Two Skeptical Arguments

Guest Contributor

15 March 2009

I’ve been claiming that there are some really powerful skeptical arguments (on the show and in response to Ken's previous post). I have also been claiming that one aspect of their force is that they do not depend on setting the standards for knowledge very high. Here are two such arguments.

1. Hume’s argument.
The first is inspired by David Hume. The argument begins with the assumption that our beliefs about the external world are at least partly based on how things appear. For example, I believe that I am presently seated at my desk at least partly because that is the way things visually appear to me. But that can’t be the whole story, the argument continues. I must also be assuming, at least implicitly, that the way things appear is a good indication of the way things really are. If I were not relying on that assumption, Hume argues, then the fact that things appear to me a certain way would not be a reason to think that they are that way. But now how am I to justify this assumption about the reliability of appearances? How can I know that the way things appear is a good indication of the way things really are? According to Hume, there is no way to justify that assumption. For example, suppose I were to rely on appearances, reasoning that, as far as I can tell, the way things appear to me appear to be a reliable indication of the way things really are. This, of course, would be to argue in a circle, taking for granted the very thing at issue. Here is Hume’s argument put more formally.

(H)

1. All my beliefs about the external world depend for their evidence on both a) the way things appear to me, and b) an assumption that the way things appear to me is a reliable indication of the way things really are.
2. But the assumption in question can’t be justified.
Therefore,
3. All my beliefs about the external world depend for their evidence on an unjustifiable assumption. (1, 2)
4. Beliefs that depend for their evidence on an unjustifiable assumption do not count as knowledge.
Therefore,
5. None of my beliefs about the external world count as knowledge. I don’t know anything about the external world. (3,4)

Clearly, a linchpin of this argument is premise (2): that an assumption regarding the reliability of appearances cannot be justified. In support of premise (2), Hume considers various possibilities for justifying the assumption in question. One consideration that Hume emphasizes is that the assumption is itself a contingent claim about the external world. That is, the assumption claims that sensory appearances are, as a matter of contingent fact, related to the way things are in a particular way. This suggests that the assumption can be justified, if at all, only in the way that contingent claims about the external world are justified in general—i.e. by relying on the way things appear! But this, of course, would be to argue in a circle, taking for granted the very thing at issue. Here again is the reasoning in support of (2).

(H2)

1. All my beliefs about the external world depend for their evidence on both a) the way things appear to me, and b) an assumption that the way things appear to me is a reliable indication of the way things really are.
2. The assumption in question is itself a belief about the external world.
Therefore,
3. The assumption depends on itself for its evidence. (1, 2)
4. Beliefs that that depend on themselves for their evidence can’t be justified.
Therefore,
5. The assumption in question can’t be justified. (3, 4)

A natural thought is that the assumption that appearances are a reliable guide to reality can be justified in some other way, perhaps by some sort of a priori reflection that proceeds independently of appearances. But Hume thinks that this line of reasoning is a dead end. This is because the assumption in question makes a contingent claim about the way things are—it is a matter of contingent fact, and not a matter of necessity, that appearances do or do not reflect the way things really are. But that sort of fact cannot be known through a priori reflection. In short, a priori reflection gives us knowledge of necessary truths rather than contingent truths.

2. Descartes’s argument.
The second skeptical argument is inspired by Descartes’s Meditation One, and in particular by Barry Stroud’s reading of that meditation. To understand the argument, consider the claim that one sees a goldfinch in the garden, based on one’s observation that the bird is of a particular size and color, and with a tail of a particular shape. Suppose now that a friend challenges one’s claim to know, pointing out that woodpeckers also are of that size and color, and also have tails with that shape. As Stroud points out, this seems to be a legitimate challenge to one’s claim to know that the bird is a goldfinch. More generally, if one’s evidence for one’s belief that the bird is a goldfinch is consistent with the possibility that it is in fact a woodpecker, then one does not know on the basis of that evidence that it is a goldfinch. Based on this sort of reasoning, the skeptic proposes the following plausible principle:

