14 Personal Identity
The question of persistence takes for granted that one and the same object may exist at more than one time. The oak tree is identical with the sapling from which it originated, even if may now look nothing like it. In this unit, we will be concerned with the question of what makes a person at one time be one and the same as a person at a another time.
The question of personal identity presupposes a distinction between qualitative and numerical identity. Two photocopies of a given document are numerically different even if they are qualitatively identical. On the other hand, the oak tree is numerically identical with the sapling from which it originated even if they remain qualitatively different. The oak tree and the sapling are one and the same object even though the oak tree not now how it used to be.
14.1 The Question
There are two issues generally associated to the question of personal identity:
- What exactly is the nature of a person?
- What is for a person at one time be numerically identical to a person at another time?
We will pay special attention to the second question, though answers to the second should illuminate the first. Candidate answers to the second question all take the form of a criterion of personal identity:
\(X\) at \(t\) is one and the same person as \(Y\) at \(u\) if and only if \(\dots \dots \dots \dots \dots\)
That is, the answer to the second question provides necessary and sufficient conditions for personal identity over time. But these presumably flow from the nature of a person. Once we have a criterion of personal identity, we will be able to deal with a variety of problem cases, which would put some strain on our concept of a person. Here is, for example, the simple teletransporter case (Parfit 1984) describes:
I enter the Teletransporter. I have been to Mars before, but only by the old method, a space‐ship journey taking several weeks. This machine will send me at the speed of light. I merely have to press the green button. Like others, I am nervous. Will it work? I remind myself what I have been told to expect. When I press the button, I shall lose consciousness, and then wake up at what seems a moment later. In fact I shall have been unconscious for about an hour. The Scanner here on Earth will destroy my brain and body, while recording the exact states of all of my cells. It will then transmit this information by radio. Travelling at the speed of light, the message will take three minutes to reach the Replicator on Mars. This will then create, out of new matter, a brain and body exactly like mine. It will be in this body that I shall wake up.
Call the person emerging on Mars after about an hour Replica. Am I, while stepping on the machine, one and the same person as Replica? Is simple teletransportation a means of travel or a means of death?
I suffer an accident, which leads to total and irreversible amnesia. Not only will I lose my memories of having particular experiences, but I will also lose memories of facts about my own past life. Once I wake up, I may still remember how do speak or how to swim, but when asked, I will not be able to give you my name or remember anything about my life.
Am I, before the accident, one and the same person as the person waking up after the accident? Or is the accident the end of me?
14.1.1 The Physical Criterion
The physical criterion of personal identity takes its cue from a common view of persistence for material objects. What makes a material object at a time one and the same as a material object at a different time? It is not uncommon to assume that the answer to that question involves spatiotemporal continuity:
\(X\) at \(t\) is one and the same material object as \(Y\) at \(u\) if and only if \(X\) at \(t\) is spatiotemporally continuous with \(Y\) at \(u\).
Material objects generally exemplify a continuous spatiotemporal path. The car I drive today is spatio-temporally continuous with the car I originally purchased many years ago, even if they are by now qualitatively different and differ even in a great many parts.
This suggests a parallel answer to the question of personal identity:
\(X\) at \(t\) is one and the same person as \(Y\) at \(u\) if and only if the brain and body of \(X\) at \(t\) is spatiotemporally continuous with the brain and body of \(Y\) at \(u\).
The criterion is too simple as stated, since some persons continue to exist even after they lose much of their body. According to a more sophisticated version of the view, what is necessary and sufficient for the continued existence of a person is the continuity of enough of their brain and body:
\(X\) at \(t\) is one and the same person as \(Y\) at \(u\) if and only if (i) enough of the brain of \(X\) at \(t\) continues to exist and is spatiotemporally continuous with \(Y\)’s brain at \(u\), and (ii) this spatiotemporal continuity is non-branching.
One immediate consequence of the physical criterion is that I do not survive being teletransported. For a proponent of the physical criterion, teletransportation, as described by Parfit, involves death. This is because the teletransporter is explicitly said to destroy my brain and body in the process. To be sure, a replica of me will emerge on Mars within an hour, but that is no consolation to me if I die in the process.
On the other hand, it follows from the physical criterion that I can survive total and irreversible amnesia.
14.1.2 The Psychological Criterion
The psychological criterion for personal identity involves a connection between the psychological states of the person at one time and the psychological states at a different time. The thought is that there should be an overlapping chain of experiences and memories connecting \(X\) at \(t\) with \(Y\) at \(u\).
\(X\) at \(t\) is psychologically connected to \(Y\) at \(u\) if, and only if, there are direct psychological connections between them. For example, some of \(X\)’s perceptions at \(t\) figure in the causal history of \(Y\)’s memories at \(u\), or \(X\)’s beliefs and desires at \(t\) result in \(Y\)’s intention at \(u\) to carry out some action.
There are different degrees of psychological connectedness. There may be a single direct connection between \(X\) at \(t\) and \(Y\) at \(u\), or there may be thousands of them. For \(X\) at \(t\) to be the same person as \(Y\) at \(u\), there must be enough direct connections between them. We will say that \(X\) at \(t\) is strongly connected to \(Y\) at \(u\) if there are in fact enough direct connections to think of them as one and the same person.
Notice that strong psychological connectedness is not a transitive relation: \(X\) at \(t\) may be strongly connected to \(Y\) at \(u\), and \(Y\) at \(u\) may be strongly connected to \(Z\) at \(v\) without \(X\) at \(t\) being strongly connected to \(Z\) at \(v\).
