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In philosophy, the topic of personal identity involves the following cluster of questions: Under what conditions is a person at one time the same person as a person at another time? In virtue of what do persons persist through time? What is it for Can a person survive Star Trek-like teletransportation as the same person? What would count as evidence that a person is the same as an earlier person? What is a person in the metaphysical sense?

These questions are regarded as intrinsically interesting, but their answers have import in other areas of philosophy: responsibility for long-ago actions, concern for stages of yourself in the distant future, the separteness of persons objection to utilitarianism, and the split-brain phenomenon, to name a few.

concerns the conditions under which a person at one time is the same person at another time. An analysis of personal identity provides a set of necessary and sufficient conditions for the identity of the person over time. This concept of personal identity is sometimes referred to as the diachronic problem of personal identity. It contrasts with the synchronic problem, which is the question of what constitutes personhood at a time - what kind of thing is a person?

The Persistence Question
The question is typically stated "What makes it the case that a person existing at one time the same as a person existing at another time?" This formulation might be too narrow, for we might be interested in knowing whether an adult is the same person as a fetus or will be the same person as a someone in a persistant vegitative state. But, as Olson points out, it is controversial whether fetuses or bodies in such vegitative states count as persons. So a better formulation of the question might be "What makes it the case that a person existing at one time is the same person as something existing at another time?" To keep the discussion simple, it's helpful to adopt the earlier, flawed formulation while keeping this caveat in mind.

In thinking about this question, it is helpful to distinguish between qualitative identity and numerical identity. Two things are qualitatively identical when they share all the same qualities (e.g., two copies of a book). Two things are numerically identical if they are one and the same thing (e.g., a copy of a book today, and the same copy tomorrow). These are independent notions: qualitative identity is not a sufficient conditioin for numerical identity (e.g., two copies of a book), nor is it necessary for numerical identity (e.g., a single book which wears out over time).

The Memory Theory
The most extreme form states: A person existing at one time is the same person as a person existing at another time if and only if the person at the later time can remember having an experience that the earlier person had. John Locke was the first modern philosopher to address the question of persistence over time at length and this was roughly his view.

There are two classical objections to the memory criterion. Thomas Reid's famous example of the brave officer shows that it is too narrow: at age 30, a soldier, while taking the standard of the enemy, remembers being flogged as a boy for robbing an orchard. At 70, he is now a general and remembers taking the standard, but does not remember being flogged. By the memory criterion, the boy who was flogged is the same person as the officer who took the standard, and the officer who took the standard is the same person as the aged general, but the boy is not the same person as the general. But that can't be right, for the relation of "being the same person" seems to be transitive.

Second, the test seems circular. For it is part of the concept of "remembering" that you can only remember your own experiences. But then the criterion becomes trivial, and even when it captures the right cases of persistence, it doesn't explain what personal identity over time consists in.

John Locke considered personal identity (or the self) to be founded on consciousness, and neither on the soul nor on the substance (or the body) (see Consciousness as the basis of personal identity (John Locke)).

The problem of personal identity is at the center of discussions about survival of death and immortality. In order to survive death, there has to be a person after death who is the same person as the person who died. So in virtue of what is the post-death individual the same person as the earlier temporal stage of the person who it is claimed is survived in the post-death individual?

There have been many thought experiments about personal identity, for example, "swamp man".

External link

 * Stanford Encyclopedia of Philosophy entry on personal identity

Personlig identitet Identidade pessoal