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1984 book by Derek Parfit

Reasons and Persons is a 1984 book by the philosopher Derek Parfit, in which the author discusses ethics, rationality and personal identity. It is divided into four parts, dedicated to self defeating theories, rationality and time, personal identity and responsibility toward future generations.

Part 1 argues that certain ethical theories are self defeating. One such theory is ethical egoism, which Parfit claims is 'collectively self defeating' due to the prisoner's dilemma, though he does not believe this is sufficient grounds to reject the theory. Ultimately, Parfit does reject "common sense morality" on similar grounds. In this section, Parfit does not explicitly endorse a particular view; rather, he shows what the problems of different theories are. His only positive endorsement is of "impersonal ethics" – impersonality being the common denominator of the different parts of the book.

Part 2 focuses on the relationship between rationality and time, dealing with questions such as: should we take into account our past desires?, should I do something I will regret later, even if it seems a good idea now?, and so on. Parfit's main purpose in Part 2 is to make an argument against self interest theory. Self interest theorists consider the differences between different persons to be extremely important, but do not consider the differences between the same person at different times to be important at all. Parfit argues that this makes self interest theory vulnerable to attack from two directions. It can be compared to morality on one side, and 'present aim theory' on the other. Parfit argues that our present aims can sometimes conflict with our long term self interest. Arguments that a self interest theorist uses to explain why it is irrational to act on such aims, can be turned against the self interest theorist, and used as arguments in favor of morality. Conversely, arguments that a self interest theorist uses against morality could also be used as arguments in support of 'present aim' theory.

Part 4 deals with questions of our responsibility towards future generations, also known as population ethics. It raises questions about whether it can be wrong to create a life, whether environmental destruction violates the rights of future people, and so on. One question Parfit raises is this: given that the course of history drastically affects what people are actually born (since it affects which potential parents actually meet and have children; and also, a difference in the time of conception will alter the genetic makeup of the child), do future persons have a right to complain about our actions, since they likely wouldn't exist if things had been different? This is called the non identity problem. Another problem Parfit looks at is the mere addition paradox, which supposedly shows that it is better to have a lot of people who are slightly happy, than a few people who are very happy. Parfit calls this view "repugnant", but says he has not yet found a solution.

Bernard Williams described Reasons and Persons as "brilliantly clever and imaginative", and commended it as part of a wave of work in analytic philosophy that deals with concrete moral problems rather than abstract meta ethics. Philip Kitcher wrote in his review of Parfit's On What Matters that Reasons and Persons "was widely viewed as an outstanding contribution to a cluster of questions in metaphysics and ethics". Peter Singer included Reasons and Persons on a top ten list of favourite books in The Guardian, stating that "Parfit's penetrating thought and spare prose make this one of the most exciting, if challenging, works by a contemporary philosopher". Writing for The New York Review of Books, the philosopher P. F. Strawson gave the book a positive review, stating "Very few works in the subject can compare with Parfit’s in scope, fertility, imaginative resource, and cogency of reasoning". In an interview, David Chalmers said that he "loved" Reasons and Persons, saying that it gave him a "sense of how powerful analytic philosophy can be when done clearly and accessibly."