1. A person knows that p on the basis of evidence E, only if E rules out alternative possibilities to p.

Further support for this sort of principle comes from reflection on scientific enquiry. Suppose that there are several competing hypotheses for explaining some phenomenon, and suppose that these various hypotheses are “live” in the sense that current evidence does not rule them out as possibilities. It would seem that one cannot know that one of the hypotheses is true until further evidence rules out the remaining ones. Again, principle (1) above looks plausible.
The second step in the skeptical argument is to point out that there are various possibilities that are inconsistent with what we claim to know about the external world. For example, it is possible that things appear to me visually just as they do now, but that I am actually lying in my bed asleep rather than sitting at my desk awake. It is possible that things appear to Descartes’s just as they do, but that he is actually the victim of an evil demon, a disembodied spirit who only dreams that he inhabits a material world and is presently seated by the fire. To be clear, it is no part of the skeptical argument that such alternative possibilities are true, or even that they are somewhat likely. The point is only that they are possibilities, and so undermine our knowledge if our evidence does not rule them out.
The third step in the skeptical argument is to claim that our evidence does not in fact rule these possibilities out. The gist of the present claim is something like this: These possibilities are consistent with all the evidence that we have or could have at our disposal. Even if, practically speaking, we don’t usually give such possibilities a thought, upon reflection we have no evidence available to us that counts against them, and in favor of our preferred beliefs.
If we put these three claims together we have the materials for a powerful skeptical argument. Here is the argument stated more formally.

(D)

1. A person knows that p on the basis of evidence E, only if E rules out alternative possibilities to p. (Principle 1 from above.)
2. It is a possibility that I am not sitting at my desk awake, but merely dreaming that I am.
Therefore,
3. I know that I am sitting at my desk only if my evidence rules out the possibility that I am merely dreaming. (1, 2)
4. But my evidence does not rule out this possibility.
Therefore,
5. I do not know that I am sitting at my desk. (3, 4)

And of course the skeptical argument is supposed to generalize. That is, it is supposed to apply to beliefs about the external world in general. We therefore have:

6. The same line of reasoning can be brought to bear against any belief about the external world.
Therefore,
7. No one knows anything about the external world. (5, 6)

One way to understand the notion of “ruling out” a possibility is as follows: A body of evidence E rules out a possibility q if and only if E supports not-q in a non-circular way. Here we can understand support as a semantic notion: Evidence E supports propositions p, in the relevant sense, just in case E entails p or E makes p probable. Putting these ideas together, we get the following interpretation of premise (4) of argument (D).

4a. My evidence for my belief that I am sitting at my desk neither entails nor makes probable (in a non-circular way) the proposition that I am not dreaming.

Why might one accept premise (4a)? One reason for accepting (4a) is the considerations put forward by Hume’s argument above. That is, one might think that my evidence for believing that I am sitting at my desk is the way things appear to me, together with my assumption that the way things appear to me is a reliable indication of the way things are. But as Hume’s reasoning shows, there is no non-circular way to justify the assumption in question, and therefore no good evidence for either that assumption or further beliefs that are based on it. In particular, my evidence cannot entail or even make probable (in a non-circular way) the proposition that I am not dreaming. Insofar as this is the reasoning behind (4a), argument (D) is parasitic on argument (H).
There is, however, another way to understand the notion of evidence ruling out alternative possibilities. On this understanding, a body of evidence E rules out alternative possibilities to p just in case E discriminates the state of affairs represented by p from alternative states of affairs. For example, hearing my wife coming in the door from work, my auditory experience rules out the possibility that it is my children coming home from school or a burglar coming in through a window. In effect, I have the capacity to “tell the difference,” so to speak, and this is what allows me to know that it is my wife who has just come in the house. On this understanding of “ruling out”, it does seem plausible that my evidence must rule out alternative possibilities in order to ground knowledge. For example, how could I know that my wife has just come home, on the basis of hearing her come through the door, if I could not discriminate that state of affairs from my daughter’s coming through the door? Moreover, premise (4) of argument (D) becomes plausible on this understanding of “ruling out.” We now have

4b. My evidence does not discriminate my sitting at my desk from my merely dreaming that I am sitting at my desk.

