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)