 over SK.

(2C) is just an arbitrary instance of (2CG). And (UP) and (2CG) entail the generalization of (3C):

(3CG) For any (O), my evidence does not justify me in believing Q.

And (3CG) entails (CJ) in just the same way (IC) and (2C) entail (ID). Is it plausible, therefore, to suppose that the intuitive appeal of (CJ) depends on the

appeal of (UP)? If that were true, our acceptance of (CJ) would depend on our acceptance of what is asserted in (3CG). But that is just the generalization of what (3D), the conclusion of (DC), asserts as well. (CJ) is suitable as the justification for premise (ID) in (DC) precisely because its considerable appeal is independent of our acceptance of the conclusion of (DC).

33 I would like to thank Brad Armendt, Tom Blackson, David Cowles, Greg Fitch, Ted Guleserian, Bernie Kobes, John O'Leary-Hawthorne Steve Reynolds, and Jonathan Vogel for helpful discussion of these issues.




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Two Kinds of Skeptical Argument