Protagoras

Protagoras of Abdera (c. 485 - c. 411 BC) was a Greek sophist, one of the best-known thinkers of the fifth century BC. Plato presents him as the inventor of the professional sophist, someone who charges fees for teaching "virtue", understood as the knowledge and skill needed to succeed in public life rather than as moral goodness. He is studied alongside Gorgias as one of the few sophists whom Plato and Aristotle treated as philosophers. His ideas about knowledge, language and political life remain debated more than twenty-five centuries later.

His life was largely itinerant. Most ancient authors place his origin in Abdera, though the playwright Eupolis considered him a native of Teos. A widely repeated story, recorded by Aulus Gellius, says that Democritus noticed him tying bundles of firewood with geometric precision and took him on as a pupil. The tale is probably legendary. Protagoras travelled through Greek cities, Sicily and Asia Minor, teaching rhetoric, good conduct and the management of one's household and public affairs. He charged high fees, which brought him fame and criticism alike. He visited Athens several times and was close to Pericles, who commissioned him to draft the constitution of the colony of Thurii, probably around 444-443 BC. That text is often credited with establishing public and compulsory education for the first time.

His standing in Athens declined after Pericles died. Diogenes Laertius reports that a book of his, On the Gods, in which he admitted he could not know whether the gods existed, was read in the house of Euripides and led to a charge of impiety. Philostratus notes that it is unclear whether a trial took place, and tradition speaks of either exile or death. In any case, his books were ordered to be burned. He drowned during a voyage towards Sicily, and sources disagree about his age at death, giving either seventy or ninety years.

None of his complete works survives. We know him through fragments and through the testimony of Plato, who makes him a character in several dialogues, and of Aristotle, who criticises him. He is credited with classifying kinds of speech acts, such as questions, commands and requests, with studying grammatical gender, and with an interest in the correct use of words. His most famous thesis, "Man is the measure of all things", appears in his work Truth and has been read in several ways. Plato understood it as a denial of objective truth. The surviving fragments suggest that Protagoras held each judgement to depend on the experience of the person making it. From this he drew a practical conclusion: communities and democracy require listening to other people's opinions and debating them openly, even though not all opinions carry equal weight, and the teacher should help students tell which ones are useful. He also held that on any question two opposed arguments can be made, an idea associated with antilogy, and for this he was criticised for making the weaker argument appear stronger.

His influence is considerable. His relativism and his attention to language place him among the forerunners of rhetoric and of later reflection on words. His name also became the emblem of the sophist, a word that gradually took on a disparaging sense.