  Thus the Unitarian Cornford argues that Plato is not rejecting the Heracleitean flux theory of perception. He is rejecting only D1’s claim that knowledge is that sort of perception. It remains possible that perception is just as Heracleitus describes it. Likewise, Cornford suggests, the Protagorean doctrine that “man is the measure of all things” is true provided it is taken to mean only “all things that we perceive.”