Derek Antony Parfit

Life and Career

Derek Antony Parfit (1942–2017) was a British philosopher widely regarded as one of the most important moral philosophers of the late twentieth and early twenty‑first centuries. He was born in Chongqing, China, where his British parents were doctors working in a wartime hospital, and was raised mainly in England and later the United States.

Parfit studied at Eton College and then Balliol College, Oxford, initially focusing on history. After a fellowship in the United States, he returned to Oxford and gradually shifted from history to philosophy, developing the distinctive style of argument and thought experiments that would characterize his work. From 1967, he was based at All Souls College, Oxford, a research‑only institution. He remained a Fellow there for most of his life, holding visiting positions at leading universities in the United States and elsewhere.

Known for his intense focus, Parfit published relatively little but at great length and with meticulous attention to detail. His major book, Reasons and Persons (1984), quickly became a central text in ethics and metaphysics. Decades later he published the multi‑volume On What Matters (2011; expanded 2017), synthesizing and revising many of his views about moral reasons. He died in London on 1 January 2017.

Personal Identity and the Reductionist View

Parfit’s most famous work concerns personal identity over time. He challenged common assumptions about what makes a person at one time the “same” person at another time, asking what actually matters in survival.

Drawing on earlier debates in analytic philosophy, Parfit defended a reductionist and psychological continuity view of identity. According to this approach, facts about a person’s existence over time reduce to more basic facts about physical and psychological continuity—such as memory connections, character, and intentions. There is no further “deep” fact about identity beyond these relations.

To clarify this, Parfit employed a series of striking thought experiments, including:

• Teletransportation: A machine records your physical and psychological information, destroys your body, and reconstructs a perfect duplicate on Mars. Is the person on Mars you, or a copy?
• Fission: Your brain is divided and each half is transplanted into a different body, with each resulting person retaining your memories and character. Which of them, if either, is you?

Parfit argued that in such cases, what matters prudentially—what gives us reason to care about the future individual—is not strict identity but psychological connectedness and continuity. Identity, he claimed, can be indeterminate or not uniquely preserved in some scenarios, yet this does not undermine what is practically important.

This leads to his well‑known conclusion that “identity is not what matters” in survival. Instead, we should care primarily about the continuation of mental life and projects, not about some metaphysically strict sameness of person. Parfit held that recognizing this can alter how individuals regard their own future, their relation to past selves, and their attitude toward death and altruism.

Critics have questioned whether identity can truly be separated from what matters in survival, suggesting that ordinary moral practices—such as responsibility and promise‑keeping—presuppose a more robust notion of personal identity. Others have argued that Parfit’s thought experiments rely on highly artificial cases that may not generalize. Supporters, however, contend that these scenarios successfully expose hidden assumptions and show that everyday thinking may overstate the importance of numerical identity.

Ethics, Reasons, and Population Theory

Parfit’s work in moral philosophy is equally influential. In Reasons and Persons and On What Matters, he examined the nature of reasons, rationality, and moral obligation.

He defended forms of moral realism and objectivism about reasons, arguing that there are facts about what we have reason to do that do not depend solely on our desires or cultural norms. Parfit engaged with consequentialism, Kantian ethics, and contractualism, attempting to show that, at a deep level, these traditions may converge on similar principles. This “triple theory” suggests that what we have most reason to do is what no one could reasonably reject, what treats persons as ends and not merely as means, and what would make the outcomes best.

Another major contribution lies in population ethics, where Parfit systematically explored ethical questions about bringing new people into existence and comparing outcomes with different numbers of people. Here he introduced and analyzed a number of paradoxes and puzzles, the most famous being the “Repugnant Conclusion”:

• On certain plausible utilitarian assumptions, a very large population of people whose lives are barely worth living could be judged better than a smaller population of people with very high quality of life.
• Parfit found this conclusion highly counterintuitive yet difficult to avoid, given seemingly attractive principles about aggregation of well‑being.

His analysis of these issues led him to distinguish among different versions of utilitarianism, to examine prioritarian and person‑affecting views, and to highlight tensions between our intuitions about equality, total welfare, and fairness to future generations. Parfit did not claim to have fully resolved these problems, but he mapped the logical space with unusual clarity, shaping almost all later work in population ethics and long‑term moral reasoning.

Influence and Reception

Parfit’s influence extends across ethics, political philosophy, decision theory, and metaphysics. His insistence on clarity, the use of elaborate thought experiments, and the systematic comparison of rival theories helped set the agenda for several fields.

Supporters regard him as having shown that moral theory can be both rigorous and imaginative, and see his work as providing powerful tools for thinking about personal responsibility, future generations, and global priorities. His discussions of population ethics have become central to debates about climate change policy, existential risk, and obligations to future people.

Critics have raised concerns about the heavy reliance on highly idealized or technologically speculative cases, questioning whether these illuminate or distort ordinary moral life. Others dispute his objectivist claims about reasons, arguing from subjectivist, expressivist, or relativist perspectives that moral reasons depend more closely on human psychology and social practices than Parfit allowed.

Despite such disagreements, there is wide agreement that Parfit’s writings transformed contemporary analytic philosophy. Reasons and Persons in particular is often credited with reshaping the fields of personal identity, rational choice, and ethical theory, while On What Matters sparked extensive discussion about the possible unity—or irreducible plurality—of moral principles.

Parfit’s legacy continues through his students, interlocutors, and critics, and his arguments remain central reference points in philosophical work on identity, reasons, and the ethics of our impact on the future.