Principia Ethica is recognised as the definitive starting point for twentieth-century ethical theory. Its influence was first largely confined to the Bloomsbury Group - Maynard Keynes wrote that it was 'better than Plato' - who took it up for its celebration of the values of art and love; but later it achieved the widespread recognition it still retains as a classic text of analytic ethical theory. It is particularly renowned for Moore's argument that previous ethical theories have been guilty of a fallacy - the 'naturalistic fallacy'. Principia Ethica is reprinted here with the previously unpublished Preface Moore wrote for a planned, but never completed, second edition.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.