}On What Matters{ is a major work in moral philosophy, and in this first volume Parfit presents a powerful new treatment of reasons and rationality, and a critical examination of Kant\'s ethics, contractualism, and consequentialism - leading to a new and groundbreaking conclusion.. }On What Matters{ is a major work in moral philosophy, and in this first volume Parfit presents a powerful new treatment of reasons and rationality, and a critical examination of Kant\'s ethics, contractualism, and consequentialism - leading to a new and groundbreaking conclusion.