Cratylus, an Athenian, was contemporary with Socrates but was probably considerably younger. He was, according to Aristotle, a follower of the doctrines of Heraclitus, and Plato, in his youth, was closely associated with him. Aristotle implies that this was before he came under Socrates' influence, although later sources put the influence of Cratylus upon Plato after the death of Socrates.

Cratylus took as his starting point the doctrine of the flux of phenomena (here assumed to have been a genuine doctrine of Heraclitus, despite G. S. Kirk's objections), and he capped Heraclitus's saying that one cannot step twice into the same river by adding "nor once either." His reason clearly was his contention that the river is changing even as you step into it. He ended by coming to the view that one ought not to say anything, but only move the finger, since no true statement can...