Knowledge entails reliability; or so claims the most widespread theory of knowledge - evidentialism. You know what time is it only if you suppose that your source of information - be it a clock in your computer or the person sitting next to you - can be trusted. Well, skepticism is precisely the attitude of doubting the reliability of one's sources. Thus, you think you know what time is it; but, can you prove that the clock is well-functioning? The skeptical stance is probably as old as Homo sapiens and it comes in different forms.
What Is Skeptical Doubting?
Skeptical doubting is peculiar, and should be distinguished from a different form of doubting. If Joe tells you that today is the 19th and you firmly believe that it is the 20th, you will doubt Joe's words because you strongly believe you have the truth otherwise. This is not skeptical doubting. Suppose instead a different scenario. Joe tells you he believes that President Nixon was a KGB spy; you do not have any way to disprove him, but you are not confident that his source is reliable. In this case, you do not believe you have the truth otherwise, but you just do not want to commit neither to the truth nor to the falsity of Joe's statement. You are now raising a skeptical doubt towards Joe's claim: its mark is the suspension of judgment.
Two Doubtin' Targets
Skeptical doubts can be directed towards two quite different targets. You may doubt that someone knows that such and such is the case. This goes under the name of epistemic skepticism, from the Greek episteme, "knowledge". Or you may doubt that something exists, as when you doubt whether God exists, or that there is a ghost in the house. This is ontological skepticism. The most famous formulations of skepticism are of the epistemic variety.
Ancient and Modern Skepticism
Philosophers who endorsed skepticism or that took its challenges in great consideration can be found in every age and location. Among the chief figures, Arcesilaus, Carneades, Aenesidemus, Sextus Empiricus, Michel de Montaigne, René Descartes, David Hume, George E. Moore, Ludwig Wittgenstein, Hilary Putnam.
The most ancient skeptic of western philosophy on record is Pyrrho of Elis (c. 360-c. 270 B.C.). He seems to have written no text and to have gone through life with a singular disposition of the soul. Pyrrho simply held common opinion in no consideration, hence attributing no relevance to the most basic and instinctive habits. No cleaning of the house, no respect of social norms, no fear. A proverbial episode has it that finding himself on a ship in the middle of a terrible sea-storm, he was the only one to maintain the most serene calm. Probably influenced also by the Buddhist tradition of his time, which he had encountered in one of his travels, Pyrrho viewed the suspension of judgment as a mean to achieve that freedom of disturbance that sole can lead to happiness. His goal was to keep each human's life in a state of perpetual inquiry. At different moments, Pyrrho's teachings exercised a deep influence on major philosophers, including Aenesidemus (1st century B.C.), Sextus Empiricus (2nd century A.D.), Michel de Montaigne (1533-1592), Renè Descartes.
In 272 B.C., about seventy years after Plato's death, Arcesilaus of Pitane took the direction of the Academy, the institution Plato had opened more than a century before. The Stoic and Epicurean schools were flourishing at the time, and to Arcesilaus their teachings rested on unwarranted grounds. He attacked them by raising skeptical doubts, establishing what now goes under the label of academic skepticism. Now, this form of skepticism is so radical that it cannot even be formulated. The gist of the position can be thus put. For any claim A, including the present one, neither affirm A nor affirm its contrary; and not even affirm or deny that you are neither affirming nor not affirming it. Yet clearly this characterization is contradictory. After all - Arcesilaus would say - any attempt to nail skepticism down would amount to committing to the truth of at least that formulation!
Skepticism was revived in Early Modern times, under the auspices of authors such as Erasmus of Rotterdam and Michel de Montaigne. It was to portrait the position of the latter, that Descartes raised three epistemic doubts at the outset of his Meditations on First Philosophy. Descartes saw a virtue in the skeptic's suspension of judgment, namely that prudence and carefulness which characterizes scientific method. On the other hand, he argued that even the most radical skeptical concerns could be acquiesced through a proper examination of matters at hand. The first doubt regards knowledge that comes from the senses; the second invests the very existence of material objects; the third is the most powerful, encompassing the previous two ones. What if you were but the creation of an omnipotent evil deceiver, who 'engineered' you so to systematically get all of our opinions wrong? This is the famous evil deceiver hypothesis, also known as the hyperbolic doubt.
Brain In a Vat?
A contemporary version of the hyperbolic doubt has been proposed by Hilary Putnam, in the first chapter of Reason, Truth, History (1981), titled "Brains in a Vat." Putnam's doubt is simple: how do you know that you are not a brain in a vat, which is 'fed' exactly the experiences that you have had thus far? How could you prove that your life is not 'virtual' in its entirety? This is precisely the idea behind the movie The Matrix (1999), which intrigued millions of viewers around the globe, and still does. Skepticism is quite appealing at times.
The ghost of epistemic skepticism dominates the contemporary debate on epistemology as well. George E. Moore and, more recently, Crispin Wright argued that skeptical hyperbolic doubts can be put aside by 'simply' realizing that their alleged falsity is evident. That now I have two hands is evident to me and I need to provide the skeptic no proof of such fact. On the other hand, contextualists such as David K. Lewis, Keith De Rose, and Stewart Cohen claimed that skeptical doubts can be discarded by pointing out that their doubts hinge on contextual features that were implicitly ruled out by the speaker. Analogous is the reply considering radical skeptical doubts among non-relevant alternatives to the actual world.
Skeptics can easily be regarded as the "bad guys" of the philosophical arena. On the other hand, if philosophy consists in raising questions that lead us to ponder and revise the very foundations of our own thinking, then skepticism is probably the chief school of philosophy. And, by all means, the efforts made in fighting the skeptics brought about some of the best philosophical ideas.