II. Protagoras was a pupil of Democritus. And he was surnamed Wisdom, as Phavorinus informs us in his Universal History.
III. He was the first person who asserted that in every question there were two sides to the argument exactly opposite to one another. And he used to employ them in his arguments, being the first person who did so. But he began something in this manner: “Man is the measure of all things: of those things which exist as he is; and of those things which do not exist as he is not.” And he used to say that nothing else was soul except the senses, as Plato says, in the Theætetus; and that everything was true. And another of his treatises he begins in this way: “Concerning the Gods, I am not able to know to a certainty whether they exist or whether they do not. For there are many things which prevent one from knowing, especially the obscurity of the subject, and the shortness of the life of man.” And on account of this beginning of his treatise, he was banished by the Athenians. And they burnt his books in the market-place, calling them in by the public crier, and compelling all who possessed them to surrender them.
He was the first person who demanded payment of his pupils; fixing his charge at a hundred minæ. He was also the first person who gave a precise definition of the parts of time; and who explained the value of opportunity, and who instituted contests of argument, and who armed the disputants with the weapon of sophism. He it was too who first left facts out of consideration, and fastened his arguments on words; and who was the parent of the present superficial and futile kinds of discussion. On which account Timon says of him:—
Protagoras, that slippery arguer,
In disputatious contests fully skilled.
He too, it was, who first invented that sort of argument which is called the Socratic, and who first employed the reasonings of Antisthenes, which attempt to establish the point that they cannot be contradicted; as Plato tells us in his Euthydemus. He was also the first person who practised regular discussions on set subjects, as Artemidorus, the dialectician, tells us in his treatise against Chrysippus. He was also the original inventor of the porter’s pad for men to carry their burdens on, as we are assured by Aristotle, in his book on Education; for he himself was a porter, as Epicurus says somewhere or other. And it was in this way that he became highly thought of by Democritus, who saw him as he was tying up some sticks.
He was also the first person who divided discourse into four parts; entreaty, interrogation, answer, and injunction: though some writers make the parts seven; narration, interrogation, answer, injunction, promise, entreaty, and invocation; and these he called the foundations of discourse: but Alcidamas says that there are four divisions of discourse; affirmation, denial, interrogation, and invocation.
V. The first of his works that he ever read in public was the treatise on the Gods, the beginning of which we have quoted above, and he read this at Athens in the house of Euripides, or, as some say, in that of Megaclides; others say that he read it in the Lyceum; his pupil, Archagoras, the son of Theodotus, giving him the aid of his voice. His accuser was Pythodorus, the son of Polyzelus, one of the four hundred; but Aristotle calls him Evathlus.
VI. The writings of his which are still extant are these: a treatise on the Art of Contention; one on Wrestling; one on Mathematics; one on a Republic; one on Ambition; one on Virtues; one on the Original Condition of Man; one on those in the Shades Below; one on the Things which are not done properly by Men; one volume of Precepts; one essay entitled Justice in Pleading for Hire; two books of Contradictions.
These are his books.