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. Parfit now presents a powerful new treatment of reasons, rationality, and normativity, and a critical examination of three systematic moral theories — Kant's ethics, contractualism, and consequentialism — leading to his own ground-breaking synthetic conclusion. Along the way he discusses a wide range of moral issues, such as the significance of consent, treating people as a means rather than an end, and free will and responsibility. On What Matters is already the most-discussed work in moral philosophy: its publication is likely to establish it as a modern classic which everyone working on moral philosophy will have to read, and which many others will turn to for stimulation and illumination. The second volume of Derek Parfit's magnum opus is in four parts. The first presents critiques of his work by four of the world's leading moral philosophers. The second contains his responses. The third and longest part is a self-contained monograph by Parfit on normativity. The final part comprises seven new essays by Parfit on Kant, reasons, irrationality, autonomy — and why the universe exists.