The Skeptical Challenge
My concern, in this and posts to follow, is to give an answer to the skeptical challenge, one that does justice to the force of skeptical arguments but allows for both knowledge and rational belief concerning the external world.
Imagine, for a moment, that the world is not as it seems to be. Imagine that the experiences which are so formative in your conception of the world are not the product of various objects they superficially indicate, but are instead the product of a carefully designed computer programme. You are a brain in a vat, and your experiences are stimulated by electrical impulses, sent from a supercomputer via electrodes attached to various parts of your brain. If that were the case, how could you know it? But perhaps it is the case. How, then, can one claim to know anything about the external world?
The above sketches a familiar skeptical line of thought. The skeptic begins with a distinction between appearance and reality, noting that what appears to be may differ from what is. He then proceeds to offer a skeptical hypothesis – that is, he describes a possible state of affairs that is both consistent with the evidence we have, and is inconsistent with our usual hypothesis based on that evidence. In the above case, the brain-in-a-vat hypothesis counts as a skeptical hypothesis because our experiences might have been the same were we brains-in-vats, but obviously the hypothesis that we are brains-in-vats contradicts our usual picture of how we relate to the world as embodied beings. This is implied to undermine our claims to know otherwise.
Just how the hypothesis does this is often left implicit, but one way the skeptical argument might accomplish this is via the “Entailment Principle” (EP):
EP: S knows that p on the basis of evidence e, only if e entails p.
As mentioned, a skeptical hypothesis is one which is consistent with our evidence. But if there is such a hypothesis, then no commonsense hypothesis about the external world would actually be entailed by our evidence, since it is always possible that the skeptical alternative is true. It seems to follow by the EP that we do not know that our experiences are representative of the world as it is, and so the large majority of what we assumed ourselves to know are casualties of the skeptical argument.
But this principle is dubious. One worry here is that if the principle is true, then skepticism follows too easily. It is quite obvious that much of what we claim to know isn’t supported by deductively strong evidence, but this means, since it is such a short step to the view that these knowledge claims must be false, that the fact should be obvious to everybody. So the very success of the strategy casts doubt on its soundness. A second, more serious, concern is that evidence doesn’t entail anything without an evidential framework, and that when we say that “p is known on the basis on e” we are invoking some framework which relates the evidence to the truth. So the entailment of a proposition by evidence is not secured by evidence alone.
I won’t push these loose ideas too far, since there is a better way for the skeptic to connect his hypothesis to the absence of knowledge. Here is another principle he might enlist, called the “Principle of Deductive Closure” (PDC). Here is one version:
PDC: If S knows that p, and S knows that p entails q, then if S believes that q, S also knows that q.
Whereas the entailment principle facilitated the denial of knowledge by emphasizing the consistency of a skeptical hypothesis with our evidence, this principle relies on the other feature of a skeptical hypothesis described, that a skeptical hypothesis would be inconsistent with our commonsense hypotheses about the world. Suppose that one knows some commonsense fact, say, that one has hands. Suppose also that one knows that if one has hands, then the brain-in-a-vat hypothesis is false. Via the principle of deductive closure, then, one must also know that the skeptical hypothesis is false, if one believes it. But, the skeptic reminds us, we know no such thing. And so one cannot know that one has hands, either.
Anybody new to these ideas is bound to experience some profound disorientation about here, their minds vacillating between the natural self-ascription of knowledge and the irresistability of the skeptic’s line of thought. And often part of the disorientation is a feeling of semantic disintegration – one begins to ask, “Well, just what is knowledge, anyway?” The task of my next post will be to give a rough answer to this question, with an eye towards resolving the aporia.