in the most common sense, the refusal to grant that there is any knowledge or justification. Skepticism can be either partial or total, either practical or theoretical, and, if theoretical, either moderate or radical, and either of knowledge or of justification. Skepticism is partial iff (if and only if) it is restricted to particular fields of beliefs or propositions, and total iff not thus restricted. And if partial, it may be highly restricted, as is the skepticism for which religion is only opium, or much more general, as when not only is religion called opium, but also history bunk and metaphysics meaningless.
Skepticism is practical iff it is an attitude of deliberately withholding both belief and disbelief, accompanied perhaps (but not necessarily) by commitment to a recommendation for people generally, that they do likewise. (Practical skepticism can of course be either total or partial, and if partial it can be more or less general.)
Skepticism is theoretical iff it is a commitment to the belief that there is no knowledge (justified belief) of a certain kind or of certain kinds. Such theoretical skepticism comes in several varieties. It is moderate and total iff it holds that there is no certain superknowledge (superjustified belief) whatsoever, not even in logic or mathematics, nor through introspection of one’s present experience. It is radical and total iff it holds that there isn’t even any ordinary knowledge (justified belief) at all. It is moderate and partial, on the other hand, iff it holds that there is no certain superknowledge (superjustified belief) of a certain specific kind K or of certain specific kinds K1, . . . , Kn (less than the totality of such kinds). It is radical and partial, finally, iff it holds that there isn’t even any ordinary knowledge (justified belief) at all of that kind K or of those kinds K1, . . . , Kn.
Greek skepticism can be traced back to Socrates’ epistemic modesty. Suppressed by the prolific theoretical virtuosity of Plato and Aristotle, such modesty reasserted itself in the skepticism of the Academy led by Arcesilaus and later by Carneades. In this period began a long controversy pitting Academic Skeptics against the Stoics Zeno and (later) Chrysippus, and their followers. Prolonged controversy, sometimes heated, softened the competing views, but before agreement congealed Anesidemus broke with the Academy and reclaimed the arguments and tradition of Pyrrho, who wrote nothing, but whose Skeptic teachings had been preserved by a student, Timon (in the third century B.C.). After enduring more than two centuries, neo- Pyrrhonism was summarized, c.200 A.D., by Sextus Empiricus (Outlines of Pyrrhonism and Adversus mathematicos). Skepticism thus ended as a school, but as a philosophical tradition it has been influential long after that, and is so even now. It has influenced strongly not only Cicero (Academica and De natura deorum), St. Augustine (Contra academicos), and Montaigne (‘Apology for Raimund Sebond’), but also the great historical philosophers of the Western tradition, from Descartes through Hegel. Both on the Continent and in the Anglophone sphere a new wave of skepticism has built for decades, with logical positivism, deconstructionism, historicism, neopragmatism, and relativism, and the writings of Foucault (knowledge as a mask of power), Derrida (deconstruction), Quine (indeterminacy and eliminativism), Kuhn (incommensurability), and Rorty (solidarity over objectivity, edification over inquiry). At the same time a rising tide of books and articles continues other philosophical traditions in metaphysics, epistemology, ethics, etc. It is interesting to compare the cognitive disengagement recommended by practical skepticism with the affective disengagement dear to stoicism (especially in light of the epistemological controversies that long divided Academic Skepticism from the Stoa, giving rise to a rivalry dominant in Hellenistic philosophy). If believing and favoring are positive, with disbelieving and disfavoring their respective negative counterparts, then the magnitude of our happiness (positive) or unhappiness (negative) over a given matter is determined by the product of our belief/disbelief and our favoring/disfavoring with regard to that same matter. The fear of unhappiness may lead one stoically to disengage from affective engagement, on either side of any matter that escapes one’s total control. And this is a kind of practical affective ‘skepticism.’ Similarly, if believing and truth are positive, with disbelieving and falsity their respective negative counterparts, then the magnitude of our correctness (positive) or error (negative) over a given matter is determined by the product of our belief/disbelief and the truth/falsity with regard to that same matter (where the positive or negative magnitude of the truth or falsity at issue may be determined by some measure of ‘theoretical importance,’ though alternatively one could just assign all truths a value of !1 and all falsehoods a value of †1). The fear of error may lead one skeptically to disengage from cognitive engagement, on either side of any matter that involves risk of error. And this is ‘practical cognitive skepticism.’ We wish to attain happiness and avoid unhappiness. This leads to the disengagement of the stoic. We wish to attain the truth and avoid error. This leads to the disengagement of the skeptic, the practical skeptic. Each opts for a conservative policy, but one that is surely optional, given just the reasoning indicated. For in avoiding unhappiness the stoic also forfeits a corresponding possibility of happiness. And in avoiding error the skeptic also forfeits a corresponding possibility to grasp a truth. These twin policies appeal to conservatism in our nature, and will reasonably prevail in the lives of those committed to avoiding risk as a paramount objective. For this very desire must then be given its due, if we judge it rational.
