Identity. I am who I am.

Personal Identity

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. ............