Protagoras of Abdera (c. 490 - c.420 BCE) is most famous for his claim that "Of all things the measure is Man, of the things that are, that they are, and of the things that are not, that they are not" (DK 80B1) usually rendered simply as "Man is the Measure of All Things". In maintaining this stance he pre-figures the existential relativism of writers like Luigi Pirandello ("It is so if you think so") by some two thousand plus years. It is curious to consider, then, how a man who claimed that what was true to each of his listeners was, in fact, true (including the idea that no one could know the gods' will objectively) could come to be the most highly paid Sophist in ancient Greece.
The Sophists were educated men who, for a price, would teach the youth the art of rhetoric or politics and the trappings of culture (it is from Sophist that we have the word sophisticated). Although looked down upon by Plato (from whom we get many accounts of the teachings of various Sophists, most of them unfavorable) the Sophists provided a valuable service to the aristocracy of Athens, especially in that they claimed to be able to provide young men with the sort of education that would give them advantage in Athenian politics and commerce. In his Apology, Plato has Socrates scorn this practice saying how public education in Athens could produce the same results as the Sophists do far more easily and cheaply.
Protagoras in Plato
In the dialogue of the Theatetus, Plato argues against Protagoras' view thusly, "If what each man believes to be true through sensation is true for him - and no man can judge of another's experience better than the man himself, and no man is in a better position to consider whether another's opinion is true or false than the man himself, but...each man is to have his own opinions for himself alone, and all of them are to be right and true - then how, my friend, was Protagoras so wise that he should consider himself worthy to teach others and for huge fees? And how are we so ignorant that we should go to school to him, if each of us is the measure of his own wisdom?" (161B). Plato argues, here and in the dialogue Protagoras, that it is impossible for everyone to know the Truth of a matter if everyone's opinions of that Truth differ, often dramatically. What Protagoras seems to be saying, however, is that the apprehension of Truth is relative to the individual perception and what one recognizes as 'true' will be True to that individual despite any evidence to the contrary.
Protagoras' Claim in Depth
Of all things the measure is man, of the things that are, that [or "how"] they are, and of things that are not, that [or "how"] they are not." The precise meaning of this statement, like that of any short extract taken out of context, is far from obvious, although the long discussion of it in Plato's Theaetetus gives us some sense of how ancient Greek audiences interpreted it. The test case normally used is temperature. If Ms. X says "it is hot," then the statement (unless she is lying) is true for her. Another person, Ms. Y, may simultaneously claim "it is cold." This statement could also be true for her. If Ms. X normally lives in Alaska and Ms. Y in Florida, the same temperature (e. g. 25 Celsius) may seem hot to one and cool to the other. The measure of hotness or coldness is fairly obviously the individual person. One cannot legitimately tell Ms. X she does not feel hot -- she is the only person who can accurately report her own perceptions or sensations. In this case, it is indeed impossible to contradict as Protagoras is held to have said (DK80a19. From Encyclopedia of Philosophy).
Whether a room is objectively cold, then, can never really be known since the experience of being cold is entirely subjective. This same claim was extended to knowledge of the gods, "Concerning the gods, I have no means of knowing whether they exist or not or of what sort they may be. Many things prevent knowledge including the obscurity of the subject and the brevity of human life" (DK80b4). For one to claim certain knowledge of the will of the gods, then, was simply to claim that one held a subjective view of what the gods were like and what they wanted of human beings; in other words, one believed in one's individual subjective understanding of the concept of the gods but could never claim to actually objectively understand them - though it would not have mattered if one did make such a claim since, if one believed it was true, it was.
Death & Vindication
Not surprisingly, considering the importance the Athenians placed on the concept of Eusebia (piety, loosely translated) Protagoras was charged with impiety and drowned in the sea while fleeing for safety to the colony at Sicily. His practices and teachings were used as a model for the `Socrates' character in Aristophanes' play The Clouds which, at Socrates' trial for impiety in 399 BCE, was cited by Socrates as a strong witness against him, even though that character had nothing to do with the actual man and philosopher Socrates. To the Athenians, however, even though they had finally condemned the teachings and writings of Protagoras, what seemed true to them about Socrates was true and, in the conviction and death sentence of Socrates, they proved Protagoras' most famous claim to be right.
Contributor's Note: A version of this article was previously published on the site Suite 101. C. 2009 Joshua J. Mark