Cratylus (Κρατυλος) is the name of a dialogue by Plato, dating to ca. 360 BC. In the dialogue, Socrates is asked by two men, Cratylus and Hermogenes, to advise them whether names are "conventional" or "natural", i.e. whether language, and by language, Plato naturally means Ancient Greek, is a system of arbitrary signs, or whether words have some intrinsic relation to the things they signify. It is the earliest text of Classical Greece to deal with matters of etymology and linguistics.
Plato's dialogue is named after the late fifth century (B.C.E.) philosopher Cratylus (of Athens). Little remains of Cratylus or his mentor Heraclitus (of Ephesus, Asia Minor). It is said that Heraclitus proclaimed that one couldn't step in the same river twice, and it is also said that Cratylus went a step further to proclaim that one couldn't even do it once. Cratylus, as a result of his profound realisation, renounced his power of speech and waggled his finger when he felt like it.