Cratylus is the name of a dialogue by Plato. Most modern scholars agree that it was written mostly during Plato's so-called middle period. In the dialogue, Socrates is asked by two men, Cratylus and Hermogenes, to tell them whether names are "conventional" or "natural", that is, whether language is a system of arbitrary signs or whether words have an intrinsic relation to the things they signify.

The individual Cratylus was the first intellectual influence on Plato (Sedley). Aristotle states that Cratylus influenced Plato by introducing to him the teachings of Heraclitus, according to MW. Riley.

The subject of Cratylus is the correctness of names, in other words, it is a critique on the subject of naming (Baxter).

The final theory of relations between name and object named is posited by Cratylus, a disciple of Heraclitus, who believes that names arrive from divine origins, making them necessarily correct. Socrates rebukes this theory by reminding Cratylus of the imperfection of certain names in capturing the objects they seek to signify. From this point, Socrates ultimately rejects the study of language, believing it to be philosophically inferior to a study of things themselves.