Derek Parfit

Derek Antony Parfit (December 11, 1942 to January 2, 2017) was a British moral philosopher and a fellow of All Souls College, Oxford, whose 1984 book Reasons and Persons reshaped the philosophy of personal identity and population ethics. His treatment of obligations to future generations, including the non-identity problem and the Repugnant Conclusion, became a load-bearing part of the case for longtermism, and his work is cited as foundational by Nick Bostrom, Toby Ord, and William MacAskill.¹²

All Souls and an Ascetic Life

Parfit was born in Chengdu, China, to British medical missionaries, educated at Eton, and read modern history at Balliol College, Oxford, graduating in 1964. He won a fellowship at All Souls College, the research-only Oxford college admitting a handful of fellows by examination, and it remained his academic home for the rest of his life, with visiting professorships at Harvard, New York University, and Rutgers in his later decades. He married the philosopher Janet Radcliffe Richards in 2010, having met her in 1982.¹³

His working life was famously austere. As Larissa MacFarquhar reported in a 2011 New Yorker profile, Parfit wore the same gray trousers and white shirt so as not to lose time choosing clothes, ate as little varied a diet as possible, and kept only one serious interest outside philosophy: photographing the same buildings in Oxford, Venice, and St. Petersburg year after year, then paying to have the images retouched toward an exacting ideal. He was a member of Giving What We Can and supported effective altruism, pledging to donate at least a tenth of his income. David Edmonds, whose 2023 biography Parfit: A Philosopher and His Mission to Save Morality drew the fullest portrait of him, wrote that Parfit single-mindedly tried to rescue secular morality by arguing it has an objective rational basis, holding that if he could not show there are objective facts about right and wrong his life would be futile.⁴⁵

Reasons and Persons

Reasons and Persons, published by Oxford University Press in 1984, was organized in four parts addressing self-defeating ethical theories, rationality and time, personal identity, and future generations. Its third part advanced a reductionist or "bundle" account of personal identity, on which what matters in survival is psychological continuity and connectedness (Parfit's "Relation R") rather than a deep further fact of identity, a view he argued should loosen the grip of self-interest on practical reason. He drew on the work of Henry Sidgwick, whose 1874 The Methods of Ethics he held to contain the largest number of true and important claims of any work in the history of ethics.²⁶

The fourth part, on future generations, introduced the non-identity problem: because policies and even the timing of a conception change which particular people come to exist, a choice that leaves future people worse off than others who might have existed harms no determinate person, since those worse-off people owe their very existence to the choice. Parfit's "depletion" example, in which present generations consume resources for a small near-term gain at the cost of distant welfare, showed that person-affecting moral views struggle to condemn outcomes most people find clearly wrong. He concluded that personal identity is in this respect irrelevant to ethics, pushing toward impersonal principles that weigh outcomes by their total value.⁷²

The Repugnant Conclusion

Within population ethics Parfit framed the Repugnant Conclusion, which he stated as the claim that "for any possible population of at least ten billion people, all with a very high quality of life, there must be some much larger imaginable population whose existence, if other things are equal, would be better even though its members have lives that are barely worth living." Total utilitarianism, by valuing the sum of welfare, implies that a vast population with lives barely worth living can be better than a smaller flourishing one, and Parfit's "Mere Addition Paradox" traced a chain of individually plausible steps that lead there. He spent much of his later life searching, without final success, for a "Theory X" that would avoid the conclusion while preserving the impartial weighing of future lives.⁸²

Parfit's argument that future people count equally and could vastly outnumber the living, so that the value of the far future dominates moral calculation, was developed by Bostrom and Ord at the Future of Humanity Institute, and MacAskill's 2022 What We Owe the Future drew its title and its core argument from Parfit's question of what the living owe to those who will exist. Parfit's final work, the multi-volume On What Matters (Oxford University Press, 2011 and 2017), argued that Kantian, contractualist, and consequentialist ethics converge on the same moral truths, what he called the view that they are "climbing the same mountain."⁹²