Personal identity over time

Scenario

Find a photograph of yourself at five years old. Almost none of the matter in that child's body is still part of you: the atoms turn over, the cells that hold long term are a small minority, and the ordinary biological estimate is that most of the body you are made of today has been replaced several times since. The beliefs have turned over too. Whatever that child thought about the world, wanted, or feared has mostly been revised, forgotten, or reversed. And yet you do not hesitate to call the child in the photograph you, the same person rather than a relative or a predecessor, entitled to take credit for that child's small accomplishments and stuck with blame for that child's mistakes in a way no stranger would be. Something is doing the work of making the claim true. The photograph does not tell you what that something is. It only forces the question: when almost everything about a person changes, what is left over that counts as them staying the same person.

Definition

Two candidate answers are already implicit in how the photograph case gets described, and pulling them out one at a time is more useful than announcing them.

Start with the fact that the body is continuous even while its matter is replaced: a five-year-old grows into an adult through a chain of physical stages, each stage overlapping the last with no gap, no year in which nothing physically links the child to the adult. Someone who says the child in the photograph is you because that body, tracked continuously through time, became this body, is applying a criterion. Call it bodily continuity: two person stages count as the same person when there is an unbroken physical chain of organism between them, whatever the turnover in the matter that composes the organism at any one stage.

Now notice that the case can also be described without mentioning the body at all. You remember being a somewhat younger version of yourself, who remembered being younger still, in a chain that reaches back toward the child in the photograph, and your present character grew out of that child's character by a similarly continuous process of revision rather than replacement. Someone who says the child is you because of that chain, memory linked to memory, trait grown out of trait, is applying a different criterion. Call it psychological continuity: two person stages count as the same person when there is an overlapping chain of memory, intention, and character between them, independent of whatever happens to the matter carrying it. John Locke was the first to state this criterion sharply, holding that personal identity consists in continuity of consciousness rather than in the sameness of any body or soul.

Both criteria explain the photograph case without strain, which is exactly why the case alone cannot tell you which one is correct. The two candidates are not competing descriptions of the same fact; they are two different facts, physical continuity and psychological continuity, that happen to run together in every ordinary life. Isolating which one actually carries your identity, rather than merely accompanying it, requires a case where the two facts come apart.

Common mistakes

The most common mistake is treating bodily continuity as though it settled the question by default, since the body is the more visible fact. It stops settling anything once you construct a case where the two criteria diverge. Suppose a person's body is gradually replaced by artificial parts, one organ and one limb at a time, at a pace slow enough that memory and character carry through each step unbroken, until no biological tissue remains. Bodily continuity, read strictly as continuity of the original organism's material, is broken by the end. Psychological continuity is not. Whether you say the resulting person is or is not the one who started the process depends on which criterion you were using, and the case shows that the two can be pulled apart rather than confirming either.

The mirror mistake is treating psychological continuity as though memory alone were airtight. It is not, for two reasons. A memory can be false: a person can seem to remember an event that never happened to them, in which case the chain looks unbroken from the inside while carrying no real link to the past self it claims. And a memory chain can be duplicated rather than merely continued: if a procedure produced two people who each carried an equally continuous psychological chain back to the same earlier person, both would satisfy the criterion, yet they cannot both be identical to that one earlier person and to each other, since two distinct people existing side by side are not numerically one thing. Derek Parfit built exactly this kind of fission case to show that psychological continuity can hold without there being a single fact about which resulting person is the original.

A further mistake is assuming identity has to be all or nothing once a criterion is chosen. The replacement case above does not have to be read as a sharp line at which the person one day stops existing and a new one starts; some who take psychological continuity seriously treat identity itself as a matter of degree under enough strain, which the gradual case is built to expose rather than settle.

Build with it

Pick whichever criterion, bodily or psychological continuity, you found more plausible while reading the definition above. Then construct your own thought experiment, using fission, gradual replacement, or a swap between the two, that forces your chosen criterion to deliver a verdict on a case you build rather than one already given to you here.

A fission version: describe a procedure that splits one person's psychological chain, unbroken, into two separate resulting people, and ask your criterion which of the two, if either, is the original. A replacement version: describe a rate and an extent of physical substitution, more gradual or more sudden than the organ case above, at which you judge the resulting person stops being the one who started, and say what changed at that point. A swap version: describe a case where a person's psychological chain, memory and character, transfers into a different body while the original body continues on with none of it, and ask which one you would treat as the same person for a purpose that matters, inheritance, punishment, a promise made before the swap.

Success on this task has two parts. First, the case you build has to force your chosen criterion to issue a definite verdict, not one you can dodge by redescribing the case. Second, you have to state plainly whether the verdict is one you accept, in which case you now hold your criterion with a clear sense of what it commits you to, or one you find hard to accept, in which case you have located a real cost in the criterion you started with rather than an abstract worry about it. Either outcome is a result. A thought experiment that leaves your criterion undisturbed was not built hard enough.