Principia Ethica is recognized as the definitive starting point for twentieth-century ethical theory. The text is reprinted here with the previously unpublished preface Moore wrote for a planned, but never completed, second edition. Though unfinished, it sets out clearly Moore’s second thoughts about his own work. The volume also includes two important pieces from his later ethical writings, “Free Will” and “The Conception of Intrinsic Value,” and a new introduction by Thomas Baldwin.