Derek Antony Parfit FBA (/ˈpɑːrfɪt/; 11 December 1942 – 2 January 2017[3][4]) was a British philosopher who specialised in personal identity, rationality, and ethics.

For his entire academic career, Parfit worked at Oxford University, where he was an Emeritus Senior Research Fellow at All Souls College at the time of his death.

He was awarded the 2014 Rolf Schock Prize "for his groundbreaking contributions concerning personal identity, regard for future generations, and analysis of the structure of moral theories.

Parfit was educated at the Dragon School and Eton College, where he was nearly always at the top of the regular rankings in every subject except maths.

One may consider an aspiring author whose strongest desire is to write a masterpiece, but who, in doing so, suffers depression and lack of sleep.

The appeal to full relativity raises the question whether a theory can be consistently neutral in one sphere of actualisation but entirely partial in another.

While Parfit did not offer an argument to dismiss S outright, his exposition lays self-interest bare and allows its own failings to show through.

Where self-interest puts too much emphasis on the separateness of persons, consequentialism fails to recognise the importance of bonds and emotional responses that come from allowing some people privileged positions in one's life.

[13] In the conclusion of the third volume, published shortly after his death, Parfit writes that the affluent have strong moral obligations to the poor: One thing that greatly matters is the failure of we rich people to prevent, as we so easily could, much of the suffering and many of the early deaths of the poorest people in the world.

The money that we spend on an evening’s entertainment might instead save some poor person from death, blindness, or chronic and severe pain.

[14] In his book On Human Nature, Roger Scruton criticised Parfit's use of moral dilemmas such as the trolley problem and lifeboat ethics to support his ethical views, writing, "These 'dilemmas' have the useful character of eliminating from the situation just about every morally relevant relationship and reducing the problem to one of arithmetic alone."

He instead suggests that more complex dilemmas, such as Anna Karenina's choice to leave her husband and child for Vronsky, are needed to fully express the differences between opposing ethical theories, and suggests that deontology is free of the problems that (in Scruton's view) beset Parfit's theory.

In some cases, he used examples seemingly inspired by Star Trek and other science fiction, such as the teletransporter, to explore our intuitions about our identity.

Parfit argued that reality can be fully described impersonally: there need not be a determinate answer to the question "Will I continue to exist?"

[17] Following David Hume, Parfit argued that no unique entity, such as a self, unifies a person's experiences and dispositions over time.

[18] A key Parfitian question is: given the choice between surviving without psychological continuity and connectedness (Relation R) and dying but preserving R through someone else's future existence, which would you choose?

Parfit described his loss of belief in a separate self as liberating:[14][19] My life seemed like a glass tunnel, through which I was moving faster every year, and at the end of which there was darkness.

Parfit explains that from this so-called "Argument from Below" we can arbitrate the value of the heart and other organs still working without having to assign them derived significance, as Johnston's perspective would dictate.

The non-identity problem arises from the observation that actions taken today can fundamentally alter which future people come into existence.

In chapter 16 of Reasons and Persons, Parfit posits that one's existence is intimately related to the time and conditions of one's conception.

"[12]: 386  The other is what he calls the "Hedonistic version"; he formulates this as "If other things are equal, the best outcome is the one in which there is the greatest average net sum of happiness, per life lived.

"[12]: 387  He then describes the other formulation, the "non-Hedonistic Impersonal Total Principle": "If other things are equal, the best outcome is the one in which there would be the greatest quantity of whatever makes life worth living.

[24] Parfit and other philosophers have explored a number of potential ways to avoid this "repugnant conclusion", many of which are discussed in Reasons and Persons.

[26] Others proposed a minimal threshold of liberties and primary social goods to be distributed, or suggested to adopt a deontological approach that looks to values and their transmission through time.

[25] Michael Huemer and Torbjörn Tännsjö endorse the conclusion, considering that the repugnance comes from intuitions that should be revised.

[27][28] In 2021, a number of philosophers said that avoiding the repugnant conclusion was receiving excessive focus and should not be considered a necessary condition for an adequate theory of population ethics.