This book shows how, in his enormously influential 'Essay concerning Human Understanding' (1689), John Locke embraces the new rhetoric of seventeenth-century natrual philosophy, adopting the strategies of his scientific contemporaries to create a highly original natural history of the human mind. With the help of Locke's notebooks, letters and journals, Peter Walmsley reconstructs Locke's scientific career, including his early work with the chemist Robert Boyle and the physician Thomas Sydenham. He also shows how the 'Essay' embodies in its form and language many of the preoccupations of the science of its day, from the emerging discourses of experimentation and empirical taxonomy to developments in embryology and the history of trades. The result is a new reading of Locke, one that shows both his brilliance as a writer and his originality in turning to science to effect a radical reinvention of the study of the mind.