The global skeptic insists that we should question everything, trusting nothing: apparent perception, memory, "rational intuition", even methods of reasoning -- anything that can be doubted must be suspended from our minds and considered 'guilty until proven innocent'. The obvious problem with this stance - if followed assiduously - is that it entails rejecting everything, leaving one with no mind at all. This is the crucial point: just as a pure 'blank slate' cannot learn from experience, so must we rely on various assumptions if we are to learn, reason, and act rationally at all.
That's not to say we should dogmatically refuse to question our beliefs and practices. It's just that we cannot question them all at once. Any individual belief or assumption may be tested in light of the other things we (provisionally) take to be true, and revised if some incoherence is found. But we have to reason from the beliefs we have -- as Neurath famously put it, our mind is like a ship at sea, and even as we replace a faulty plank we must trust our weight to others.
The mere fact that those other beliefs can likewise be questioned does not suffice to show that we can't reasonably accept them (again, provisionally) in the meantime. The skeptic's claim to the contrary is itself a questionable belief, and not one we have any reason to accept. (It may seem intuitive at first, but once we fully understand the implications of this claim, it would be crazy to accept it -- or so I argue.)
To begin with, it's worth emphasizing that global skepticism is self-defeating, insofar as it implies that we should not trust its recommendation to trust nothing. Or, as Railton writes (in 'How to Engage Reason', Reason and Value, p.186):
Hume observed that [the sceptic] displays a touchingly unsceptical attitude towards the power of argumentation and his own powers of thought and memory. We might add: toward his own command of language and the content of this thoughts. Remove this default confidence, and he can no more declare his words to be 'giving an argument for scepticism'--an intentional action--than 'giving a recipe for haggis' or 'scat-singing without a tune'.
Hence my earlier claim that one who truly internalizes global skepticism is no longer capable of intentional thought or action at all. To accept global skepticism is to forsake any hope of rationality. It is the ultimate intellectual black hole. Railton (p.187) draws an important lesson:
Default trust, however 'blind', is not inherently blinding. On the contrary. If the sceptic trusts his ability to speak English and draw the conclusions demanded of his premisses, and I trust my own appreciation of his argument, we will both see (no longer be blind to) a problem that I have in defending my beliefs: Where I previously had hoped to be able always to have a reason for whatever I believe, taking nothing 'blindly' or 'without reason', I now realize that this hope is impossible--some things cannot, without regress or circularity, be argued for.
I trust the reader will by now agree that global skepticism is a non-starter, as any rational agent must take some things 'on trust' if they are to be capable of reasoning at all. Still, one might ask, what of slightly restricted forms of skepticism? Couldn't one consistently hold the more traditional skeptical view that it's just our sensory experiences and/or inductive practices that shouldn't be trusted? The traditional skeptic is willing to trust in reasoning as much as anybody is. They simply have different expectations about the external world (or "affirm a different prior") from the rest of us. In particular, they hold that all possible worlds are (a priori) equally probable, whereas we anti-skeptics consider some (perhaps simpler or more regular-seeming) distributions of properties across space and time to be more probable than others.
Sure, there may not be anything inconsistent about traditional skepticism, so defined. (Though such a skeptic has no reason to expect that they will continue to exist long enough to finish their thought.) But nor is there much reason to accept it, or indeed to find it any more credible than the claim that the world just came into existence 5 minutes ago. Admittedly, traditional skepticism is motivated by a premise that seems plausible at first glance, namely: "the reasonable 'default' view is to start off by assigning each possible world an equal probability of being actual." That sounds fairer and more reasonable than an a priori bias in favour of, say, worlds where memories are typically true--representing times and events that really did happen. But I suspect the intuitive plausibility of this is an instance of us being misled by overly-abstract principles. When we consider every more particular judgment or knowledge-attribution we are inclined to make, is it really plausible to think that the one abstract principle is more credible than all our conflicting particular judgments - and practices - combined? Colour me (cough) skeptical.
Upon reflection - in light of our actual beliefs - it seems we have most reason to reject the skeptic's principle. Though it seemed plausible at first, it is inconsistent with other claims that most of us find much more plausible. Here's another: rational agents should learn from experience. Skeptics -- even of the merely 'traditional' variety -- can't. As Railton writes (in 'Rational Desire and Rationality in Desire'):
It is well known that in order to learn about one's environment (whether one be human, animal, or teachable computer) it is not enough to have ample sensory input and plenty of memory registers to fill. The learner must also bring some expectations--such as expected dimensions of similarity (the "implicit quality space"). Otherwise, experience will simply accumulate in its infinite diversity, and all experiences will be equally relevant or irrelevant to one another. No lessons will be extracted.
Carnap gave an elegant demonstration of this point within the theory of logical probability. He asked us to consider a confirmation function that began (sensibly, it would seem) with no "prior bias", i.e., that assigned the same non-zero probability to every possible state of the world (what he called "state descriptions"). This function would, even given indefinitely large amounts of information about past states of the world, still assign the same probability to every logically possible way of extending this history into the future. In a fundamental sense, it could not learn from experience.
So I think it's pretty safe to conclude that (even the traditional form of) radical skepticism isn't rational. There's no guarantee that the rest of us are any better off, of course, but at least we have a chance. We do the best we can -- and we're yet to see any good reason to think that some other way is better.
One may be left feeling unsatisfied: the best we can do may not seem good enough. But since turning to skepticism is even worse, it seems we will just have to learn to live with tentatively trusting in the reliability of our perceptions, despite the uncertainty of it all. At least we can learn such things -- and that is something to be thankful for.