Naturalization of the Soul charts the development of the concept of soul in western thought, from Plato to the present. The authors place particular emphasis on the eighteenth century which witnessed an enormous intellectual transformation in the way theorists perceived self and personal identity and paved the way for contemporary philosophical and psychological debates. The present work fills a very important gap in intellectual history by being the first book to trace the evolution of theories of self and personal identity throughout the eighteenth century. Martin and Barresi initially examine the history of Western thought on self and identity from Plato to Locke. The central chapters analyse the evolution of these concepts from Locke in the seventeenth century to Hazlitt, at the beginning of the nineteenth, exploring the two critical transitions which marked this period: the move from a religious conception of soul to a philosophical conception of self, and then to a scientific conception of mind. This rigorous and erudite study becomes even more compelling by proving that contemporary theories on self which have, since the 1960s, been debated by philosophers and psychologists were widely discussed by theorists in the eighteenth century. These include issues such as the implications of fission examples for personal identity theory and the thesis that personal identity is not what matters primarily in survival. Martin and Barresi also break new ground by recognising the status of William Hazlitt as one of the most significant theorists of the English Enlightenment, on a par with Locke and Hume, for his direct relevance to contemporary thinking. Naturalization of the Soul makes a major contribution to our understanding of one of the most radical transformations in the history of intellectual thinking and is essential reading for all who are interested in the issues which lie at the core of the western philosophical tradition.