In this article I contrast 17th and 18th explanations of hysteria including those of Sydenham and Willis with those given by Plato and pre-modern medicine. I show that beginning in the second decade of the 17th century the locus of the disorder was transferred to the nervous system and it was no longer connected with the womb as in Hippocrates and Galen; hysteria became identified with hypochondria, and was a disease contracted by men as well as women. I discuss the purely mechanical explanation of hysteria given by Robert Boyle who attributed its cause to corporeal ideas as well as overly sensitive disposition of the nervous system. I relate this the mechanical theory of the nervous system prominent in Descartes' writings on physiology. The paper closes with a discussion of the contrast between early modern explanations of hysteria and the nature of man with those of Freud in the early 20th century.