Presents the first complete account of the thought of David Hartley, one of the most original minds of the eighteenth century. In this first complete account of Hartley's thought, Richard Allen explains Hartley's theories of physiology, perception and action, language and cognition, emotional development and transformation, and spiritual transcendence. By drawing a biographical portrait of its subject, the book explores the relationship of mind and body in Hartley's system, and surveys Hartley's influence upon later scientists and social reformers, particularly Joseph Priestley.