Protagoras (Greek: Πρωταγόρας) (ca. 490– 420 BC) was a pre-Socratic Greek philosopher and is numbered as one of the sophists by Plato. In his dialogue Protagoras, Plato credits him with having invented the role of the professional sophist or teacher of virtue.
Plutarch relates a story in which the two spend a whole day discussing an interesting point of legal responsibility, that probably involved a more philosophical question of causation. "In an athletic contest a man had been accidentally hit and killed with a javelin. Was his death to be attributed to the javelin itself, to the man who threw it, or to the authorities responsible for the conduct of the games?
His most famous saying is: "Man is the measure of all things: of things which are, that they are, and of things which are not, that they are not". Like many fragments of the Presocratics, this phrase has been passed down to us without any context, and its meaning is open to interpretation. Plato ascribes relativism to Protagoras and uses his predecessor's teachings as a foil for his own commitment to objective and transcendent realities and values. Plato also ascribes to Protagoras an early form of phenomenalism, in which what is or appears for a single individual is true or real for that individual.
Protagoras was a proponent of agnosticism. In his lost work, On the Gods, he wrote: "Concerning the gods, I have no means of knowing whether they exist or not or of what sort they may be, because of the obscurity of the subject, and the brevity of human life. (80B4 DK)
Very few fragments from Protagoras have survived, though he is known to have written several different works: Antilogiae and Truth. The latter is cited by Plato, and was known alternatively as 'The Throws' (a wrestling term referring to the attempt to floor an opponent). It began with the "man the measure" pronouncement. The Protagoras crater on the Moon was named in his honor.
Protagoras knew that the less appealing argument could hide the best answer, which is why he stated that it was constantly necessary to strengthen the weakest argument. Having been born before Socrates himself, this progressive viewpoint in the development of consensual truth could conceivably have contributed to the progressive styles of many of the other great minds which followed him. His most recent defender is Joseph Margolis, especially in the latter's The Truth About Relativism (Blackwell's, 1991).