Dylan Matthews in Vox:
Derek Parfit, who died at age 74 on Sunday evening, was not the most famous philosopher in the world. But he was among the most brilliant, and his papers and books have had a profound, incalculably vast impact on the study of moral philosophy over the past half century.
His work did not dwell on topics of merely academic interest. He wrote about big topics that trouble everyone, philosopher and layperson alike: Who am I? What makes me “me”? What separates me from other people? How should I weigh my desires against those of others? What do I owe to my children, and to the future in general? What does it mean for an action to be right or wrong, and how could we know?
Parfit was not a prolific author; he tended to write his books over the course of decades, refining them repeatedly after discussions with colleagues and students. In the end, he wrote only two: 1984’s Reasons and Persons, and 2011’s On What Matters, a two-volume, 1,440 page tome whose third volume is still yet to be published. But both are classics, the latter generating such furious debate that a volume of essays discussing it was released two years before the book itself even came out (most of the key arguments had circulated in draft form for some time).
For an excellent overview of Parfit’s life and the major themes of his work, I highly recommend Larissa MacFarquhar’s beautiful and incisive New Yorker profile, published as On What Matters finally hit shelves. But perhaps the best way to experience Parfit’s writing, and understand why both his ideas and his method of articulating them proved so influential, is to dig into a few of his most important and fascinating arguments.
If there’s a single idea with which Parfit is most strongly identified, it’s the view that personal identity — who you are, specifically, as a person — doesn’t matter. This argument, made in the 1971 paper “Personal Identity” and in the third section of Reasons and Persons, is jarring at first, but his case is persuasive, and the implications are profound.
Parfit asks us to imagine that he is fatally injured in an accident, but his brain is mostly unharmed. His two brothers are also in the accident, and emerge brain-dead, but with otherwise healthy bodies. Doctors then split his healthy brain in half, and implant a half in each of his brothers’ bodies. “Each of the resulting people believes that he is me, seems to remember living my life, has my character, and is in every other way psychologically continuous with me,” Parfit writes in Reasons and Persons. “And he has a body that is very like mine.”
He then asked: What happened to Derek Parfit in all this? Did he die? That can’t be right; if anything, he doubled.
More here.