Descartes’ Skeptical Challenge
Intro to Philosophy, Spring 2012
Handout 3
Torrey Wang
I. Cartesian skeptical argument from ignorance (CSA)1
1. You don’t know that you’re not being induced by an evil genius into falsely believing that the
world is as you perceive it to be.
2. If you don’t know that you’re not being induced by an evil genius into falsely believing that
the world is as you perceive it to be, then you don’t know that the world is as you perceive it
to be.
C1. Therefore, you don’t know that the world is as you perceive it to be. (Modus Ponens)
3. If you don’t know that the world is as you perceive it to be, then you don’t that there is an
external world.
4. You don’t know that the world is as you perceive it to be. (From C1)
C2. Therefore, you don’t know that there is an external world. (Modus Ponens)
II. General skeptical argument from ignorance (GSA)
Notice that the above argument is generalizable into the following schema:
1. You don’t know that not-q.
2. If you don’t know that not-q, then you don’t know that p.
C. Therefore, you don’t know that p. (Modus Ponens)
(Where ‘q’ represents some skeptical scenario and ‘p’ represents an ordinary proposition about the
external world whose truth is logically incompatible with q.)
this.
How is the inference from premises 1 and 2 to C justified? The following sections examine
III. The closure principle
The principle behind CSA, and skeptical arguments from ignorance in general, exploits our
intuition that our knowledge of any proposition about the world, e.g., that there exist other minds,
hinges on our knowledge of the fact that we aren’t being deceived into having false beliefs about the
world, that the veil of perception which seals us off from the world beyond does in fact reflect
reality. (Remember, Descartes is a representationalist about perception, like almost all of his
contemporaries.) This principle is known in the literature as the Closure principle.
Let us state Closure in more useful general terms. What the principle essentially captures is
that since any proposition logically implies the truth of other propositions, to lay claim to a
1
Rene Descartes, Meditations on First Philosophy, 3d ed., trans. Donald A. Cress (Indianapolis: Hackett, 1993), 16-7.
1
proposition as an item of knowledge, one must also know, or be in a position to know, the
propositions which are implied by it. Moreover, insofar as this is true, one must also know, or be in
a position to know, that the propositions which are logically incompatible with it are false. Returning
to GSA, we see that this principle is indeed what licenses the move from premises 1 and 2 to C: it is
tacitly assumed that if p is logically incompatible with q, then knowing that p is not possible without
one’s knowing also that not-q. As you will learn throughout the course of Intro to Philosophy, the
draw of skeptical challenges from ignorance are often taken to be parasitic on our implicit
acceptance of Closure.
Let us now state Closure in more formally perspicuous terms. Two forms of the Closure
principle are given in the chart below. (Note the parenthesized contrapositive equivalents!)
Closure principle
Strong Closure
Weak Closure
Description
If S knows that p, and p entails q, then S knows (or is in a position to
know) that q. (If S does not know that q, and p entails q, then S does
not know that p.)
If S knows that p, and S knows that p entails q, then S knows (or is in
a position to know) that q. (If S does not know that q, and S knows
that p entails q, then S does not know that p.)
Question: which form of the Closure principle do you think Descartes accepts?
If you chose Weak Closure, you are right. Why do you think this is? Epistemologists tend to
reject Strong Closure because it entails logical omniscience for knowledge about any given
proposition, no matter how trivial the claimed item of knowledge is. For example, it may turn out on
a fully spelled out description of the physical world and the laws of logic that the truth that I see
myself in the mirror entails the truth of Godel’s completeness theorem. Surely, we needn’t know the
latter to know the former. Descartes, on the other hand, requires only certainty (though this in itself
is asking a lot) for knowledge. So he accepts Weak Closure.
IV. Barry Stroud’s principle of exclusion
Now, that skeptical arguments of Descartes’ sort depend on some form of Closure for their
pull is only half the story. Let me fill in the rest of the picture that is missing by way of an example.
Take the following contemporary twist on Descartes’ “dream” hypothesis, that we could be stuck in
a lifelong dream.
Lifelong dream. On the distant planet Alpha Centauri live molecular duplicates of
all of us Earthlings, including mine, Torrey*. In fact, Alpha Centauri mirrors Earth in all of
its happenings and its people. Just about all, anyway. There is one exception: all Alpha
Centaurians are confined by their unknown leader, Evil Genius, to a perpetual dream-state,
and believe things based on the contents of their dreams. So, while I am presently creating
this handout at my computer at t2, and hence form the belief that p, I am typing at my
computer, at t2, Evil Genius induces in Torrey* the dream that he is also doing the same
thing at t2, leading him to form the same belief that p at t2. Since Alpha Centauri is just like
2
Earth in every other respect, Torrey*’s belief that p at t2 is true, like mine: he is typing away,
just as I am. Furthermore, we both have the same reasons for all of our beliefs.
But this is not all. Notice that according to Weak Closure, Torrey*, in addition, knows that p,
since the fact that he’s dreaming that p as p plays out just the way he dreams it is not logically
incompatible with p.2 According to Weak Closure, then, Torrey* needn’t know or believe one way or
another whether he is dreaming that p. Intuitively, however, Descartes would deny this, since
Torrey*’s beliefs evidently lack the appropriate causal relations to the relevant features of his world.
