Reasons and Persons

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Reasons and Persons
Reasons and Persons.jpg
Author Derek Parfit
Country United Kingdom
Language English
Subjects Ethics, rationality, personal identity
Publisher Oxford University Press
Publication date
Media type Print
Pages 560 pages (paperback)
ISBN 0-19-824908-X
OCLC 9827659

Reasons and Persons is a 1984 philosophical work by 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.

Self-defeating theories[edit]

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. Ultimately, Parfit rejects "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.

Rationality and time[edit]

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.

One of Parfit's arguments is as follows: self-interest theorists consider the differences between different persons at the same time as significant in terms of rationality, but do not consider the difference between the same person at different times to be as significant. But if, as Parfit argues, a reductionist theory of personal identity holds, then the difference between different persons at the same time is more like the difference between the same person at different times. So, if reductionism is true, self-interest theorists are inconsistent in viewing spatial relations as significant but temporal relations insignificant. Thus, the foundations of the self-interest theory are undermined by non-reductionism, which lends support to the present-aim theory of rationality, the critical version of which Parfit favours.

Personal identity[edit]

Part 3 argues for a reductive account of personal identity; rather than accepting the claim that our existence is a deep, significant fact about the world, Parfit's account of personal identity is like this:

At time 1, there is a person. At a later time 2, there is a person. These people seem to be the same person. Indeed, these people share memories and personality traits. But there are no further facts in the world that make them the same person.

Parfit's argument for this position relies on our intuitions regarding thought experiments such as teleportation, the fission and fusion of persons, gradual replacement of the matter in one's brain, gradual alteration of one's psychology, and so on. For example, Parfit asks the reader to imagine entering a "teletransporter," a machine that puts you to sleep, then destroys you, breaking you down into atoms, copying the information and relaying it to Mars at the speed of light. On Mars, another machine re-creates you (from local stores of carbon, hydrogen, and so on), each atom in exactly the same relative position. Parfit poses the question of whether or not the teletransporter is a method of travel—is the person on Mars the same person as the person who entered the teletransporter on Earth? Certainly, when waking up on Mars, you would feel like being you, you would remember entering the teletransporter in order to travel to Mars, you would even feel the cut on your upper lip from shaving this morning.

Then the teleporter is upgraded. The teletransporter on Earth is modified to not destroy the person who enters it, but instead it can simply make infinite replicas, all of whom would claim to remember entering the teletransporter on Earth in the first place.

Using thought experiments such as these, Parfit argues that any criteria we attempt to use to determine sameness of person will be lacking, because there is no further fact. What matters, to Parfit, is simply "Relation R," psychological connectedness, including memory, personality, and so on.

Parfit continues this logic to establish a new context for morality and social control. He cites that it is morally wrong for one person to harm or interfere with another person and it is incumbent on society to protect individuals from such transgressions. That accepted, it is a short extrapolation to conclude that it is also incumbent on society to protect an individual's "Future Self" from such transgressions; tobacco use could be classified as an abuse of a Future Self's right to a healthy existence. Parfit resolves the logic to reach this conclusion, which appears to justify incursion into personal freedoms, but he does not explicitly endorse such invasive control.

Parfit's conclusion is similar to David Hume's view, and also to the view of the self in Buddhism, though it does not restrict itself to a mere reformulation of them. For besides being reductive, Parfit's view is also deflationary: in the end, "what matters" is not personal identity, but rather mental continuity and connectedness.

Future generations[edit]

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?

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.[1]

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".[2]

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".[3]


  1. ^ Williams, Bernard (7 June 1984). "Personal Identity". London Review of Books. 6 (10): 14–15. Retrieved 6 April 2012. 
  2. ^ Kitcher, Philip (11 January 2012). "The Lure of the Peak". The New Republic. Retrieved 6 April 2012. 
  3. ^ Singer, Peter (6 April 2001). "Peter Singer's top 10 books". The Guardian. Retrieved 6 April 2012. 

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