Protagoras is the earliest known sophist of ancient Greece. He was born at Abdera, in Thrace, probably about 480 BCE. It is said that Protagoras was once a poor porter carrying large bundles of wood on his shoulders. He attracted the attention of Democritus who took a liking toward him and instructed him in philosophy (Diog. Laert. ix. 53; x. 8; Gell. v. 3). This well-known story, however, appears to have arisen from the statement of Aristotle that Protagoras invented a sort of porter's knot for the more convenient carrying of burdens. In addition to this, Protagoras was about twenty years older than Democritus. Protagoras was the first who called himself a Sophist, and taught for pay; and he practiced his profession for forty years. Pericles debated moral problems with him, and he was employed to draw up a code of laws for the Athenian colony of Thurii in 445 BCE. Thus he arrived in Athens at least by that year. We are not informed about whether he accompanied the colonists to Thurii, but at the time of the plague (430) we find him again in Athens. Between his first and second visit to Athens he had spent some time in Sicily, where he had acquired fame. He brought with him to Athens many admirers from other Greek cities through which he had passed. His instructions were so highly valued that he sometimes received 100 minae from a pupil; Plato says that Protagoras made more money than Phidias and ten other sculptors.

Protagoras wrote a large number of works, of which the most important were entitled Truth (Alethia) and On the Gods (Peritheon). The first contained the theory refuted by Plato in the Theaetetus. In 411 he was accused of impiety by Pythodorus, one of the Four Hundred. The charges were based on his book On the Gods, which began with the statement, "Respecting the gods, I am unable to know whether they exist or do not exist" (Diog. Laert. ix. 52). The impeachment was followed by his banishment, or, as others affirm, only by the burning of his book. His doctrine was, in fact, a sort of agnosticism based upon the impossibility of attaining any absolute criterion of truth. Plato gives a vivid picture of the teaching of Protagoras in the dialogue that bears his name. Protagoras was especially celebrated for his skill in the rhetorical art. By way of practice in the art he was accustomed to make his pupils discuss theses (communes loci), an exercise which is also recommended by Cicero. He also directed his attention to language, and tried to explain difficult passages in the poets. He is said to have been the first to make the grammatical distinctions of moods in verse and of genders in nouns. Protagoras died about 411 at the age of nearly seventy years, when he was lost at sea on his way to Sicily.

Protagoras was the author of the famous saying, "Man is the measure of all things; of what is, that it is; of what is not, that it is not." This saying puts in a nutshell the whole teaching of Protagoras. Indeed, it contains the essence of the entire thought of the sophists. By "man" he did not mean humankind at large. He meant the individual person. By "measure of all things," he meant the standard of the truth of all things. Each individual person is the standard of what is true to himself. There is no truth except the sensations and impressions of each person. The earlier Greek philosophers made a clear distinction between sense and thought, between perception and reason, and had believed that the truth is to be found, not by the senses, but by reason. The teaching of Protagoras rests on denying this distinction.