Protagoras: truth is whatever each person perceives it to be

Protagoras claimed 'man is the measure of all things' - of what is, that it is, and of what is not, that it is not. If the same wind feels cold to you and warm to me, he argued, there's no further fact about what it 'really' is beyond each person's perception. Plato spent much of the Theaetetus trying, and largely failing, to refute this early statement of relativism.