From Sextus Empiricus, Outlines of Pyrrhonism:
Protagoras holds that "Man is the measure of all things--of things that are, that they are; of things that are not, that they are not." . . . So he says, in effect, that man is the standard of judgment of all things, of those that are that they are, and of those that are not that they are not. And for this reason he posits only what appears to the individual, thus introducing relativity. . . . What he states then is this--that matter is in flux, . . . and the senses are transformed and altered according to the times of life and to all other conditions of the bodies. He says also that the reasons (logoi) of all the appearances are present in matter, so that matter, so far as depends on itself, is capable of being all those things which appear to all. And men, he says, apprehend different things at different times owing to their differing dispositions; for he who is in a natural state apprehends those things present in matter which are able to appear to those in a natural state, and those who are in a non-natural state the things which can appear to those in a non- natural state. Moreover, precisely the same account applies to variations due to age, and to the sleeping or waking state, and to each several kind of condition. Thus, according to him, man becomes the criterion of real existences; for all things that appear to men also exist, and things that appear to no man have no existence either.
From Diogenes Laertes, Lives of the Eminent Philosophers:
Protagoras was the first to say that on every issue there are two arguments opposed to each other; these he made use of in arguing by the method of questioning, a practice he originated. One of his works, moreover, begins in this way: "Of all things the measure is man, of things that are, that they are, and of things that are not, that they are not." He said too that the soul was nothing apart from its sensations . . . and that all things are true. Elsewhere he begins a work in this fashion: "Concerning the gods, I cannot know either that they exist or that they do not exist; for there is much to prevent one's knowing: the obscurity of the subject and the shortness of man's life." Because he began his book in this way he was expelled by the Athenians, and they also burned his books in the marketplace, having first collected them by public messenger from all who owned copies.
Protagoras was the first man to exact a fee of a hundred minas. He was also the first to distinguish the tenses of the verb, to expound the importance of the right moment, to conduct debates, and to introduce disputants to the tricks of argument. Moreover,, in neglecting meanings and concerning himself with mere words, he fathered the present shallow tribe of quibblers--to the point that Timon can speak of him as "
Protagoras, avid in combat, master of wrangling." He it was who first introduced the Socratic type of argument. And he was the first to adopt in discussion the argument of Antisthenes which attempts to prove that contradiction is impossible. . . And he was the first to introduce the methods of attacking any thesis.
From Plutarch, Pericles:
When a competitor in the pentathlon, without meaning to, struck and killed Epitimus of Pharsalus with a javelin, [Pericles, the leading politician in Athens] spent a whole day with
Protagoras trying to decide whether, according to the most correct judgment, one ought to regard as the cause of the mishap the javelin, or the man who through it, or rather the directors of the games.
From Aristotle, Rhetoric:
[Regarding arguments from probabilities.] It is of this line of argument that Corax's Art of Rhetoric is composed. If the accused is not open to the charge--for instance, if a weakling be tried for violent assault--the defense is that he was not likely to do such a thing. But if he is open to the charge--i.e., if he is a strong man--the defense is still that he was not likely to do such a thing, since he could be sure that people would think that he was likely to do it. And so with any other charge: the accused must be either open or not open to it: there is in either case an appearance of probable innocence, but whereas in the latter case the probability is genuine, in the former it cannot only be asserted in a limited sense. This sort of argument illustrates what is meant by "making the worser argument appear the better." Hence people were right in objecting to the training Protagoras undertook to give them. It was a fraud; the probability it handled was not genuine but spurious.
From Plato, Protagoras.
For I say that the truth is as I have written: each one of us is the measure of what is and what is not, and one man differs vastly from another--the difference being just this: that different things are and appear to different men. And I am far from denying the existence of wisdom and of wise men; it is rather that I call "wise" precisely that man who, by working a change in us, makes what is good appear and be to any one of us to whom what is evil appears and is. But, I think, when a man's soul is in an evil state, and so has thoughts which suit that state, then a good state of soul makes him think other thoughts, natural to that state, which some men ineptly call "true," whereas I call them better than the former kind but in no way truer. . . . And the wise and good orators make what is beneficial rather than what is harmful appear just to the cities. For whatever sort of thing appears just and honorable to each city is so for that city for as long as it so deems. But the wise man makes what is beneficial rather than what is harmful for them in any particular case appear and be just.