Derek Antony Parfit (11 December 1942 – 1 January 2017) was a British philosopher. He was the author of Reasons and Persons and On What Matters.

In an obituary, Jeff McMahan referred to Parfit as "one of the most important philosophers of the past half century and, in the view of many, the single best moral philosopher in more than a century." (McMahan 2017) Reasons and Persons is "widely regarded as the most important work in utilitarian moral philosophy in the twentieth century" (Mulgan 2013: 404), and has been described as "perhaps the most argument-filled book ever to have been written" (Baier 1984: 220) and as "contain[ing] the highest ratio of important insights per page of any philosophical work." (Chappell 2021)

Parfit's students include Jeff McMahan and Toby Ord.

Human extinction

In the concluding section of the concluding chapter of Reasons and Persons, Parfit draws a comparison between three possible outcomes:

1. Peace.
2. A nuclear war that kills 99% of the world's existing population.
3. A nuclear war that kills 100%.

Everyone agrees that (3) is worse than (2), and that (2) is worse than (1). But Parfit notes that, although most people believe the difference between (1) and (2) to be greater than the difference between (2) and (3), the reverse is in fact the case. Although many more additional people will die in (2) relative to (1), than in (3) relative to (2), only (3) involves the death of everyone alive, and hence of the species as a whole. (2) kills almost everyone in the present generation, but (3) prevents all future generations from ever being born. Since the vast majority of potential people do not yet exist, (3) represents a far greater loss than (2). Appreciation of this critical insight has motivated many effective altruists to focus on reducing the risk of human extinction, as well as existential risk more generally.

Effective altruism

In his later years, Parfit became increasingly interested in effective altruism. Simon Beard, a postdoctoral researcher at the Centre for the Study of Existential Risk, summarizes Parfit's involvement with the effective altruism community (Beard 2020):

Derek had hoped to write a 5th book, On What Matters Volume 4, in which he would present the theories that he had been working on concerning the ethics of future generations and in which he could finally achieve the goal he had set out to achieve back in 1968, of applying the principles of moral philosophy to real-world problems. He would never complete it. However, by inspiring others to work in Effective Altruism, and giving them sound arguments to support their convictions that people can, and should, do a lot more good than they currently do, this does not seem to matter so much. His work was, ultimately, worthwhile.