Derek Antony Parfit (/ˈpɑːrfɪt/; 11 December 1942 – 2 January 2017) was a British philosopher who specialised in personal identity, rationality, and ethics. He is widely considered one of the most important and influential moral philosophers of the late 20th and early 21st centuries.

Parfit rose to prominence in 1971 with the publication of his first paper, "Personal Identity". His first book, Reasons and Persons (1984), has been described as the most significant work of moral philosophy since the 1800s. His second book, On What Matters (2011), was widely circulated and discussed for many years before its publication.

For his entire academic career, Parfit worked at the University of Oxford, where he was an Emeritus Senior Research Fellow at All Souls College at the time of his death. He was also a visiting professor of philosophy at Harvard University, New York University, and Rutgers University. He was awarded the 2014 Rolf Schock Prize "for his groundbreaking contributions concerning personal identity, regard for future generations, and analysis of the structure of moral theories."

Parfit was born in 1942 in Chengdu, China, the son of Jessie (née Browne) and Norman Parfit, medical doctors who had moved to Western China to teach preventive medicine in missionary hospitals. The family returned to the United Kingdom about a year after Parfit was born, settling in Oxford. Parfit was educated at the Dragon School and Eton College, where he was nearly always at the top of the regular rankings in every subject except mathematics. From an early age, he endeavoured to become a poet, but he gave up poetry towards the end of his adolescence.

He then studied modern history at Balliol College, Oxford, graduating in 1964. In 1965–66, he was a Harkness Fellow and took courses at The New School, New York University, Columbia University, and Harvard University. During these years, he met, among others, John Rawls, Stuart Hampshire, and Robert Paul Wolff. He abandoned historical studies for philosophy during the fellowship.

After his Harkness Fellowship, Parfit returned to Oxford to start his BPhil in philosophy, and became a fellow of All Souls College. He held this position until age 67, at which point university policy mandates retirement. He remained a regular visiting professor at Harvard, New York University, and Rutgers.

In Reasons and Persons, Parfit suggested that nonreligious ethics is a young and fertile field of inquiry. He asked questions about which actions are right or wrong and shied away from meta-ethics, which is more concerned with logic and language.

In Part I of Reasons and Persons Parfit discussed self-defeating moral theories, namely the self-interest theory of rationality ("S") and two ethical frameworks: common-sense morality and consequentialism. He posited that self-interest has been dominant in Western culture for over two millennia, often making bedfellows with religious doctrine, which united self-interest and morality. Because self-interest demands that we always make self-interest our supreme rational concern and instructs us to ensure that our whole life goes as well as possible, self-interest makes temporally neutral requirements. Thus it would be irrational to act in ways that we know we would prefer later to undo.