Personal Identity

You will wake tomorrow with, at best, a hazy continuation of today's thoughts, in a body that has replaced a good share of its cells and altered its shape by a hair. Almost everyone is certain that the person who wakes will be you. The question is what that certainty rests on, and the answer matters because a great deal depends on it: who may be punished for a crime, who should be anxious about a coming operation, and what, if anything, survival after death would have to involve.

Two questions about persons

The phrase "personal identity" covers two different questions, and much confusion comes from running them together.

The other question is the one people have in mind when they speak of "finding their identity". That concerns which values, projects and traits characterise a person, and it is a question about character, not about whether there is one person or two. Marya Schechtman has called the first the reidentification question and the second the characterisation question, and the distinction is useful because some theories answer one and are then criticised for failing to answer the other.

Notice how the ship of Theseus bears on the first. A ship persists through replacement of its timbers, and the puzzle was what, in place of matter, secures that. For a person the same puzzle arises with higher stakes, because we do not merely classify the persisting thing but care about its fate. John Locke, in the Essay Concerning Human Understanding (1690, Book II, chapter 27), made the modern version of the question explicit by separating the identity of a man, an animal of a certain kind, from the identity of a person, a thinking being able to regard itself as itself at different times and places. Whether the two come apart is the first thing to settle, and the different theories divide over it.

Three families of answer are worth taking in turn, and each is then tested against the same kind of case.

Psychological continuity

Locke's own proposal was that a person is the same as long as consciousness extends backwards: you are the person who did a past deed just in case you can remember doing it. The thought behind it is that persons are the kind of thing defined by their mental life, and that what ties a person's days together is the way later states of mind carry the earlier ones forward.

The simple memory criterion has a famous refutation, due to Thomas Reid in Essays on the Intellectual Powers of Man (1785). A boy is flogged for robbing an orchard. As a young officer he takes a standard from the enemy, and remembers the flogging. As an old general he remembers taking the standard, but has forgotten the flogging. On Locke's criterion the general is the officer, and the officer is the boy, but the general cannot remember being the boy, so the general is not the boy. Identity is transitive, so the criterion has produced a contradiction.

The standard repair, proposed by H. P. Grice in 1941 and developed by Derek Parfit, is to stop requiring direct memory and to count chains.

The general and the boy are not directly connected by memory, but each link between them is, and the chain overlaps. Continuity is transitive even when direct connection is not, so the repair removes the contradiction. It also handles the ordinary fact that people forget a great deal and nonetheless stay themselves.

A second objection is older. Joseph Butler, in 1736, held that memory presupposes personal identity, since remembering an experience is partly constituted by its having been your own. If so, the criterion is circular: it explains identity by memory and memory by identity. Sydney Shoemaker and Parfit answered by introducing quasi-memory, a state like memory in how it is caused by an earlier experience and how it represents it, but not required to be of one's own experience. The criterion can then be stated in terms of quasi-memory without presupposing identity.

Even so repaired, the theory has unusual consequences. If what matters is the continuity of mental life, nothing in the account requires that the continuity run through one body. Shoemaker's brain-transfer cases, from Self-Knowledge and Self-Identity (1963), imagine your brain being moved into another body. The psychological theorist says you go where your mind goes, and that if the complete pattern of your mind were copied into a new brain and the old one destroyed, you would survive in the new one.

Biological continuity and animalism

The rival to the psychological view begins from the fact that each of us is an organism. That seems a modest claim, and its supporters take it to be the most natural account of what we are.

Eric Olson's The Human Animal (1997) is the central modern defence. His most direct argument is about counting. There is a human animal in your chair: it has a brain, it is awake, and it is reading these words. If the animal is thinking and you are not it, then two thinking beings occupy your chair, you and the animal, each with the same thoughts. That seems absurd. If instead you are the animal, the problem vanishes.

The second consideration is about what we once were and may become. You were once a foetus with no mental life to speak of, and on the psychological view you were never a foetus, since no chain of psychological connections reaches back that far. That seems a strange result about our early history. Equally, if the cerebrum were destroyed but the body kept alive, the animal would continue, and the animalist says that you would too, living on without a mind, a consequence the psychological theorist denies.

The two views therefore give opposite verdicts on the transplant case. Suppose your cerebrum, which carries your psychology, is removed and placed in an empty skull. The psychological theorist says you go with the cerebrum. The animalist says you remain with the organism, now mindless, and that the recipient is someone else with your memories.

Narrative identity

The third family comes from outside the metaphysics of persistence, and it is best understood as an answer pitched at the characterisation question that is also offered as a theory of what makes someone the person they are.

