What conditions must be met for person A in 1984 to be the same person as person B in 2000?

First published: 13 March 2024

Last modified: 01 June 2024

This content was originally written in February 2024.
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Essay

Person A in 1984 is the same person as person B in 2000 if and only if there is non-branching psychological continuity between the persons ${P_A|_{1984}}$ and ${P_B|_{2000}}$. Psychological continuity is present when there exists an overlapping series of strong causal psychological connections between persons at different time periods. The non-branching condition is imposed in order that our conditions produce the result that identity is a one-to-one relation, as it is possible for two individuals (i.e. non-identical persons) to have psychological continuity between them. In this essay, I first consider rival physical continuity accounts of personal identity, and explain why we should reject them on the basis of their apparently incorrect judgements in several thought experiments. I then present the psychological account and defend it against objections relating to the personhood of foetuses and fission cases, concluding that non-branching psychological continuity is both a necessary and sufficient condition for persistence of identity.

In ordinary life, we often use physical assessments to determine whether persons we have observed on different occasions are the same. There is, of course, an important difference between providing a set of conditions whose satisfaction provides evidential grounds for concluding that certain persons are the same, and identifying a logical criterion for personal identity. Whilst the former task can be achieved by a specification of identity which generally yields the correct answer, successfully completing the latter undertaking demands an analysis of identity which is never mistaken. Nevertheless, the usefulness of a physical approach in providing epistemic conditions for personal identity makes it a natural place to start for the metaphysical problem too. A simple physical analysis of identity is as follows (Kind 2015):

$P_A |_{1984}$ is the same person as $P_B |_{2000}$ if, and only if, the body of $P_A |_{1984}$ is numerically identical to the body of $P_B |_{2000}$.

The first problem with this account, though, is its vagueness in referring to numerical identity. One cannot simply appeal to the fact that we can generally tell when bodies are numerically identical, because that is far below the perfect standard of reliability required for a logically true condition. How, then, to specify the requirements for numerical identity? It is clearly incorrect to say that bodies are numerically identical only if they are made up of exactly the same matter – cells in a human are constantly dividing and dying, and yet this is not an obstacle to the persistence of personal identity (Parfit 1984, 204). We may have the intuition that it is physical continuity which matters, rather than qualitative similarity. This is the key insight of animalism, which presents a modified physicalist account of identity (Kind 2015):

$P_A |_{1984}$ is the same person as $P_B |_{2000}$ if, and only if, $P_A$ and $P_B$ are biologically continuous.

This modified condition is an improvement: it can accommodate the changes to bodies which we see over the course of an ordinary life, as well as (for instance) more abrupt changes such as organ transplants or artificial, so long as the human organism’s metabolic and other processes continue (Olson 1997, 16). But though this refined physical condition is more precise and fully-specified than the original formulation, is it correct?

Examination of both actual and possible cases in which the criterion appears to give the wrong judgement suggests not. First, consider the phenomenon of dissociative identity disorder (DID). Most of those with DID exhibit several personalities controlling their bodies with only one apparent at any given time, and only able to access memories from when that personality was last in control of the body (Brook & Raymont 2021). On any intuitive understanding of personhood, one would conclude that these discrete personalities are separate identities cohabiting in one persistent body. Therefore, it is false that biological continuity between $P_{A}$ and $P_{B}$ logically entails them being the same person. It is also false that biological continuity is required for persistence of identity. To see why, we can turn to the following thought experiment (Kind 2015):

Gemma is involved in a car accident, and her body is injured in such a way that she could not possibly remain alive, though her brain is undamaged. George has been hospitalised in a persistent vegetative state for months, with his brain wholly incapable of thought, but his body still performing its essential functions. An ambitious doctor decides to transplant Gemma’s brain into George’s body, resulting in a fully living and thinking being.

It seems strongly like Gemma, not George, is the individual who has survived this ordeal – yet the body which remained intact and performing its metabolic processes was George’s. Williams (1970) attempts to rebut the negative implications of this experiment for the physical account with a structurally identical scenario designed to elicit the opposite intuitions:

You are kidnapped by a different doctor, who informs you that you will shortly be painfully tortured. However, he assures you that before this happens, he will remove all your memories, beliefs, and personality traits, and replace them with those of another individual. Once this has begun, he will begin the torture.

According to Williams, we still feel a sense of fear for and attachment to the person about to be tortured, suggesting that the persistence of the body is what matters for identity. However, I do not believe this story succeeds at undermining the earlier case against the physical account of identity. For one thing, I do not share Williams’s intuitions of prudential concern for the person to be tortured – whilst I would no doubt feel sorry for that person just as I would for any human who was about to undergo such an experience, I do not think that they would be me. Moreover, I suspect that for those who are pulled in the other direction by Williams’s scenario, the decisive factor is the use of the personal pronoun “you” when describing who is to be tortured, rather begging the question of whether identity persists in this instance. On the basis of DID and brain transplants, we can therefore say that the condition of biological continuity appears to be neither necessary nor sufficient for identity to hold between persons, and thus this account cannot be the logical condition we are in search of.

