The history of Greek thought might be less problematical were it possible to assume that the figure we know as Protagoras was in reality two persons of that name, who lived separate existences in the fifth century b.c., but managed somehow to become hopelessly confused in the mind of later antiquity. One of them was the pattern and prototype of that disreputable breed the sophist, the other a serious political thinker of considerable insight and originality. It is the former, not the latter, who, on this assumption, should be credited with the relativist doctrine that man is the measure of all things and that things are always just what each person imagines them to be, as well as with the destructive corollaries of this doctrine: that no act is wrong if the door does not think it so and that false statement and false opinion are impossible. He is also the man who taught his students how to make the worse cause appear the better, and how to argue both for and against the same proposition, alleging in justification of the latter practice that the contrary of every statement is just as true as the statement itself. His professed agnosticism led to the burning of his books at Athens and to his own expulsion from the city.