The great schools of ancient philosophy — Platonists, Aristotelians, Stoics, Epicureans — put forward certain knowledge (dogma) about how the world is. Another tradition opposed precisely this claim to certainty: skepticism. The physician and philosopher Sextus Empiricus (c. 160–210 CE) is the most comprehensive surviving compiler of this tradition — especially of the Pyrrhonism descending from Pyrrho. His "Outlines of Pyrrhonism" and other works carry to us the skeptical arguments of otherwise lost centuries.

The skeptic's method is not to refute but to balance. Against every claim he sets an equally convincing counter-claim. The senses deceive — a stick in water looks bent; but reason deceives too — philosophers reach exactly opposite conclusions on the same question. When arguments balance one another this way, one reasonable stance remains: *epoché*, the suspension of judgement. Even "I do not know" is too definite a claim; the skeptic says only "I cannot determine."

This is not despair but a path to tranquillity. For Sextus the dogmatic person is forever restless with the anxiety of "I must find the truth"; whereas the one who suspends judgement is freed from wearing himself out on unsolvable questions and unexpectedly attains an inner peace (*ataraxia*). Skepticism is thus an "art of living" like Stoicism and Epicureanism — but it reaches tranquillity not through certain knowledge but by giving up on knowledge.

When Sextus's texts were rediscovered in the Renaissance they had great impact; they lit the debate running from Montaigne's "What do I know?" to Descartes's search for firm ground by doubting everything, and on to modern epistemology. Alongside Stoicism and Epicureanism, this skeptical tradition is the third great Hellenistic philosophy of life.