Identity. I am who I am.
Personal identity deals with questions about ourselves qua people (or persons). The most common question is what it takes for us to persist from one time to another. What is necessary, and what is sufficient, for some past or future being to be you? But there are other questions of equal interest and importance. Many are familiar thoughts that occur to everyone at some time: What am I? When did I begin? What will happen to me when I die? Philosophical discussions of personal identity go right back to the origin of the discipline, and most major figures have had something to say about it.
I will first survey the main philosophical questions that go under the heading of personal identity. Most of the entry will then focus on the question of personal identity over time: what it means and the main proposed answers. I will try to show how these answers relate to some of the other questions about personal identity, and to more general questions in metaphysics and the philosophy of mind.
1. The Problems of Personal Identity
There is no one problem of personal identity, but a range of loosely related problems. Discussions that go by the name of personal identity are most often about questions like these:
Who am I? We often speak of one's "personal identity" as what makes one the person one is. Your identity in this sense consists roughly of those attributes that make you unique as an individual and different from others. Or it is the way you see or define yourself. Your individual psychological identity is a property that you might have for a while and then lose. You could acquire a new identity, or perhaps carry on without one.
Persistence. What does it take for a person to persist--for the same person to exist at different times? What sorts of adventures could you possibly survive? What sort of thing would necessarily bring your existence to an end? What determines which future being, or which past one, is you? You point to a girl in an old photograph and say that she is you. What makes you that one--rather than, say, one of the others? What is it about the way she relates to you as you are now that makes her you? For that matter, what makes it the case that you existed at all back then? An answer to this question is an account of our persistence conditions, or a criterion of personal identity over time (a constitutive rather than an evidential criterion: see the Evidence Question below).
Historically this question often arises out of the hope that we might continue to exist after we die. Whether this is in any sense possible depends on whether biological death is the sort of thing that one could survive. Imagine that after your death there really will be someone, in the next world or in this one, who is a bit like you. How would that being have to relate to you as you are now in order to be you--rather than me, say, or a new person who didn't exist before? What would the Higher Powers have to do in order to resurrect you? Or is there anything they could do?
Evidence. How do we find out who is who? What evidence bears on the question whether the person here now is the one who was here yesterday? What ought we to do when different kinds of evidence support opposing verdicts? One source of evidence is memory: if you can remember doing something, or at least seem to remember it, it was probably you who did it. Another source is physical continuity: if the person who did it looks just like you, or even better if she is in some sense physically or spatio-temporally continuous with you, that is reason to think she is you. Which is more fundamental? Does memory supply evidence all by itself, for instance, or does it count as evidence only insofar as it can be checked against third-person, "bodily" evidence?
This question dominated the philosophical literature on personal identity from the 1950s to the 1970s (Penelhum 1970 is a good example). Though it is sometimes confused with the Persistence Question, the two are not the same. What it takes for you to persist through time is one thing; how we find out whether you have is another. If the criminal had fingerprints just like yours, the courts may conclude that he is you. But even if it is conclusive evidence, having your fingerprints is not what it is for some past or future being to be you.
Population. If we think of the Persistence Question as having to do with which of the characters introduced at the beginning of a story have survived to become the characters at the end of it, we can also ask how many characters are on the stage at any one time. What determines how many of us there are now, or where one person leaves off and someone or something else begins?
You may think that the number of people is simply the number of human animals--members of the primate species Homo sapiens (perhaps discounting those in a defective state that don't count as people). But this is disputed. Surgeons sometimes cut the nerve bands connecting one's cerebral hemispheres (commissurotomy), resulting in such peculiar behavior as simultaneously pulling one's trousers up with one hand and down with the other. Does this give us two people--two thinking, conscious beings? (See e.g. Nagel 1971. Puccetti 1973 argues that there are two people within the skin of every normal human being.) Could a human being with split personality literally be the home of two, or three, or seven different thinking beings? (Wilkes 1988: 127f.)
This is sometimes called the problem of "synchronic identity", as opposed to the "diachronic identity" of the Persistence Question (and the "counterfactual identity" of the How could I have been? Question below). But we shouldn't take this to imply that identity comes in two kinds, synchronic and diachronic. There are, rather, two kinds of situations where questions about the identity and diversity of people and other concrete things arise: synchronic situations involving just one time and diachronic ones involving several times.
Personhood. What is it to be a person? What is necessary, and what is sufficient, for something to count as a person, as opposed to a non-person? At what point in your development from a fertilized egg did there come to be a person? What would it take for a chimpanzee or a Martian or an electronic computer to be a person, if they could ever be?
What am I? What sort of things, metaphysically speaking, are you and I and other human people? What metaphysical category, if you like, do we fall under? For instance, are we material or immaterial? Are we substances, attributes, events, or something different still? Are we made of matter, or of thoughts and experiences, or of nothing at all?
Here are some possible answers to this question. We might be human animals. Surprisingly, most philosophers reject this answer. We will return to it in Sections 6 and 7. We might be partless, immaterial souls (or, alternatively, compound things made up of an immaterial soul and a material body: see Swinburne 1984). Hume said that each of us appears to be "a bundle or collection of different perceptions, which succeed each other with an inconceivable rapidity, and are in a perpetual flux and movement" (1888: 252; see also Quinton 1962 and Rovane 1998: 212). A modern descendant of this view says that you are a sort of computer program, a wholly abstract thing that could in principle be stored on magnetic tape (a common idea in science fiction). A popular view nowadays is that we are material objects "constituted by" human animals: you are made of the same matter as a certain animal, but you and the animal are different things because what it takes for you to persist is different (Wiggins 1967: 48, Shoemaker 1984: 112-114, Baker 2000). Another is the idea that we are temporal parts of animals (or in science fiction of more than one animal). There is even the paradoxical view that we don't really exist at all. The existence of human people is a metaphysical illusion. Many philosophers have denied their own existence (e.g. Russell 1985: 50 and Unger 1979).
How could I have been? How different could I have been from the way I actually am? Which of my properties do I have essentially, and which only accidentally or contingently? Could I have had different parents? Frank Sinatra and Doris Day might have had children together. Could I have been one of them? Or could their children only have been people other than me? Could I have died in the womb before I acquired any mental features? Are there possible worlds just like the actual one except for who is who--where people have "changed places" so that what is in fact your career is my career and vice versa? Whether these are best described as questions about personal identity is debatable. (They are not about whether beings in other worlds are identical with people in the actual world: see van Inwagen 1985.) But they are often discussed in connection with the others.
What matters? What is the practical importance of facts about our identity and persistence? Imagine that surgeons are going to put your brain into my head, and that neither of us has any choice about this. Will the resulting person (who will think he is you) be responsible for my actions, or for yours? Or both? Or neither? To whose property will he be entitled? Suppose he will be in terrible pain after the operation unless one of us pays a large sum in advance. If we were both entirely selfish, which of us has a reason to pay?
The answer to these questions may seem to turn entirely on whether the resulting person will be you or I. Only you can be respo