  In 155c–157c the flux theory is used to develop a Protagorean/Heracleitean account of perception, to replace accounts based on the object/property ontology of common sense. Socrates notes the subversive implications of the theory of flux for the meaningfulness and truth-aptness of most of our language as it stands. (He returns to this point at 183a–b.) The ontology of the flux theory distinguishes kinds of “process” (kinêsis), i.e., of flux, in two ways: as fast or slow, and as active or passive. Hence there are four such processes. On these the flux theory’s account of perception rests.