Our intuitions in the brain transplant case suggest that a psychological criterion may be a better fit. Following Parfit (1984, 206), I argue:

$P_A |_{1984}$ is the same person as $P_B |_{2000}$ if, and only if, $P_A |_{1984}$ is psychologically continuous with $P_B |_{2000}$.

In this context, psychological continuity exists when there is a series of overlapping psychological connections across time, such that the memories, beliefs, personality traits, and intentions of $P_{1}|_{t_{1}}$ are causally linked with those of $P_{2}|_{t_{2}}$, and also from $P_{2}|_{t_{2}}$ to $P_{3}|_{t_{3}}$, and so on. By relying on continuity rather than simply connectedness, this condition is a transitive relation and thus a legitimate candidate for an equivalence relation such as identity (Parfit 1984, 207). The role played by causal links in connectedness also means that periods of temporary sleep, amnesia, or comas do not mean that a person ceases to be the person they used to be upon awaking, provided that there do remain psychological connections. If, perhaps as a result of brain damage, an individual loses their memories and has a dramatic change in personality after losing consciousness, the psychological account would however conclude that they no longer are the same person, as seems entirely intuitive.

An animalist would try to deny that personhood can fundamentally consist of psychological identity, by asserting that humans are organisms, whose persistence is determined by biological, not psychological, conditions. To support this, the animalist may gesturing to the fact that someone could reasonably say “I was once a foetus” even though no person has ever been psychologically continuous with any foetus (Olson 2023). Yet this argument equivocates on its use of the verb “to be”: someone might also reasonably say “I was once stardust” without it being true that they Really ought to talk instead about Shoemaker (1984: 92-97) “Personal Identity: A Materialist’s Account” instead, and the argument that there are no thinking organisms (in response to the too-many-thinkers objection). are the same entity as that stardust was. This line of argument therefore lacks force, and is not a reason to reject the psychological criterion.

A more compelling potential challenge for the psychological account is the fact that psychological continuity is not an inherently one-to-one relation in the same way that numerical identity of bodies is, as illustrated by Parfit’s (1984, 199) Teletransporter:

Henrietta steps into a sophisticated Teletransporter, a device which scans its users, disintegrates them at their location of departure, and recreates them exactly at their desired destination using the scanned information. However, this time the Teletransporter malfunctions somewhat, and in addition to recreating Henrietta at her destination, also recreates her in another city.

Since Henrietta cannot possibly be the same person as two other people, and yet both are psychologically continuous with her, the psychological account appears flawed. However, adding a non-branching clause to our criterion of identity which rules out identity in cases where psychological clones have been created repairs the account. This may seem an unsatisfying and ad-hoc solution. To some extent, it is – but that is no indictment of the psychological account. The judgements given by rival theories in this case are considerably worse: the bodily and animalist accounts would not only conclude that Henrietta no longer exists due to the duplication, but that any teleportation would destroy her identity due to the physical discontinuity it introduces. This does not seem like a natural conclusion to arrive at. Surely for Henrietta, what matters prudentially is whether or not there exist agents with her beliefs, memories, and intentions. This suggests that psychological continuity, rather than the identity relation of whether any person is the same as her, is what is most important (Parfit 1984, 271).

So, to conclude, for person A in 1984 to be the same person as person B in 2000, there must be psychological continuity between the persons $P_{A}|_{1984}$ and $P_{B}|_{2000}$, with no branching having occurred. Psychological continuity is understood as the existence of overlapping causal connections between the mental states of persons at different points in time, a condition which may be met even if there is no physical continuity between those persons. Our intuitions in thought experiments where there is psychological continuity but no physical continuity tend to judge that the persons may still be the same. This means we ought to reject brute physical analyses of personal identity, and instead favour a psychological account. The inclusion of a no-branching clause means that this account is not logically invalidated by instances of fission, and such cases demonstrate that identity is not what prudentially matters. Rather, we should be concerned about the survival of agents with our beliefs, memories, and intentions – something which is determined by psychological continuity alone.

Bibliography

Brook, A., & Raymont, P. (2021). The Unity of Consciousness. In The Stanford Encyclopedia of Philosophy. https://plato.stanford.edu/archives/sum2021/entries/consciousness-unity/

Kind, A. (2015). Persons and personal identity. Polity.

Olson, E. T. (1999). The Human Animal. In Personal Identity Without Psychology. https://doi.org/10.1093/0195134230.001.0001

———. (2023). Personal Identity. In The Stanford Encyclopedia of Philosophy https://plato.stanford.edu/archives/fall2023/entries/identity-personal/

Parfit, D. (1984). Reasons and persons. https://doi.org/10.1093/019824908X.001.0001

Williams, B. (1970). The self and the future. The Philosophical Review, 79(2), 161. https://doi.org/10.2307/2183946