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.