Charles Darwin is known as a biologist, geologist, and naturalist, but he was also a philosopher. This book uncovers Darwin's forgotten philosophical theory of emotion, which combines earlier associationist theories with his theory of evolution. The British associationists of the eighteenth and nineteenth centuries argued that the mind operates primarily through the association of ideas, and that emotions are strings of thoughts, feelings, and outward expressions, connected by habit and association. Charles Darwin's early notebooks on emotion reveal a keen interest in associationist philosophy. This book shows that one of his final works, The Expression of the Emotions in Man and Animals (1872), is a work of associationist philosophy, and analyzes Darwin's revolutionary idea: that if the associations that produce emotions can be inherited, then the theory of evolution can explain how emotions first occurred in simpler organisms and then developed and were compounded into the complex experiences humans have today.