Two Types of Skepticism
It is common for philosophers to use the phrase “skepticism with regard to the external world” or “skepticism about other minds”. This is quite misleading, because it conflates two distinct questions: one relating to reality, the other to knowledge of reality. The dictionary definition of “skeptic” is “a philosopher who denies the possibility of knowledge, or even rational belief, in certain areas” (OED). This is the right definition: a philosophical skeptic is concerned to deny or doubt that we have knowledge about the external world (say), not to deny or doubt the external world. It is of course possible to deny or doubt the existence of the external world: a philosopher could (like Berkeley) claim that the notion of matter or unperceived existence is incoherent. So one can be skeptical about whether the external world exists for reasons having to do with the nature of the external world. We could call this “existence skepticism” and contrast it with “knowledge skepticism”. One could also be skeptical about the existence of a particular person or place (Santa Klaus, Atlantis) or about knowledge of a particular person or place. These are quite distinct kinds of claim. It is therefore a question whether one kind of skepticism has any bearing on the other.
It is clear that existence skepticism does not imply knowledge skepticism. Whether X exists is quite independent of whether we can have knowledge of X, except in the trivial sense that if X does not exist there is nothing to have knowledge of (though we can have knowledge of fictional entities). Doubts about the coherence of the notion of mind-independent matter have nothing to do with the possibility of our knowing about such a thing. They are ontological not epistemological. But neither does knowledge skepticism lead to existence skepticism: denial or doubt of knowledge of something does not entail denial or doubt of that thing. This is very clear from the third-person point of view: I can deny that someone else has knowledge of X without in any way casting doubt on X itself—my claim is purely epistemological. Likewise, I can cast doubt on my own knowledge of something without casting doubt on that thing—I may be quite neutral about the thing itself. I may think that I have reasons to doubt that I have knowledge of X without thinking that I have reasons to doubt X (such as the possibility that I am a brain in a vat). The fact that I lack knowledge of X is not the fact that X is not real.
So the success of skepticism with regard to our knowledge of the external world does not entail skepticism with regard to the external world. Reasons to doubt knowledge of external reality are not reasons to doubt external reality. It may be replied that knowledge skepticism about X entails that X is doubtable, since we can no longer assert with certainty that X exists. We can coherently doubt the existence of the external world, since we do not have conclusive reasons to believe in it. That seems right, but it doesn’t follow that the existence of the external world is doubtful: no reason against its existence has been given at all, so it is not doubtful. It would be a misuse of language to say that the existence of New York is doubtful, given only that it is possible for us to doubt that New York exists, i.e. its existence is not a certainty. Compare the words “dubitable” and “dubious”: the former means “uncertain” or “capable of being doubted”, but the latter means “questionable” or “objectionable”. There is a relativity in “dubitable” that is not present in “dubious”, since what can be doubted by one person may not doubtable by another (e.g. the proposition that I am in pain); while what is dubious is dubious for everyone, it being inherent in the proposition in question. Merely being open to doubt (by person x) is not the same as being doubtful or dubious tout court. So knowledge skepticism is quite neutral with respect to existence skepticism: we can deny or doubt knowledge of X without giving any reason to deny or doubt X. Arguments against knowledge of X are not arguments against X.
Why does this distinction matter? First, it is good to be very clear about the difference between the two questions, since it can be easy to confuse them. Second, the phrase “skepticism about the external world,” used to refer to knowledge skepticism about the external world, is a logical solecism (and similarly for other minds, the past, etc). Third, people seem routinely to suppose that knowledge skepticism about X shows that the existence of X is “doubtful” or has somehow been “cast into doubt” or has been impugned in some way. What is true is that such skepticism implies that the existence of X is doubtable, i.e. not certain (and hence, allegedly, not known); but it doesn’t follow from this that X is (inherently) doubtful or somehow objectionable. There is nothing dubious about the external world or other minds or the past—nothing discreditable or suspect or defective or objectionable. Our habitual beliefs about these things may be suspect and defective, but not what the beliefs are about. It is simply that we don’t have knowledge of these things—we are defective, not the thing itself. God can be certain of these things—they are not dubitable for him–but we can’t; and that is a problem we have. There is nothing dubious or discreditable about the proposition that the external world exists; it is just that we cannot believe that proposition with certainty (or perhaps at all).
The right thought to have when confronted by knowledge skepticism is: “Oh, so I can’t know that X exists, but that doesn’t mean X is somehow intrinsically doubtful or problematic”. That is, existence skepticism doesn’t follow from knowledge skepticism—not even a tiny bit. To put it differently, the proposition that we have knowledge of the external world might reasonably be criticized, but that has no tendency to show that the proposition that there is an external world can be criticized. Asserting that proposition might be criticized, since assertion is a claim to knowledge; but that does not show that the proposition itself can be criticized. The skeptic is not challenging the truth of the proposition that there is an external world, only our ability to know it. If you find yourself unable to suspend belief in the face of skepticism, you may be being irrational, but what you believe might be completely unimpeachable. There is nothing inherently doubtful about the external world or other minds, even granting the truth of skepticism. We thus have no reason to be skeptical about the external world and other minds (unless we have direct arguments against these things—which we don’t have). There are reasons to doubt the existence of things like Santa Klaus and Atlantis, or God and the Devil, but the external world and other minds are not in this category (pace Berkeley). One can rightly be a skeptic about those dubious entities, but in that sense one cannot rightly be skeptic about the external world and other minds. In the latter case, the skepticism is purely of an epistemic kind, and reflects poorly only on us not on them.