Note (return) — Chapter X
Reading edition
This doctrine had its foundation in the physical philosophy of Heraclitus. He had taught that all things are in a perpetual flux and change. "Motion gives the appearance of existence and of generation." "Nothing is, but is always a becoming" 499 Material substances are perpetually losing their identity, and there is no permanent essence or being to be found. Hence Protagoras inferred that truth must vary with the ever-varying sensations of the individual. "Man (the individual) is the measure of all things." Knowledge is a purely relative thing, and every man's opinion is truth for him. 500 The law of right, as exemplified in the dominion of a party, is the law of the strongest; fluctuating with the accidents of power, and never attaining a permanent being. "Whatever a city enacts as appearing just to itself, this also is just to the city that enacts it, so long as it continues in force." 501 "The just, then, is nothing else but that which is expedient for the strongest." 502