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THREE: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 the88 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 Theaet��tus. 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 in the great controversy of Nature versus Law, Protagoras sided with the latter, his position will at once become clear. When the champions of Nature credited her with a stability and an authority greater than could be claimed for merely human arrangements, it was a judicious step to carry the war into their territory, and ask, on what foundation then does Nature herself stand? Is not she, too, perpetually changing, and do we not become acquainted with her entirely through our own feelings? Ought not those feelings to be taken as the ultimate standard in all questions of right and wrong? Individual opinion is a fact which must be reckoned with, but which can be changed by persuasion, not by appeals to something that we none of us know anything about. Man is the measure of all things, not the will of gods whose very existence is uncertain, nor yet a purely hypothetical state of Nature. Human interests must take precedence of every other consideration. Hector meant nothing else when he preferred the obvious dictates of patriotism to inferences drawn from the flight of birds.