Your body is destroyed, but only after it has been scanned and the blueprint beamed to Mars, where an organic replica of you is created. Is that replica you? It is physically and psychologically indistinguishable from what you were – yet suppose several such replicas are made?

Invoking this and other ingenious thought experiments, the philosopher Derek Parfit, who has died aged 74, transformed the centuries-old question of personal identity – what makes some future person me? – by subtly sabotaging and resetting it. He also reframed the agenda in moral philosophy, helped to replace the ideal of equality with the principle of prioritising the worst-off, and established a new philosophical discipline, population ethics.

He wrote only two books, Reasons and Persons (1984) and the hefty On What Matters (2011), but their originality, brilliance and provocativeness not only inspired philosophers all over the world, but also influenced discussion of practical and political strategies in tackling poverty, inequality, welfare economics, ageing and global warming. A senior research fellow of All Souls College, Oxford (1984-2010), Parfit was a visiting professor at Harvard, Rutgers and New York University, and elected a fellow of the British Academy (1986). In 2014 he received philosophy’s equivalent of a Nobel prize, the Rolf Schock prize in logic and philosophy.

He was born in Chengdu, western China, where his parents, Jessie (nee Browne) and Norman Parfit practised preventive medicine in Christian missionary hospitals. The family moved to Oxford a year after Derek’s birth. At the age of seven, he wanted to be a monk, and prayed fervently that his parents, who had by then lost their faith, should return to it.

However, perturbed by the problem of evil, he lost his own faith at the age of eight, and turned to poetry-writing. He was educated at Eton, and won the top history scholarship of his year to read history at Balliol College, Oxford (1961-64). After graduating, he held a Harkness fellowship for two years at Columbia and Harvard universities, then, in 1967, gained a prize fellowship to All Souls, where he changed from history to philosophy. Within a year of taking up the subject, and without even a degree, let alone a doctorate, in it, he had acquired an international reputation, and his first paper (Philosophical Review, 1971) became famous instantly.

In it, as in Reasons and Persons, Parfit proposed a solution to the problem of personal identity by disentangling the question “What makes it true that some person in the future will be me?” from “What makes it rational for me to care in an egoistic way about some future individual?” Before him, philosophers had thought that the answer to the second question depended on the answer to the first. But, using thought experiments involving brain transplantation, Parfit maintained that “all-or-nothing” identity is not the point. Just as a nation continues to exist by virtue of certain relations among people, institutions and territory, so a person’s continuing to exist is just a matter of certain relations among mental states over time. There is no “deep further fact”, no time-spanning, sealed-in entity. So if it is rational for me to care not only about myself-right-now but also varyingly attenuated degrees of psychological continuity with it, then the same reasoning compels me to extend my arena of caring.

By thinning out the connection between my present and future selves, Parfit hoped to reciprocally fatten up the connection between me and (at least some) other people. Although treating personal identity as a separate issue, he nicely enmeshed it in ethics. He helped to dislodge the view – so troublesome for morality, but so entrenched in philosophy, economics and common sense – that the rational action is necessarily the one that best serves my self-interest. Self-interest is partial as regards persons (me), but impartial as regards time (it forces me to consider the long-term effects of present pleasures). But if, argued Parfit, I can have reason to take care of my future self (by not drinking copious whisky, say, even if to do so is my greatest immediate desire), then I can also have reason to take care of other people, even if I now feel strongly disinclined to.

It can in fact be rational to do what is against my self-interest – to throw myself on the hand grenade if what I most want is to save my comrades’ lives. A “critical present-aim theory” – that, rationally, I should further my present aims if there are sufficient reasons to have such aims – is Parfit’s suggested rival to the self-interest view of individual rationality that has dominated western thinking since Socrates.

The age-old self-interest theory, Parfit argued, is anyway problematic: if each person does what is best for themselves, often the outcome is worse for everyone than it would have been had they all acted altruistically. By the same sort of “moral mathematics”, it is clear that many “harmless torturers”, each of whom inflicts minute amounts of suffering, will jointly cause a lot. When we see ourselves as less separate, ethics becomes more impersonal.