One might think that this claim is obviously right. To be clear-- I assume that the skeptical argument must be wrong somewhere. My point here is that it isn't obvious where, or that the argument is invoking some very high standard for knowledge.

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Comments (24)

Guest

Sunday, March 25, 2007 -- 5:00 PM

Excellent synopsis. I think I agree with Professor

Excellent synopsis. I think I agree with Professor Taylor's post that the challenge of skepticism cannot be met, and so that our knowledge talk reduces to talk about belief formation strategies tailored to our phenomenal world.
Hume probably gave the only answer to the skeptical challenge one could hope to give, namely, that at the end of the day we all (do and probably should) set aside the assumptive and discriminative problems you've limned and go about "knowing" things about our world (such as it seems to us), with more or less fitting results. I might be trapped in a dream or a simulation, but then if so my best hope for escape would seem to lie in rational belief formation. For what other strategy is there?

Guest

Sunday, March 25, 2007 -- 5:00 PM

The two skeptical arguments above are interesting,

The two skeptical arguments above are interesting, but I think Ken
was asking a question in his previous posts that has not
yet been addressed: are there powerful arguments
against rational (or justified or warranted) belief?
For instance, can we modify the above two arguments to produce
compelling arguments that no one can have justified beliefs about
the external world?
This had some plausibility:
H.4 Beliefs that depend for their evidence on an unjustifiable assumption do not count as knowledge.
but what about this?
H.4* Beliefs that depend for their evidence on an unjustifiable assumption do not count as justified.
A similar change for D is changing this
D.1 A person knows that p on the basis of evidence E, only if E rules out alternative possibilities to p.
to this:
D.1* A person is justified in believing that p on the basis of evidence E, only if E rules out alternative possibilities to p.
As far I can see, the reasons given in support of D.1 (on the second
sense of ruling out) support D.1* too. I am unsure about H.4*.

Guest

Monday, March 26, 2007 -- 5:00 PM

Nice post, Patrick. I think here we need to make

Nice post, Patrick. I think here we need to make a distinction between epistemic and practical justification (this is relevant to Q's comment and Ken Taylor's post as well). I think there is a familiar sense in which an action A or belief B is practically rational if it is my best or only option. But then the skeptic is not challenging the practical rationality of either believing or acting on those beliefs. Rather, he/she is challenging the belief's epistemic standing, and claiming that it is not based on sufficiently good epistemic (truth-indicating) reasons. My point is now this, in favor of H.4*: I can see where it is sometimes practically rational to believe or act on the basis of epistemically unjustified assumptions. But it is much less plausible that one can be epistemically justified in believing on the basis of epistemically unjustified assumptions. If one can be, it should come as a big surprise.

MIke

Monday, March 26, 2007 -- 5:00 PM

John, Doesn't the Humean argument depend on kn

John,
Doesn't the Humean argument depend on knowing a priori that, necessarily, it is possible that the way things appear are not the way they are? If that proposition is not necessary, then it is not knowable a priori (on Hume's view). But it certainly does not seem to express a 'relation of ideas'. On the other hand it is not a matter of fact, either.
But setting that aside, I guess I don't see why the epistemological standards are not high in this argument. To know, I'm required to rule out every deceptive/misleading alternative. That seems like a fairly high epistemic standard.

aaron

Monday, March 26, 2007 -- 5:00 PM

The skeptical arguments are useless because they c

The skeptical arguments are useless because they cannot be met. A creative person can always come up with a reason why an answer isn't the only one - Hume's denial of reality could be the basis for refuting ANY question. There has to be context to the question, or it becomes meaningless.
Consider the desk argument. We assume the following:
1) dreaming is different from reality
2) I am an individual
3) when I use the word "sit", it means the same thing to me as it does to you
4) that I am not quantumly sitting at one desk, while standing in an alternate universe
Etc, etc. The skeptic can find an infinite number of reasons not to believe that a person sits at his desk. At its worst, this line of thinking becomes semantic, rather than philosophical.
The point of philosophy, I believe, is to ask questions that are in context, that have certain parameters that are mutually agreed upon. If two people cannot agree about the most basic of facts to begin a question, then the question loses all meaning.