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.

Why personal identity doesn’t matter

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. Two people are now walking around with his memories and experiences and thoughts and attitudes, rather than one. Did he continue to exist? If so, as who? Is he one of the brothers, but not the other? Why just that one and not the other? Is he each of them — are two people simultaneously Derek Parfit? That can’t be right either. If that were true, then if only one of them died, then it would be the case that Derek Parfit is both alive and has died. That’s ridiculous.

His answer is that there would be no one identical to him after the split. “There will be two future people, each of whom will have the body of one of my brothers, and will be fully psychologically continuous with me, because he has half of my brain,” he writes. “Knowing this, we know everything.”

If identity mattered, then this result would be just as bad as death, since both erase his identity. But this clearly isn’t as bad as death; his psychological being gets to keep on going, twice! So identity isn’t what matters. “Since I cannot be one and the same person as the two resulting people, but my relation to each of these people contains what fundamentally matters in ordinary survival, the case shows that identity is not what matters,” he concludes. “What matters is … psychological connectedness and/or psychological continuity, with the right kind of cause.”

This implies a lot of counterintuitive things about people’s relationships to their future selves, and to each other. For one thing, it makes selfishness a lot less defensible. When I decline to give $500 to charity and instead go on a vacation to Mexico, I’m privileging my future self above another person. But if personal identity isn’t what matters, then the fact that going on vacation benefits a future-person who happens to be physically and psychologically continuous with present-me seems less important. I’m separated from my future self by time; I’m separated from the people the charity would help by space. Why treat the latter separation as more important?

This view — that our distance from our future and past selves is greater than we might imagine, and our distance from other people’s present selves is smaller — has a lot in common with traditional Buddhist teaching. An appendix to Reasons and Persons is devoted to drawing out this similarity. After being introduced to the book by Harvard philosopher Dan Wikler, a Tibetan monastery in northern India began chanting passages from Reasons and Persons along with more traditional material.

And this argument also influenced how Parfit thought about his own death. One particularly poignant passage from the book has circulated among former students and colleagues on social media since his passing:

When I believed [that personal identity is what matters], I seemed imprisoned in myself. My life seemed like a glass tunnel, through which I was moving faster every year, and at the end of which there was darkness. When I changed my view, the walls of my glass tunnel disappeared. I now live in the open air. There is still a difference between my life and the lives of other people. But the difference is less. Other people are closer. I am less concerned about the rest of my own life, and more concerned about the lives of others. When I believed [that personal identity is what matters], I also cared more about my inevitable death. After my death, there will be no one living who will be me. I can now redescribe this fact. Though there will later be many experiences, none of these experiences will be connected to my present experiences by chains of such direct connections as those involved in experience-memory, or in the carrying out of an earlier intention. Some of these future experiences may be related to my present experiences in less direct ways. There will later be some memories about my life. And there may later be thoughts that are influenced by mine, or things done as the result of my advice. My death will break the more direct relations between my present experiences and future experiences, but it will not break various other relations. This is all there is to the fact that there will be no one living who will be me. Now that I have seen this, my death seems to me less bad.

The “repugnant” conclusion

Ignoring personal identity makes a lot of ethical problems a lot easier. Consider the dilemma posed by a prospective mother who’s informed that she has an asymptomatic disease that poses no risk to her well-being, but would result in her offspring having a considerably shorter lifespan. But if she receives a couple of shots, free of charge, over six months to cure the disease, then any children she has thereafter will live a full life. Intuitively, most people would say that it’s better to wait — the would-be kid is better off for having a doubled lifespan.

If personal identity mattered, though, saying that becomes harder. After all, the possible kid six months from now and the possible kid right now are two totally different kids — just as actual siblings are different individuals. By delaying, the mother isn’t making any currently existing kid better or worse off; she’s just causing one to exist, and preventing the other from ever existing. If personal identity matters, this is a conundrum. But since Parfit has argues that personal identity doesn’t matter, it suffices to say that if some individual is brought into existence, it’s better that said individual live a longer rather than shorter life.

But, as Parfit showed in the concluding section of Reasons and Persons, this creates new difficulties. In 1984, when he was writing, fears of overpopulation and of a “population bomb” that led to exponential growth in the human population, with average living standards plummeting until such a point that the whole world is living in a miserable slum together, were very prominent. Amidst that debate, he offered an argument suggesting that a nightmare overpopulation world might actually be an improvement, so long as the people in it were still better off alive than dead. He thought this conclusion was horrifying and worked for decades on a way to avoid it, but he concluded it’s very difficult indeed to avoid.

He asks us to compare four possible worlds: A, A+, B, and “divided B” (or B-). Each has a different-sized population, indicated by the bars’ widths, and a different level of average well-being for each population and sub-population contained within it, as indicated by the bars’ heights:

A is a world with a relatively small population with very high average well-being; A+ is that world, plus an equally-sized group of somewhat less well-off individuals. B- is a world with two A-sized groups with high average well-being, but not quite as high as A. And B is just the two groups in B- squished together.

