Cratylus c. 460 BC – c. 380 BC

Cratylus was a Greek philosopher of late fifth and early fourth century BC Athens, an Heraclitean who carried the doctrine of universal flux to its extreme conclusion. According to Aristotle, whereas Heraclitus had said that one cannot step twice into the same river, Cratylus held that one cannot step into it even once, and in his old age he is said to have abandoned speech as a stable medium of meaning, communicating only by silently moving his finger. He gave his name to Plato's dialogue on the philosophy of language and is reported to have been Plato's first teacher.

Cratylus of Athens was born around the middle of the fifth century BC. Almost everything we know of him comes from Plato and Aristotle, and the dramatic dating of Plato's dialogues places him in Athens during the last decades of the century. He is reported to have been a follower of Heraclitus and, according to Aristotle's Metaphysics A 6, the teacher of the young Plato in his pre-Socratic Athenian period before Plato's encounter with Socrates.

He is the title character of Plato's Cratylus, in which he defends the thesis that names belong to their objects by nature against Hermogenes' conventionalism, with Socrates as their critical interlocutor; the dialogue is the earliest extended treatment of the philosophy of language. No writings by Cratylus himself survive.

Aristotle records that Cratylus pushed Heraclitus's flux doctrine to its limit: where Heraclitus had said one cannot step into the same river twice, Cratylus insisted one cannot step into it even once, and finally refused to speak at all, merely wagging his finger, on the grounds that no name could fit a reality already different by the time it was uttered. Aristotle treats Cratylus's radicalisation as the provocation that drove Plato to seek a stable object of knowledge in the unchanging Forms. He is presumed to have died early in the fourth century BC.