This is so, and I stress this, even though Torrey*’s beliefs are such that they match up with his
world. So, it is crucial to distinguish the question whether someone can believe truths while
dreaming from whether he can come to know truths while in a dream. Weak Closure thus issues the
wrong verdict, from Descartes’ perspective, regarding Torrey*’s lack of knowledge.
So, because Torrey*’s dream-induced belief in Lifelong dream that he is typing at his
computer at t2 is not logically incompatible with its being the case that he is, in fact, typing at his
computer at t2, Weak Closure has the absurd result that Torrey* knows that p. Clearly, however,
since Descartes’ program of hyperbolic doubt is to cast out as unknown all of his beliefs which lack
full certainty,3 the deficiency just now examined of Weak Closure demands correction. We need a
principle that is wide enough in its scope to deny Torrey* knowledge that p. Barry Stroud argues that
the following principle not only accurately expresses the principle behind Descartes’ dream
hypothesis and other skeptical scenarios akin to it, but also, our ordinary intuitions about when
someone actually knows.4
Principle of Exclusion
If S knows that p, and S knows that q is incompatible with S’s
knowing that p, then S knows that q is false. (If S does not know that
q, and S knows that q is incompatible with his knowing that p, then S
does not know that p.
Intuitively, this principle makes sense. If I claim to know that I am looking at a zebra in a
cage at a zoo (p), and furthermore, also know that the possibility that the zebra is really a mule
cleverly painted to resemble a zebra (q) is incompatible with my knowing the former, I must be able
to rule out the latter to maintain knowledge of it. How, then, does the Exclusion principle obviate
the problem we ran into in Lifelong dream when we had only Weak Closure to work with? That is,
how is Torrey*, on this strengthened variant of Weak Closure, denied knowledge?
In this way: while Torrey*’s belief that p happens to map on to the way that his world
actually is, it fails to provide him with access to any sort of privileged information which would
2 We are assuming, then, the plausibility that all of Torrey*’s beliefs are just as justified as mine, even though his beliefs
aren’t causally related to his world in the “natural” way that mine are. This strikes me as rather unobjectionable.
3 Hence his lamentation in the first Meditation: “… [T]here is nothing among the things I once believed to be true which
it is not permissible to doubt—and not out of frivolity or lack of forethought, but for valid and considered reasons”
(16). The possibility of doubt which attaches to all his beliefs leads Descartes, in the second Meditation, to uproot
“whatever can be weakened even to the slightest degree…so that eventually all that remains is precisely nothing but what
is certain and unshaken” (18).
4 Barry Stroud, “The Problem of the External World,” in Epistemology: An Anthology, ed. Ernest Sosa and Jaegwon Kim
(2000; repr., Oxford: Blackwell, 2003), 17-19.
3
show that q, “I am dreaming that I am typing at my computer,” is false. Most of us, I think, would
err on the side of denying someone knowledge whose belief, though true, was the result of a
hallucinatory or veridical experience, as in a dream-state. Thus, Torrey* doesn’t satisfy the Exclusion
principle’s requirement that S know that any state of affairs incompatible with his knowing that p
doesn’t obtain.
So, under the knowledge constraint of the Exclusion principle, to know anything, one must
know criteria which enable him to accurately and reliably distinguish between a possible experience
that connects him in a causally appropriate way to the actual world from another that is
phenomenally indistinguishable from the former, but which doesn’t connect him thus.
V. Conclusion
So, what is the upshot of the foregoing discussion? Certainly, one indubitable result that we
have obtained is that Torrey* just doesn’t know. But, if you’re especially sharp this morning, you
might have noticed that this isn’t the real upshot. It isn’t my molecular duplicate whose knowledge I
should be worrying about. It is mine. For if Torrey* cannot escape the Exclusion principle, how do I
know that I’m not actually the one who is stuck in a world like his? I do, after all, obtain my reasons
for my beliefs in exactly the same way he does.
One tack philosophers have resorted to in order to resist outright skepticism is to deny the
knowledge constraint in the Exclusion principle, and argue instead for a different account of Weak
Closure in which one does not need to know that all scenarios incompatible with his knowing a
certain proposition are false, or do not obtain—just some relevant subset of them as determined by,
say, certain salient features of one’s conversational context. But this salvage has struck many
philosophers as implausible, too, as it implies a sort of epistemic relativism: suppose under one such
account all the salient conversational features at present allow me to ignore all hypothetical skeptical
scenarios. I can then know that my car is in the parking lot, without knowing that it hasn’t been
stolen while I went into McDonald’s to grab a hamburger to go. But can anyone truly maintain
knowledge of these two facts at the same time—that my car might have been stolen, and that I
know that my car is nevertheless still where I parked it? Either my car has been stolen, or it hasn’t. If
it hasn’t, lucky me. If it has, then something has gone terribly awry. How can any account have it
that I do know that my car is parked outside even though it isn’t actually?
In short, even if we don’t like either Weak Closure or the knowledge constraint of the
Exclusion principle, we still think that one or both of them normally tracks our ordinary judgments
about when someone actually possesses knowledge. To get rid of them both would seem to make
knowledge too easy. The other apparent horn has been to withdraw into full-blown skepticism.
Neither option seems terribly appealing. Descartes’ skeptical challenge is deeply unsettling. Stroud
puts this unease nicely: “the consequences of accepting Descartes’ conclusion as it is meant to be
understood are truly disastrous.”5 Unless, of course, you understand Descartes’ proof that God
exists. This is a topic, however, for a different handout.
5
Stroud, op. cit., 23.
4