Protagoras (c. 481 BC - c. 420 BC) was an ancient Greek philosopher. He was a pre-Socratic philosopher and is numbered as one of the sophists by Plato, who in his dialogue of the same name credits him with having invented the role of the professional sophist or teacher of "virtue".
- Man is the measure of all things: of things which are, that they are, and of things which are not, that they are not.
- As quoted in Theaetetus by Plato section 152a
- There are two sides to every question.
- As quoted in Lives of Eminent Philosophers, by Diogenes Laertius, Book IX, Sec. 51
- As to the gods, I have no means of knowing either that they exist or do not exist, or what they are like.
- Let us hold our discussion together in our own persons, making trial of the truth and of ourselves.
- No intelligent man believes that anybody ever willingly errs or willingly does base and evil deeds; they are well aware that all who do base and evil things do them unwillingly.
- The art of measurement, by showing us the truth would have brought our soul into the repose of abiding by the truth, and so would have saved our life.
- The Athenians are right to accept advice from anyone, since it is incumbent on everyone to share in that sort of excellence, or else there can be no city at all.
- When it comes to consideration of how to do well in running the city, which must proceed entirely through justice and soundness of mind.
- You, Socrates, began by saying that virtue can't be taught, and now you are insisting on the opposite, trying to show that all things are knowledge, justice, soundness of mind, even courage, from which it would follow that virtue most certainly can be taught.