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.