Greek medicine under the Roman empire was divided into rival schools. Sextus Empiricus was a doctor of the Empiricist school, whose physicians went by what experience had shown to work and declined to reason about the hidden causes of disease. Yet in the Outlines of Pyrrhonism, his account of the skeptic way, he faults his own school. The Empiricists, he objects, assert that what is unclear cannot be known, and that assertion is itself a claim about the unclear. He suggests that a skeptic would sooner side with the Methodists, a rival school of medicine; scholars still treat the passage as a puzzle. The refusal behind it runs through everything he wrote: he would not affirm that the truth can be found, and he would not affirm that it cannot.
His central claim fits in a sentence. On any question about how things really are, an argument of equal force can be set against every answer; a person who sees this stops judging, and calm follows. The Greek words are isostheneia, the equal force of opposing arguments, epochē, suspension of judgment, and ataraxia, tranquility.
None of this began with him. The tradition took its name from Pyrrho of Elis, who lived in the fourth and early third centuries BCE. Aenesidemus, a skeptic of the first century BCE, revived it and is credited with ten standard arguments, the Ten Modes, for suspending judgment; five more came from later skeptics, and an ancient biographer attributes them to Agrippa. Sextus Empiricus collected and ordered these tools, and his books are the only full account of the tradition to survive.
Of his own life almost nothing is known. Reference works place him anywhere between the second century and the first half of the third, and where he lived and practised is unknown. He mentions notes of his own on medicine and on the soul; they are lost.
His opponents were the dogmatists, above all the Stoics and the Epicureans, who claimed a test of truth and, with it, knowledge of gods, causes, bodies and the good. In Against the Dogmatists he sets out their positions question by question and answers each with an argument of equal weight. Because he reported his opponents so fully, his pages are now a main source for Greek philosophers whose own books are lost.
For more than a thousand years he was little read, though never wholly unknown. In 1562 Henri Estienne, a Calvinist printer in Geneva, issued the first Latin translation of the Outlines. Seven years later Gentian Hervet, a Catholic theologian, added Latin versions of his other books and wrote in his preface that they held the answer to the Calvinists. Montaigne had skeptical sayings, most of them from Sextus, inscribed on the beams of his library. Richard Popkin, an American historian of philosophy, argued that the doubts released by these translations set off a crisis that shaped early modern philosophy.
The standing objection to the skeptic is that nobody can live without beliefs. Sextus Empiricus answered that hunger still leads him to food and thirst to drink, that he keeps the laws and customs of his country, and that he says there are gods and takes part in worship as his ancestors did, all without claiming to know how things are by nature. Whether that leaves him any beliefs at all is still disputed. In 1979 Michael Frede, a German scholar of ancient philosophy, argued that the skeptic keeps ordinary everyday beliefs and gives up only those reached through philosophical argument. Jonathan Barnes and Myles Burnyeat, both British scholars of ancient philosophy, replied that he gives up every belief. Barnes called that thoroughgoing skeptic rustic and Frede's milder one urbane. Neither reading has prevailed.