Persons across time could have identity, unity, or connectedness:
- Identity: Person at time T1 and person and time T1+n are the same person. They are identical.
- Unity: Person at time T1 and person and time T1+n re two parts of one larger cross-temporal entity. The two “time slices” are not identical to each other, but are component parts of one, four-dimensional thing.
- Connectedness: Person at time T1 and person and time T1+n have some other relation which connects them: they may share memories, or bodily parts, or be connected by a chain of overlapping memories, or overlapping bodily parts, or they may hold the same “office,” that is, they may hold the same place in social or political or legal relations. They are not strictly identical, but they may be the same person for specific purposes: for example, perhaps psychological and physical continuity would be sufficient for legal responsibility; or mere physical continuity would be sufficient for continued ownership of goods, etc.
On some level, 1 is impossible. Nothing has absolute sameness across time; the platinum-iridium kilogram bar, for example, seems to have grown lighter (or its duplicates grew heavier.) People under go much greater change than platinum-iridum bars. Absolutely strict identity won’t work for them, so we have to decide if personal identity is carried by some subset of the things that the person is made up of, or is, contra the strict notion of identity, amenable to some changes in virtual any part, etc.
The unity answer doesn’t really help, because it already assume that person at T1 and person at T1+n are slices of the same person, and that’s what we need to get at. That is: how do I know that these two person-slices are slices of the same person? Surely, even a four-dimensional person has conditions which he or she cannot survive, and must come to be and cease to be at various points in time. The rotting corpse of Johan is, for most purposes, not Johan. Nor are the various bits of matter, floating around in the early universe, which will eventually make up Johan’s body.
Connectedness seems to be the area of investigation for sameness of persons. What are the conditions of connection needed for a person to be the same person as something that exists at another time? This is, indeed, where most of the theories come down: animalism, physicalism, psychological theories, all say that some part of their preferred section of the person must be preserved, though by no means the entirety of that part. So no animalist claims that the entire body must be preserved; they focus on continuity conditions for living organisms, and must make some arbitrary choices for beginning and end points. Thus, Olson claims that a person is dead even if her brain is still functioning and we have some system of communicating with that brain (say, a neural implant hooked up to audio input and output systems that allow speech and hearing) if her body has died. So a brain transplant, on this account, is the end of a person. A sudden and complete memory wipe is probably the end of a person on psychological accounts, even if the body persists and can be rehabilitated.
The question then becomes: why are we asking about personal identity or survival? Is it to apportion blame, legal punishment, ownership of goods? To re-identify loved ones? To know if my memories are in fact my memories, and not the memories of some other that I’ve inherited?
Notably, when we do legally punish, under the vast majority of systems of laws we only punish people believed to be, in some important sense, identical to the person who committed the crime. I don’t punish a twin for her sister’s crime, for example. And we want to be our friend, the very person who has identity with the person we previously identified as our friend, not someone who merely looks and acts like him.
So it seems that, for some purposes, a kind of identity is called for, though it will not be strict identity. Strict identity may be, as David Shoemaker says, “the reddest of herrings.” But identity, more broadly construed, is central to our concerns.