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Gordon Kennedy

Personal Identity and Psychological Reductionism

When we tackle the question of 'What makes us the individual persons that we are?', one approach that we can take is to seek an answer to the question of what it is that is required for a person to continue to exist over time. If we could agree on what is required for it to be true that you continued to exist, then we would have good grounds to believe that we had discovered what makes someone the particular person they are, and by extension, what makes any person the person they are. In essence, what we are searching for are the necessary and sufficient conditions for personal identity over time.

In this essay we will focus on the claim that it is in fact, only the psychological characteristics of a person that are essential to personal identity over time. These characteristics include memory, beliefs, intentions and personality. It might also be the case that persons require some kind of body, or at least a physical means of sustaining thought, but it is the thought, not the physical basis of it, which matters. This stance, known as 'Psychological Reductionism', argues that all other features, be it physical or otherwise, are neither necessary nor sufficient for personal identity over time.

Looking at the history of Psychological Reductionism may be useful in helping us understand how this view came to be considered as a possible model for personal identity over time.

Descartes, in a way, set the scene for Psychological Reductionism by identifying thinking as the essential characteristic of the 'self'. His famous "I think therefore I am" placed at the core of the 'I' the capacity to think. However, by no stretch of the imagination, could we label Descartes a Psychological Reductionist, since he believed that the 'self', the 'I', was an indivisible, thinking, non-material substance and that personal identity consisted in the continuing existence of this substance. Where Psychological Reductionists believe that Descartes went wrong was to suppose that personal identity was fixed by the substance that was doing the thinking, rather than the thoughts themselves. To illustrate this point we can look at John Locke's thought at experiment which, for many, dealt a fatal blow against the idea that the self is an immaterial substance. Locke's experiment is presented by Joseph Chandler as follows:

Let anyone reflect upon himself and conclude that he has in himself an immaterial spirit, which is that which thinks in him and in the constant change of his body keeps him the same. Let him also suppose it to be the same soul that was in Nestor or Theristes at the siege of Troy, but he now having no consciousness of any of the actions of either Nestor or Theristes, does he or can he conceive of himself to be the same person with either of them? Can he be concerned in either of their actions, attribute them to himself, or think of them his own, more than the actions of any other men that ever existed?

According to Chandler, in the above thought experiment we have to consider what would be the case if we had 'immaterial spirits', which were thought to be the basis of our personal identity. If this was so, then the kind of reincarnation described in the experiment would certainly be possible. So, according to Locke, if all this were true, it would nonetheless be absurd to say that you were the same person as Theristes. The reason being that because the whole idea of being the same person implies sameness of consciousness i.e. having the same thoughts, plans, feelings and memories. Now since this sameness of consciousness is not preserved in reincarnation, the continued existence of an immaterial spirit cannot be sufficient for the continued existence of a person.

John Locke is regarded by many as the originator of the psychological theory. He argued that a person at an earlier time is the same as a person at a later time if the later person remembers all the experiences of the earlier one. This is asking a lot. A more plausible definition would be based on chains of remembering — today you can remember most of yesterday's experiences and tomorrow you will remember most of today's, and the day after that most of tomorrow's, and so on. So in a year's time there will be a chain of remembering leading back to yesterday, and indeed leading back to your birth, which you will now have no memory at all. Locke clearly believed that memory was not the only factor and that a person at an earlier time is psychologically continuous with a person at a later time if from that later person there is a chain of linked memories and emotions and plans and character going back to the earlier time. So that what really matters to the psychological reductionist is that there is a continuity of mental life, so that although our beliefs, desires, plans and personalities do change, they do so gradually. Even though we do sometimes undergo some quite dramatic changes, these do leave a great deal preserved.

We can now look at some of the objections and arguments against Psychological Reductionism. One argument that is commonly used against the view that psychological continuity is necessary for identity is the so called 'torture-and-amnesia' argument, which Adam Morton presents as follows:

Suppose that you have been convicted of a crime and you will be sentenced to one of two punishments. Either you will simply be painlessly killed, or you will first be given a drug that induces total amnesia and then slowly tortured to death over a period of 24 hours. Which punishment do you hope for and which do you dread, for your own sake alone? Most people imagining the situation find that they hope for a speedy death. But if the psychological theory were true it would not be you who were being tortured, so although you might think that it would be awful that this was going to happen to someone, your reaction would not really be dread for your own future.