Skepticism is instrumental in the birth of modern epistemology, and modern philosophy, at the hands of Descartes, whose skepticism is methodological but sophisticated and well informed by that of the ancients. Skepticism is also a main force, perhaps the main force, in the broad sweep of Western philosophy from Descartes through Hegel. Though preeminent in the history of our subject, skepticism since then has suffered decades of neglect, and only in recent years has reclaimed much attention and even applause. Some recent influential discussions go so far as to grant that we do not know we are not dreaming. But they also insist one can still know when there is a fire before one. The key is to analyze knowledge as a kind of appropriate responsiveness to its object truth: what is required is that the subject ‘track’ through his belief the truth of what he believes. (S tracks the truth of P iff: S would not believe P if P were false.) Such an analysis of tracking, when conjoined with the view of knowledge as tracking, enables one to explain how one can know about the fire even if for all one knows it is just a dream. The crucial fact here is that even if P logically entails Q, one may still be able to track the truth of P though unable to track the truth of Q. (Nozick, Philosophical Explanations, 1981.)
Many problems arise in the literature on this approach. One that seems especially troubling is that though it enables us to understand how contingent knowledge of our surroundings is possible, the tracking account falls short of enabling an explanation of how such knowledge on our part is actual. To explain how one knows that there is a fire before one (F), according to the tracking account one presumably would invoke one’s tracking the truth of F. But this leads deductively almost immediately to the claim that one is not dreaming: Not D. And this is not something one can know, according to the tracking account. So how is one to explain one’s justification for making that claim? Most troubling of all here is the fact that one is now cornered by the tracking account into making combinations of claims of the following form: I am quite sure that p, but I have no knowledge at all as to whether p. And this seems incoherent.
A Cartesian dream argument that has had much play in recent discussions of skepticism is made explicit (by Barry Stroud, The Significance of Philosophical Scepticism, 1984) as follows. One knows that if one knows F then one is not dreaming, in which case if one really knows F then one must know one is not dreaming. However, one does not know one is not dreaming. So one does not know F. Q.E.D. And why does one fail to know one is not dreaming? Because in order to know it one would need to know that one has passed some test, some empirical procedure to determine whether one is dreaming. But any such supposed test – say, pinching oneself – could just be part of a dream, and dreaming one passes the test would not suffice to show one was not dreaming. However, might one not actually be witnessing the fire, and passing the test – and be doing this in wakeful life, not in a dream – and would that not be compatible with one’s knowing of the fire and of one’s wakefulness? Not so, according to the argument, since in order to know of the fire one needs prior knowledge of one’s wakefulness. But in order to know of one’s wakefulness one needs prior knowledge of the results of the test procedure. But this in turn requires prior knowledge that one is awake and not dreaming. And we have a vicious circle. We might well hold that it is possible to know one is not dreaming even in the absence of any positive test result, or at most in conjunction with coordinate (not