Plato's Cratylus is a brilliant but enigmatic dialogue. It bears on a topic, the relation of language to knowledge, which has never ceased to be of central philosophical importance, but tackles it in ways which at times look alien to us. In this reappraisal of the dialogue, Professor Sedley argues that the etymologies which take up well over half of it are not an embarrassing lapse or semi-private joke on Plato's part. On the contrary, if taken seriously as they should be, they are the key to understanding both the dialogue itself and Plato's linguistic philosophy more broadly. The book's main argument is so formulated as to be intelligible to readers with no knowledge of Greek, and will have a significant impact both on the study of Plato and on the history of linguistic thought.