So, which is the best of these groups? Well, it seems like A+ is better than A. All that happened was a new group of people was added, people who presumably enjoy their lives and would rather exist than not exist. If that’s the only change, it seems like a good one! B-, in turn, seems better than A+. The average level of well-being for both groups in B- is higher than for the combined population of A+, and the total amount of well-being, if you add it all up, is higher as well. And B- seems just as good as B, since they’re identical; B- is just the split-up version of B.

The conclusion you’re left with is that B > B- > A+ > A, and, accordingly, that B > A. It’s better for there to be twice as many people, even if each of them is a little bit worse off than in a world with a lower population.

This is where things get tricky. Presumably there’s a group C, that’s twice as big as B with slightly lower average happiness, that’s better than B. And then a group D in turn. And then a group E. And then on and on until you get group Z:

Z is a world with vastly, vastly more people — 100 billion, or 200 billion, say — all living lives that are just barely worth living. Parfit’s reasoning suggests that this is better than a much smaller world where people are, on average, much happier.

This ludicrous-sounding suggestion he deemed the “repugnant conclusion,” and it’s extremely difficult to figure out a way to think about ethics that avoids it. You could say that what matters is average well-being, not total, and so say that A+ is in fact worse than A because the addition of less well-off people brings down the average. But then you’ve backed yourself into saying that it’s bad to bring perfectly happy people into existence, even when doing so makes no one else worse off. That’s also ridiculous.

One of the more common responses is to say that world Z can’t be better, because it’s not making any specific, actual people better off; it’s making everyone on Earth now worse off, and then adding another 93 billion on top of that. That argument is compelling until you remember that personal identity doesn’t matter. If you buy Parfit’s argument about identity, then it shouldn’t matter much whether specific people are made better or worse off, it just matters that there are people, whoever they are, who collectively are better or worse off. Some philosophers have responded by just accepting the repugnant conclusion as true and trying to think up explanations for why it might not be so implausible (for instance, maybe a life “barely worth living” isn’t that much worse than the most happy life possible).

At the time of his death, Parfit was working on fleshing out his “theory X,” the term he’s used since Reasons and Persons for a theory that can compellingly avoid the repugnant conclusion. I read an early draft as a college senior back in 2012 during a seminar Parfit conducted; it was striking, then as now, how profound and original a puzzle Parfit had managed to pose that he and many of his colleagues were still trying to sort out possible answers three decades later.

Why we enjoy music

As befits its title, Parfit’s last and longest book On What Matters sprawled across a great variety of topics. It’s broadly interested in what reasons people have to act in certain ways, or hold certain beliefs, or desire certain things. A lot of those questions have to do with morality, but some don’t. Perhaps the greatest joy of reading it is spotting the occasional diversions, the odd moments here and there where he makes an aside from the main narrative, often concisely expressing what would take others of us pages and pages to articulate.

One of my very favorite passages of the book is about art, and music specifically, and whether we can have reasons to enjoy or be moved by a piece of art. Parfit was himself a rather accomplished artist; he was a poet in his youth, and published a poem in the New Yorker in 1962, when he was 19 and working as a researcher for the magazine. He later took up photography.

Parfit was hardly primarily interested in aesthetics, but it was still fascinating to see him apply his approach to ethics to the analysis of art:

It is sometimes claimed that we have reasons to enjoy, or be thrilled or in other ways moved by, great artistic works. In many cases, I believe, this claim is false. We can have reasons to want to enjoy, or be thrilled or moved by, these artistic works. But these are not reasons to enjoy, or to be thrilled or moved by, these works. We do have reasons to admire some novels, plays or poems, given the importance of some of the ideas that they express. But poetry is what gets lost in the translation, even if this translation expresses the same ideas. And we never have reasons to enjoy, or be moved by, great music. If we ask what makes some musical passage so marvelous, the answer might be ‘Three modulations to distant keys’. This answer describes a cause of our response to this music, not a reason. Modulations to distant keys are like the herbs, spices, or other ingredients that can make food delicious. When someone neither enjoys nor is moved by some great musical work, this person is not in any way less than fully rational, by failing to respond to certain reasons. In comparing music with food in this way, I am not belittling music, ranking it below novels, plays, or poems. Music is at least as great as the other arts. Without music, Nietszche plausibly (though falsely) said, life would be an error. But music is also the lost battlefield and graveyard of most general aesthetic theories.

Why we should give to the poor, and preserve the far future

While much of his work had profound practical implications, Parfit was not primarily a public intellectual concerned with convincing the public to change how it thinks about morality, the way that his friend and colleague Peter Singer is.

But Parfit, like Singer, participated in the effective altruism movement, which argues that people have an obligation to do what they can to improve the world through actions like adopting vegetarianism, donating at least 10 percent of your income to effective charities, and so forth. After his passing, Singer shared the conclusion of the soon-to-be-published third volume of On What Matters, which touches on these topics, and in particular argues for the importance of staving off existential risks that threaten the future of humanity, risks like global warming, pandemics, nuclear annihilation, and so on: