An anecdote.

A few years back, a young man I know took far too much of a powerful psychoactive substance for his own good. I didn't know him at the time, but the story is that he "flipped out" and became a different person, or a different personality. This state lasted for a few months, and from what I'm told, he never fully recovered. But he was still himself, in that he maintained his own memories, and his own skills (eg, he was a poet both before and after his experience, and while his personality changed quite a bit, his poetry stayed very similar in content and style). A year later, once I had met this fellow, he had a relapse. I want to make clear that this isn't a matter of the oft fabled "flashback", but rather a case of the drug having raised issues from his childhood, and a year later those issues resurfacing once more, to be re-evaluated once more. These were related to an intense childhood trauma (the untimely death of his father and brother). Here's the rub-- when these things resurfaced the second time, the man I knew, in terms of mannerism, personality, and modes of interaction, completely disappeared, in lieu of another, purely carnal being. It was, among other things, frightening. Same body, different person, wondering when my friend is coming back (which he did.)

This story might be used as evidence to suggest that identity is based in the physical (he had the same body, so even though his mind had changed completely, he was still himself) or that identity is based in the mental (the mind that we knew had left his body, so he was not himself.) My personal belief, however, is that identity is not a black and white thing. When his mind and body are together, he is himself. When his mind and body are separate, he is only half of himself. That identity is based in either the mind, or the body, is based in the assumption that there is only one form of personal identity. This may be a category mistake. I can look at myself in the mirror, and see myself. I can also read a story, poem, or letter which I have written a long time ago, and see it as a reflection of myself.

What I mean to say is, we do not have a static physical identity which houses an ever changing mental state, because our bodies change over time. Nor do we have a static mental state which provides a center for an erratic corpus, because minds also change. Identity is not static. It is, by definition, a process of change.