In 1760 Earl Ferrers attempted to convince his peers in the House of Lords that he was suffering from "occasional insanity" on the day he killed his servant. A medical witness—or mad-doctor—participated in Ferrers's trial, testifying about the symptoms of lunacy. The physician's opinion marked the early stirrings of forensic psychiatry, a form of expert testimony that would eventually challenge the fundamental tenets of criminal responsibility.This intriguing book by Joel Eigen is the first systematic investigation of the evolution of medical testimony in British insanity trials from its beginnings in 1760 to 1843, when the Insanity Rules were formulated during the trial of Daniel McNaughtan. Based on verbatim testimony of courtroom participants—the ordinary as well as the notorious—the book shows how the conception of madness changed over time, how ambitious defense attorneys began to make use of medical opinion on madness, how the self-proclaimed specialists distanced themselves from lay witnesses, and how defendants offered the court a glimpse of madness "from the inside."Eigen goes beyond existing accounts of famous trials to analyze the elements and development of the insanity defense in hundreds of ordinary prosecutions ranging from burglary and forgery to sheep stealing. Drawing on recent scholarship on eighteenth-century crime and punishment, he sheds new light on the manner in which the legal system adapted to a novel definition of insanity that blurred traditional conceptions of human will, self-control, and criminal responsibility.