Through most of the history of Western philosophy, skepticism has been the specter haunting epistemology. I am teaching an introductory course in epistemology, and every introductory textbook has a chapter, usually near the beginning of the book, on how to “deal” with skepticism. The assumption seems to be that skepticism is not so much a specter as a bothersome insect that has to be swatted before you can get down to the proper business of epistemology, like specifying conditions of justification (“S is justified in believing that P if and only if…”). Or maybe skepticism is viewed more as epistemology’s loony uncle who has to be stowed out of sight in the attic. Either way, the assumption seems to be that skepticism is a sort of particularly pestiferous or embarrassing problem that creates special difficulties that cannot just be ignored and has to be managed somehow.
For some of the major philosophers, on the other hand, skepticism was a useful tool. Descartes and Hume used skepticism for very different purposes. For Descartes, skepticism was the first stage of his method. You engage in hyperbolic doubt, questioning everything that can be questioned. If anything is dubious in any degree, even upon outlandish scenarios like all-powerful deceiving demons, then you regard it as false. The point is not to establish global skepticism, but, just the opposite, to ground knowledge upon absolute certainty. If we engage in the most corrosive and comprehensive doubt, but find that some truths (“Cogito ergo sum”) cannot be doubted even by those means, then we have a solid foundation of certain truth upon which we may base our whole edifice of knowledge.
For Hume, skepticism was useful to show that very much of what we believe is not held on the basis of reason, but is due to habit, custom, or conditioning. We expect the sun to rise tomorrow, but we have no rational basis for believing that it will. Any projection from the past into the future presupposes the uniformity of nature. That nature is uniform is not necessarily true since it is not self-evident nor can it be demonstrated a priori. Any argument intended to establish that, in fact, nature is uniform will have to appeal to that very principle, and so will be circular. Hume concluded that since our expectation of uniformity has no rational basis, it must be due to a habitual expectation repeatedly reinforced. In other words, we are like Pavlov’s slobbering dogs.
My view is that skepticism should not be viewed as confronting us with a particularly troublesome set of problems that have to be swatted away, or at least sealed off before we can get down to our epistemological business. Neither do I see skepticism as the acid bath that dissolves the tarnish and reveals the pure gold of apodictic knowledge, as did Descartes. I do not think that the evil demon can be completely exorcised. Though I am a great admirer of Hume, I think the range of human cognitive competence is much greater than Hume admitted. Hume was far too quick to say what we could not know (he should have applied a bit more skepticism to his skepticism). Two centuries and a half centuries of spectacular scientific progress has belied his epistemic pessimism.
Skepticism is either a doctrine, i.e. a set of claims, or it is a tactic. If it is a doctrine, then it may be critically discussed like any other philosophical doctrine, and its defenders may be held to the same standards of consistency and coherence whereby we judge any other philosophical theory. If it is a tactic, then we may meet a tactic with a tactic, one that disrupts the skeptic’s uneven game and levels the playing field.
As a doctrine skepticism could, for instance, deploy the ancient problem of the criterion: Any truth claim must be evaluated by criteria, but how can we know that the criteria we invoke are the right ones? Either we justify those criteria by appealing to those very criteria, or we invoke new criteria. The former tactic argues in a circle and the latter threatens an infinite regress, therefore we have no basis for claiming to know truth. Perhaps skeptics, like the ancient Academics, will argue that nothing is known with certainty, not even what seem to be the immediate deliverances of our senses. Since, then, nothing is certain, all must be held in doubt, and suspension of belief is the rational option.
Skeptics’ claims may be questioned and must be defended like any other philosophical claim. We may ask, for instance, why we should agree that judgments of truth always require a criterion. Do we not, for instance, rightly judge that some utterances are meaningless even though we might have no criterion of meaning? Might I not know that an expression of condolence or empathy is right in a given situation even though I lack a general theory of human interactions? Are not some things simply evident, and so knowable without appeal to a criterion? The defender of the criterion argument will then have to justify the claim that all judgments of truth, without exception, require a criterion, and such arguments do not require special treatment of any sort, nor should they provoke any special anxiety. They may simply be evaluated as we would any other philosophical claims.
One particular problem that skeptical doctrines must face is the problem of reflexivity. Are the skeptic’s claims incoherent in the sense that they are self-defeating? Do they fail to live up to their own standards? For instance to the proposition that the certainty required for knowledge is not to be had, we may ask the proposer how certain he is of that. If he says that he is certain, he provides a counterexample to his own claim that nothing is certain. If he says that he is not certain, but he knows it anyway, then he is rejecting his claim that certainty is necessary for knowledge.
While “hoist with your own petard” arguments are always fun, skeptics are not stupid, and a clever skeptic can always deflect the burden of reflexivity. One way to do so is to deny that skepticism is a positive doctrine of any sort, making any sort of positive claims at all. Rather it is a tactic, perhaps the Socratic tactic of rigorously cross-examining every claim to show that it cannot meet its own standards. Thus, Socrates presented Euthyphro with his famous dilemma: Either we define piety in terms of what the gods happen to love, or we say that the gods love pious acts because they are pious. Neither horn of the dilemma was acceptable to Euthyphro, so he did not reply but hurried away, probably wishing he could administer some hemlock to that smartass Socrates.
Another, simpler tactic would simply be for the skeptic to pose endless iterations of the question “How do you know that?” Whenever a claim is made, justification is required. When justification is offered, the justification of the justification is demanded. Such a skeptic is a one-trick pony, but it is a pretty good trick, and you cannot beat it as long as you play the skeptic’s game.
If skepticism is a tactic, you can confront it with another tactic. This is what I think G.E. Moore was doing with his famous “proof” of an external world. Moore held up one hand and then another, thereby showing that there are physical objects in an external world. Of course this does not really prove anything to the skeptic, who says that Moore is begging the question against him. It is precisely such supposed perceptual truths that the skeptic is questioning. What Moore does do is to deploy a tactic to disrupt the skeptic’s tactic of endless questioning. Moore’s tactic is to shift the burden of proof to the skeptic. Here is a hand. Do you doubt it? If so, tell us why you doubt it. It seems evident to me, so tell me why it is not evident to you, why you would say that there only appears to be a hand. If the skeptic replies at all, then he accepts a burden of proof, and then we get the pleasure of subjecting him to a Socratic cross-examination.
As for Descartes, when in Mediation Three he fails to prove the existence of God, then he fails to perform the full and final exorcism of the evil demon. The evil demon we will have with us always. Could you be a brain in a vat? Sure. It cannot be disproven. On the other hand, we need to know why, just because we cannot definitively rule it out, we need to take it as a serious hypothesis. Who says that absolute certainty is a requirement for knowledge? Why can’t we be fallibilists, that is, say that we can know that p even if, possibly, not-p? Why should the possibility that I am in the Matrix or a vat be of any concern to me at all? There is zero evidence for such scenarios (or any other such scenario, as that we are living in a computer simulation) that is set up to prevent us from knowing that the scenario is true. Why waste time on a scenario that sets you up to fail?
As for Hume, he delighted in reflecting on the narrow limits that, he said, nature has placed upon us. We can only know the external appearances of things, never the hidden, secret natures whereby they supposedly have the powers that they have. For instance, Hume says that we can only know the texture, color, and taste of bread, but never why it should nourish us. That we will never know. But we do. We know how polysaccharides are metabolized in the body—the Krebs Cycle, mitochondria, ATP, ADP, and the whole story. We know loads of other things Hume thought we could not. The lesson seems to be to be skeptical of your own skepticism and not write off too quickly what we can know. We may not be as certain as Descartes thought we could be, but neither are we as ignorant as Hume held.