This book investigates the specific brain damage that causes aphasia, a condition that deprives individuals of the ability to articulate speech. Groundbreaking when it was first published in 1861, the book argues that damage to the third left frontal convolution is the culprit behind the condition. This discovery was a huge step forward in localizing brain function, and the book remains an essential text in the field of neurology, having laid the foundation for modern aphasia research and our understanding of the brain's role in language production.