Derek Antony Parfit (11 December 1942 – 1 January 2017) was a British philosopher. He is 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."^[1] Reasons and Persons is "widely regarded as the most important work in utilitarian moral philosophy in the twentieth century,"^[2] and has been described as "perhaps the most argument-filled book ever to have been written"^[3] and as "contain[ing] the highest ratio of important insights per page of any philosophical work."^[4]

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 argues 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, according to Parfit (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, and of existential risk more generally.

Effective altruism

Towards the end of his life, Parfit became increasingly interested in effective altruism and supportive of it. In one of his last talks—an event at the Oxford Union organized by Giving What We Can—he remarked:^[5]

I am immensely heartened by the way in which, here at Oxford and some other places—Harvard, Rutgers, elsewhere—, various groups have been started whose aim is to relieve the great suffering and early deaths of some of the world’s 2 billion poorest people. I really admire this, and applaud you for doing it. It is the greatest moral problem, I think, that we rich people are likely to face in our lives, and most likely to act in ways that are seriously wrong.

In the final pages of the third volume of On What Matters, Parfit expressed the intention of writing a final volume applying his views on metaethics and normative ethics to practical issues, including global poverty and existential risk. Although Parfit died before the book could be written, he arguably inspired others to carry out that project in some form. As Simon Beard, a Senior Research Associate at the Centre for the Study of Existential Risk, writes:^[6]

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.

Parfit's disciples include Jeff McMahan and Toby Ord.