Alasdair MacIntyre argued in After Virtue (1981) that an action is intelligible only against a narrative setting, and that a human life has the unity of a story. Schechtman, in The Constitution of Selves (1996), developed this into the narrative self-constitution view, on which a person is one who understands their experiences as the experiences of a single subject with a history, and who is held to that history by the demands of the community. Daniel Dennett offered a deflationary cousin, in which the self is a centre of narrative gravity, a useful fiction in the way that a centre of gravity is a useful abstraction.

The attraction is clear. It accounts for what the other views miss: why a person who forgets nothing and whose body is intact can still feel they are no longer the person they were after a profound change, and why responsibility seems tied to the history we can own. It also explains why a story is something that can be revised and that others can distort.

Its difficulties mirror its attractions. Galen Strawson, in "Against Narrativity" (2004), argues that many people do not live their lives as stories and are no less selves for it. More basically, a story is something told, and what makes a story about the same person across its parts seems to be exactly what was to be explained. If the narrative has to be true, the theory depends on some prior criterion of who the events happened to. If the narrative can be false or incomplete, it cannot be what identity consists in, since people can be wrong about their own lives. Many narrative theorists accept this and settle for a more modest role: the narrative view tells us what a person is like and what responsibility attaches, while leaving the question of reidentification to the other criteria.

Fission cases

All three theories share a feature that makes them vulnerable to one kind of example.

David Wiggins discussed such cases in 1967, and Parfit gave the best-known version in "Personal Identity" (1971) and in Reasons and Persons (1984). The human brain has two hemispheres, and people have survived with one. Suppose your brain is divided and each half is transplanted into a different body. Each recipient wakes up with your memories, your character and your intentions, and each is psychologically continuous with you in just the way that an ordinary future self would be.

This is the structure of Hobbes's version of the ship of Theseus, with the repaired ship and the reassembled ship replaced by two recipients. As there, the argument is valid, so some premise or the assumption behind it has to go. The cases are fictions, but the question they pose is whether the theory states what identity requires, or states something that normally goes along with identity and can come apart from it.

The pattern hits the other criteria too, in different ways. The biological criterion is unaffected by the hemisphere case, since the original organism, not either recipient, is the animal the animalist follows. But organisms do divide: an early human embryo can split into identical twins. The embryo's life seems equally continuous with both, and the biological criterion has the same difficulty. Narrative identity is not made contradictory by fission, because stories can fork, but it is made indeterminate: the history could be told as leading to either recipient, and no fact about the life settles which. That leaves it unable to answer the reidentification question in the very case where we most want an answer.

Responses to fission

There are several ways to respond, and each can be run against the psychological criterion.

You do not survive. Both recipients are new people, and fission is a form of death. The difficulty is that each recipient is, taken alone, exactly what a successful single transplant would give, and a single successful transplant looks like survival. Adding a second recipient seems an odd way to turn survival into death, since surely a double success cannot be a failure.

You survive as one of them. The difficulty is that nothing favours one over the other, so any choice is arbitrary.

A non-branching condition. Robert Nozick, in Philosophical Explanations (1981), proposed that identity goes to the closest continuer, and only if there is no equally close rival. In fission there is a tie, so neither is you. This is a version of the first reply, and it brings the same problem plus a new one: whether you survive depends on whether something happens elsewhere, which is the oddity the earlier article noted about the ship.

You survive as both. David Lewis, in "Survival and Identity" (1976), argued that this is what the perdurantist account makes available. Before the division there were already two people, sharing all their temporal parts up to the point of fission, so that they overlapped as two roads can share a stretch before forking. The case then involves no contradiction: you are not one thing identical to two, because there were two all along, and our ordinary counting by what is wholly there at a time gives the wrong total. The price is the perdurantist ontology, and the consequence that before the fission there were two people where we thought there was one. An endurantist, for whom a person is wholly present at each time, cannot take this route, because there is nothing in the earlier person for two to share.

Identity is not what matters. Parfit's own response was that the conflict is a sign that we have been asking the wrong question. What matters in survival, he argued, is not identity but psychological continuity and connectedness, which he called relation R, and relation R holds between you and each recipient. Fission is then not as bad as death and not the same as ordinary survival; it is about as good as ordinary survival, because it preserves what we care about even though it does not preserve identity. On this view the puzzle shows that the question "which one is really me?" may have no answer, and need not be asked.

What the debate leaves is a choice among costs. Psychological continuity handles the transplant cases best and is threatened by fission and by the circularity worry. Biological continuity respects our nature as organisms but gives odd advice about what we should care about. Narrative identity captures the human significance of a life and fails to settle who is who. All of this has taken for granted that a person at one time stands in some relation to a person at another, and so presupposes a before and an after through which a person is carried.

The next article asks what time itself is, and whether the past and the future exist in the way the present does.