On What Matters is a major work in moral philosophy. It is the long-awaited follow-up to Derek Parfit's 1984 book Reasons and Persons, one of the landmarks of twentieth-century philosophy. In this first volume Parfit presents a powerful new treatment of reasons and rationality, and a critical examination of three systematic moral theories -- Kant's ethics, contractualism, and consequentialism -- leading to his own ground-breaking synthetic conclusion...