Jean-Paul Sartre, whose first novel, "Nausea," had a biographer as its hero, spent the last 10 years of his working life on a massive psychobiography of a writer he had always detested for his estheticism and his reactionary opinions—Gustave Flaubert. He customarily explained this curious project as an attempt to synthesize what can be understood today about an individual life, given what we have learned from a century of work in psychoanalysis, social psychology, linguistics, anthropology and the symbolic analysis of culture and individual behavior. But for Sartre, understanding always involved the discovery of that point at which all constraints—external accidents, the miseries of psychic determinism and social conditioning—are suddenly transformed into the active gestures and free choices of an individual—what he called "praxis." It is never easy to reach that magical point. "The Family Idiot" takes some 3,000 pages to get there….

Sartre called "The...