Since personal identity is indeed a transitive relation, it follows that personal identity is not the same as strong psychological connectedness.
\(X\) at \(t\) is psychologically continuous with \(Y\) at \(u\) if and only if there are overlapping chains of strong connectedness between them.
According to the psychological criterion of personal identity, what is necessary and sufficient for the continued existence of enough psychological connections with an appropriate causal history:
\(X\) at \(t\) is one and the same person as \(Y\) at \(u\) if and only if (i) \(X\) at \(t\) is psychologically continuous with \(Y\) at \(t\), (ii) this psychological continuity has the right kind of cause, and (iii) this psychological continuity is non-branching.
The appeal to the right kind of cause is supposed to set aside cases in which the continuity has a deviant, maybe non-reliable causal history. Consider Donald Davidson’s swamp man: a person walks into a swamp and is struck by lightning and destroyed. Simultaneously, another bolt rearranges the swamp matter into a molecular replica of the person, complete with all her psychological states, memories, dispositions, etc. This swamp man emerges with mental states not caused by any prior experiences, brain processes, or memories of the original person. He is, in a word, psychologically continuous with the original person through a deviant causal history.
What is the right kind of cause? The normal causal mechanisms involved in ordinary psychological continuity will do. However, he extends them to include reliable non-standard mechanisms such as those involved in teletransportation. The key observation is that this mechanism, unlike that involved in the case of swamp man, dependably produces and sustains psychological continuity.
Proponents of the psychological criterion will think that absent branching, it is in fact me who emerges on Mars one hour after I enter the teletransporter. I should confidently step into the teletransporter.
On the other hand, total and irreversible amnesia is for proponents of the psychological criterion no better than death.
14.1.3 Non-Reductive Views
(Parfit 1984) takes both the physical and psychological criteria of personal identity to be reductionist views of personal identity. For they both share two features:
personal identity facts reduce to particular facts, e.g., facts to do with the continued existence of a brain and a body or facts to do with the existence of overlapping chains of strong connectedness, and
these facts can be described without either presupposing the identity of the person or by explicitly claiming the facts concern the given person. That is, they can be described in an impersonal way.
They further agree that:
At least in ordinary cases, the continued existence of a person just consists in the continued existence of a brain and body, and the occurrence of a series of interrelated physical and mental events.
This in contrast to non-reductionist criteria of personal identity, which appeal to further facts over and above physical and psychological continuity.
Even in ordinary cases, the continued existence of a person involves the existence of something distinct from the brain, body, and the series of interrelated physical and mental events.
That is, non-reductionsists posit the existence of a separate Cartesian Ego or a soul or some irreducible further entity. What is necessary and sufficient for the continued existence of a person is the continuity of the self:
\(X\) at \(t\) is one and the same person as \(Y\) at \(u\) if and only if the Cartesian Ego of \(X\) at \(t\) is the same as the Cartesian Ego of \(Y\) at \(u\).
But notice that we have no independent access to the Cartesian Ego of either \(X\) at \(t\) and \(Y\) at \(u\). The criterion takes for granted that a person is something over and above and separate from \(X\)’s brain and body at \(t\) or \(X\)’s mental life at \(t\). So, whether or not \(X\) at \(t\) is the same as \(Y\) at \(t\) involves a further fact, one that is no more basic that the fact that \(X\) at \(t\) is the same as \(Y\) at \(t\).
14.2 The Branch-Line Cases
Let us return to what (Parfit 1984) calls the branch-line variant of the teletransporter case:
In Simple Teletransportation, I am destroyed before I am replicated. This makes it easier to believe that this is a way of travelling—that my Replica is me. At the end of my story, my life and that of my Replica overlap. Call this the Branch‐Line Case. In this case, I cannot hope to travel on the Main Line, waking up on Mars with forty years of life ahead. I shall stay on the Branch‐Line, here on Earth, which ends a few days later. Since I can talk to my Replica, it seems clear that he is not me. Though he is exactly like me, he is one person, and I am another. When I pinch myself, he feels nothing. When I have my heart attack, he will again feel nothing. And when I am dead he will live for another forty years.
If we believe that my Replica is not me, it is natural to assume that my prospect, on the Branch Line, is almost as bad as ordinary death. I shall deny this assumption. As I shall argue later, being destroyed and replicated is about as good as ordinary survival. I can best defend this claim, and the wider view of which it is part, after discussing the past debate about personal identity.
The transitivity of identity precludes one from identifying you with now with Replica on Mars, but while your Replica can look forward to decades of experiences you know your life will end in just a few days.
- You a few days after the event are not identical to Replica on Mars a few days after the event.
- You now are identical to you a few days after the event.
- Therefore, you now are not identical to Replica a few days after the event.
On the other hand, you seem to be psychologically continuous and strongly connected to both you and Replica forty years after the event, and, moreover, this strong continuity appears to have an appropriate causal history.
14.3 What Follows
There are two ways to assess your prospects in the branch-line case. On the one hand, you may regard them as akin to imminent death: you know you will cease to exist just a few days after you step into the device. On the other hand, you may be encouraged by the fact that Replica, who bears an important relation to you, will continue to lead a rich and fruitful life on Mars for many decades.
We may now distinguish two views:
What matters is whether someone in the future will be numerically identical with me now.
This is the view on which as described, the branch-line scenario provides you with a case for grave concern: after just a few days, no one will any longer be numerically identical with you now.
The rival view focuses on the relation of strong psychological connectedness and continuity with an appropriate causal history, which we may call relation \(R\):
What matters is whether someone in the future will be psychologically continuous and connected with me now with an appropriate causal history whether they are numerically identical with me or not.