Reasons and Persons

Reasons and Persons is Derek Parfit's magnum opus, published in 1984, and one of the most influential works in contemporary moral philosophy. The book is divided into four parts: the first examines rationality and self-interest; the second explores consequentialism and contractualism; the third introduces Parfit's famous arguments about personal identity and what matters in survival; and the fourth develops his theory of population ethics, including the controversial "Repugnant Conclusion."

Parfit argues that our ordinary understanding of personal identity is mistaken—that we are not separately existing entities but rather continuations of psychological and physical continuity. This insight reshapes how we should think about rationality, morality, and our obligations to future people. He contends that many widely held moral views lead to absurd conclusions when applied to questions about population size and quality, forcing a fundamental rethinking of utilitarian ethics.

The work has profoundly influenced academic philosophy, particularly in ethics, metaphysics, and practical reasoning. It introduced concepts and arguments—such as the non-identity problem and the distinction between person-affecting and impersonal views—that remain central to debates in bioethics, climate ethics, and existential risk. Parfit's arguments about future generations have become increasingly relevant to discussions of long-term technological and environmental risks.