Perhaps, Parfit argued, we should ditch the intuition that morality is essentially concerned with how our individual acts harm or benefit particular individuals. If we squander energy, for instance, the people who suffer the effects of climate change will be different people from those who would have existed had we properly conserved it, since, thanks to our actions, quite other couplings and conceptions will have occurred. But their lives will be worse.

Surely, then, we should be guided by a more impartial principle requiring us to do what will produce the most wellbeing. Unfortunately, though, given the maths, that argument would compel us to prefer a massive population whose lives were barely worth living to a tiny population where everyone was extremely well-off – the so-called repugnant conclusion that Parfit was trying to rebut until his death.

In On What Matters, Parfit’s massive aim was to try to make systematic sense of three ethical approaches always assumed to be incompatible – Kant’s categorical imperative (deriving moral principles from universalisable impartial reasoning), TM Scanlon’s contractualism (basing them on informed general agreement), and rule consequentialism (focusing on how they achieve the best outcomes) – and combine them into “the triple theory”. Philosophers are divided as to how successful he was in either task, but a huge literature on the book anticipated its publication, and volume 2 contains objections by four eminent philosophers, with Parfit’s rejoinders. (Volume 3 is soon to be published, but volume 4 remains unfinished.)

Since Darwin, it has been hard to fit values into our naturalistic account of the world. Hating the sceptical notion that morality is ultimately just based on what we desire, Parfit dexterously argued that if we accept that there are non-scientific truths about belief (when it is raining, I ought to believe that it’s raining), and about prudence (I should avoid having unnecessary pain), then that opens the possibility to there being moral truths, too. I have reason to give to starving people, just as I have reason to jump out of the path of a speeding car or to stop smoking, whatever my desires in either case. Parfit’s Kantian rule consequentialism asserted that “everyone ought to follow the principles whose universal acceptance would make things go best”.

An obvious objection to his theories, Parfit admitted, is the psychological impossibility of viscerally feeling as selfless as they assert we are, or ought to be, which renders them effectively unbelievable. Another moral philosopher, his friend Bernard Williams, said that their plausibility relied on the external, third-person way in which they were presented. They chime with Buddhism, however. At one Tibetan monastery, monks intersperse chanting the usual sutras with intoning memorised passages from Reasons and Persons. Parfit himself also somehow seemed to live his theories, helped by perhaps having – as his wife, the philosopher Janet Radcliffe Richards, said – Asperger syndrome.

He said that dying became increasingly unregrettable as his selves successively vanished. When his friend Larry Temkin, phoning from America, asked, “How is Janet?” there was a baffled silence before Parfit demanded, “Why do you ask that?” Once informed that friends normally inquire about one another’s nearest and dearest, he made sure to remember that from then on.

But, despite his obsessive, unremitting industry, he would give exhaustive, invaluable commentaries on other philosophers’ work that were often far longer than the essay or book commented on. And he was financially generous, too, a member of the effective altruism movement, which enjoins everyone to give 10% of their income to charity. He was very emotional, prone to weeping when talking about global disasters or his dead sparring partner, Williams.

Terrified of wasting time, even on choosing what to eat or wear, he always had identical types of meal and kept duplicate sets of clothes, and he more often ran than walked. He drew people to him as a bright light draws moths, said a friend, but found mere chit-chat perplexing, wanting only to talk about philosophy.

Even in his one recreation, architectural photography, he was ruthlessly perfectionist. A highly specialised photograph shop, and, later, computers, enabled him to create meticulously modified, bespoke photos. Friends visiting St Petersburg and Venice would find fewer gas lamps, more telegraph poles and people, higher steeples and narrower squares than Parfit’s photos had led them to expect.

He met Janet in 1982, and they married in 2010. She survives him, as does his sister Theodora. Another sister, Joanna, predeceased him.

• Derek Antony Parfit, philosopher, born 11 December 1942; died 1 January 2017