Skepticism is a philosophical attitude that, in its most extreme form, questions the possibility of obtaining any sort of knowledge. It was first articulated by Pyrrho, who believed that everything could be doubted except appearances. Sextus Empiricus (2nd century AD), skepticism's most prominent advocate, describes it as an "ability to place in antithesis, in any manner whatever, appearances and judgments, and thus ... to come first of all to a suspension of judgment and then to mental tranquility."
Skepticism so conceived is not merely the use of doubt, but is the use of doubt for a particular end: a calmness of the soul, or ataraxia. Skepticism poses itself as a challenge to dogmatism, whose adherents think they have found the truth.
Sextus noted that the reliability of perception may always be questioned, because it is idiosyncratic to the perceiver. The appearance of individual things changes depending on whether they are in a group: for example, the shavings of a goat's horn are white when taken alone, yet the intact horn is black. A pencil, when viewed lengthwise, looks like a stick; but when examined at the tip, it looks merely like a circle.
Skepticism was revived in the early modern period by Michel de Montaigne and Blaise Pascal. Its most extreme exponent, however, was David Hume. Hume argued that there are only two kinds of reasoning: what he called probable and demonstrative (cf. Hume's fork). Neither of these two forms of reasoning can lead us to a reasonable belief in the continued existence of an external world. Demonstrative reasoning...