Tradition recognizes two schools of ancient skepticism: the Academics and the Pyrrhonists. The ancient Greek term "skeptic" was used by the Pyrrhonists to describe themselves. They denied that it described the Academics, but this point could be and was disputed, and later in antiquity the word may have been used as a common designation for both schools. Our use of the term in this way goes back to the seventeenth century.

The term itself is derived from a verb in common use meaning "to inquire" or "to investigate"—hence the skeptic as inquirer. This is a surprise. We take skepticism, roughly speaking, to imply a denial of the possibility of knowledge. Yet Sextus Empiricus, the second-century CE Pyrrhonist—and the only member of the school whose works have survived intact and in bulk—is quite firm on this point. In the opening chapter of his Outlines...