The idea that some aspects of language are ‘natural’, while others are arbitrary, artificial or derived, runs all through modern linguistics, from Chomsky’s GB theory and Minimalist program and his concept of E- and I-language, to Greenberg’s search for linguistic universals, Pinker’s views on regular and irregular morphology and the brain, and the markedness-based constraints of Optimality Theory. This book traces the heritage of this linguistic naturalism back to its locus classicus, Plato’s dialogue Cratylus. The first half of the book is a detailed examination of the linguistic arguments in the Cratylus. The second half follows three of the dialogue’s naturalistic themes through subsequent linguistic history — natural grammar and conventional words, from Aristotle to Pinker; natural dialect and artificial language, from Varro to Chomsky; and invisible hierarchies, from Jakobson to Optimality Theory — in search of a way forward beyond these seductive yet spurious and limiting dichotomies.