What is madness?"" asks Porter, who attempts to provide a partial answer through the memorabilia of demented people throughout history, George III, Nietzsche, and Nijinksy among them. Co-editor of The Anatomy of Madness (1985), Porter contends that insanity is a ""subculture that differs from age to age,"" and that the mad are sort of a mirror image of society, ""challenging the accepted views of 'truth and falsehood, reality and delusion.'"" In medieval times, no one considered Margery Kempe a hysteric (a modern diagnosis, says Porter), even though she was convinced God spoke to her regularly. The question then was whether she should be burned at the stake as a witch or revered as a holy woman. The ""demented"" were not separated from society until the 17th century, when madhouses began to dot the landscape. The 18th-century poet William Cowper, who believed God had doomed him to eternal perdition, was confined several times to ""cure"" the ghastly depression that plagued his later years. The Romantic Movement generated the concept that genius and madness are inextricably intertwined. Committed to an asylum, composer Robert Schumann starved himself to death rather than endure the treatment designed to rest his ""fevered"" brain: he was totally isolated from work, wife, and friends. In the 20th century, Freud ""psychoanalyzed"" a 17th-century contributor to this volume who believed he had made a nine-year pact with the devil. Not so, said Freud: the poor chap was actually repressing a homosexual longing for his own father and a desire to bear a child by him. The nine-year pact represented the nine months of gestation. As Porter says, ""It is often hard to tell when the psychiatrist is speaking, and when the patient:"" Dense slogging, but fascinating stuff and worth the effort.