Back in the 1960s, or even early seventies, if you asked the average intelligent person to name a philosopher, the answer would as likely have been Jean-Paul Sartre as it would have been Aristotle or Plato. “The Pope of Existentialism,” as people called him, enjoyed household-name status. By the time of his death in 1980, however, Sartre’s star had already darkened; the 50,000 mourners who shuffled after his casket to Montparnasse cemetery in Paris represented a last flare from his vanishing fame, not a sign of real influence. Structuralism, post-structuralism, feminism, colonial studies — as new radical enthusiasms swept through...