Protagoras was born about 480 B.C. He was a fellow-townsman of Democritus, and has been represented, though not on good authority, as a disciple of that illustrious thinker. It was rather by a study of Heracleitus that his philosophical opinions, so far as they were borrowed from others, seem to have been most decisively determined. In any case, practice, not theory, was the principal occupation of his life. He gave instruction for payment in the higher branches of a liberal education, and adopted the name of Sophist, which before had simply meant a wise man, as an honourable title for his new calling. Protagoras was a very popular teacher. The news of his arrival in a strange city excited immense enthusiasm, and he was followed from place to place by a band of eager disciples. At Athens he was honoured by the friendship of such men as Pericles and Euripides. It was at the house of the great tragic poet that he read out a work beginning with the ominous declaration, ‘I cannot tell whether the gods exist or not; life is too short for such difficult investigations.’[66] Athenian bigotry took alarm directly. The book containing this frank confession of agnosticism was publicly burned, all purchasers being compelled to give up the copies in their possession. The author himself was either banished or took flight, and perished by shipwreck on the way to Sicily before completing his seventieth year. The scepticism of Protagoras went beyond theology and extended to all science whatever. Such, at least, seems to have been the force of his celebrated declaration that ‘man is the measure of all things, both as regards their existence and their non-existence.’[67] According to Plato, this doctrine followed from the identification of knowledge with sensible perception, which in its turn was based on a modified form of the Heracleitean theory of a perpetual flux. The series of external changes which constitutes Nature, acting on the series of internal changes which constitutes each man’s personality, produces particular sensations, and these alone are the true reality. They vary with every variation in the factors, and therefore are not the same for separate individuals. Each man’s perceptions are true for himself, but for himself alone. Plato easily shows that such a theory of truth is at variance with ordinary opinion, and that if all opinions are true, it must necessarily stand self-condemned. We may also observe that if nothing can be known but sensation, nothing can be known of its conditions. It would, however, be unfair to convict Protagoras of talking nonsense on the unsupported authority of the _Theaetêtus_. Plato himself suggests that a better case might have been made out for the incriminated doctrine could its author have been heard in self-defence. We may conjecture that Protagoras did not distinguish very accurately between existence, knowledge, and applicability to practice. If we assume, what there seems good reason to believe, that