Another example of an argument that can be used against the view that psychological continuity is sufficient for identity is if we were to imagine beings who had a life expectancy of a thousand years. During this time, the chain of linked memories, emotions and plans could 'link' together individuals who had nothing in common whatsoever.

Further, we can object to the Lockean criterion on the grounds that it fails to provide a criterion for 'true' or 'false' memory claims. A person would not be considered to be Hitler brought back to life just because he remembered accurately the things that Hitler did 'as' things that he did. The question could still be raised as to whether there was the appropriate causal link between his present memories and the events remembered.

Other philosophers hold that the identity of a person consists in some sort of physical continuity, for example, the identity of a living organism, or the identity of the brain. David Wiggins argues for a combined physical and psychological criterion where what is required is the continuity of the physical parcel that is the functional basis for personality and memory — the brain. A thought experiment that illustrates this point is one from 'Self Knowledge and Self Identity' by Sydney Shoemaker. It is summarised by Joseph Chandlers as follows:

Two men, a Mr Brown and a Mr Robinson, had been operated on for brain tumours, and a brain extraction had been performed on both of them. At the end of the operation, however, the assistant inadvertently put Brown's brain in Robinson's head, and Robinson's brain in Brown's head. One of these men immediately dies, but the other, the one with Robinson's head and Brown's brain, eventually regains consciousness. Let us call the latter 'Brownson'..... When asked his name he automatically replies 'Brown'. He recognises Brown's wife and family.

In Chandler's analysis this simple thought experiment is taken to be one of the most persuasive in the literature. Most people would agree that Brown is the same person as Brownson, so that Brown survives his ordeal as Brownson. This being the case, then surely it is enough to show that the continued existence of a particular body is not sufficient to ensure the continued existence of a particular person. Only the continued existence of a particular brain is required for the continued existence of the person. So this thought experiment fits the Wiggins criterion for personal identity.

More recently there has been a trend to shift attention from the question of what personal identity consists in to questions about its importance. Thought experiments about hypothetical cases of 'fission' have led us to a view that we can have survival without personal identity. In these experiments it is envisaged that a brain divided into its two hemispheres is transplanted, one hemisphere for each person, into the bodies of two other persons. If we assume that each hemisphere is able to perform similar psychological functions, the original person would have become two different persons. Neither of the resulting persons would be identical with the original but the two descendants would retain a relation of psychological continuity with the original self. A leading proponent of this type of theory that asserts that we can separate the notion of survival from that of continued personal identity is Derek Parfit. Parfit asserts that psychological continuity can guarantee identity when it is 'one-one'. But if psychological continuity took a 'one-many' form, as in the case of successful brain fission, no coherent use of identity judgements could possibly correspond to, or be used to suggest the 'one-many' form of this relation. According to Parfit , what we should do in such a case, is deny the importance that would be associated to an identity judgement and attribute this importance directly to each branch of the 'one-many' relation resulting from brain fission. This case would then help to show that personal identity judgements derive their relevance from the fact of their suggesting mere psychological continuity and not personal identity.

This then suggests that our self interested concern for the future is really a concern for whatever future persons are psychologically continuous with us. According to Derek Parfit it simply doesn't matter whether or not there is some fact of the matter as to whether the future person is me or not.


Audi, Robert.(Ed). The Cambridge Dictionary of Philosophy. Cambridge University Press.(1995). pp. 574-575.

Baggini, Julian. All in the Mind? The Philosophers' Magazine 12Autumn 2000. pp. 42-43.

Chandler, Joseph. In Search of the Self. The Philosophers' Magazine 12, Autumn 2000, pp. 36.

Chandler, Joseph. There's a Thought....... . The Philosophers' Magazine 12, Autumn 2000. pp. 44-45.

Klaushofer, Alex. Taylor-made Selves. The Philosophers' Magazine 12, Autumn 2000. pp. 37-40.

Mautner, Thomas, (Ed). Penguin Dictionary of Philosophy. Penguin Reference.(1996). pp. 410.

Ibid. pp. 417-418.

Morton, Adam. Philosophy in Practice — An Introduction to the Main Questions. Blackwell.(1996). Chapter 14. pp. 407-415.

Palmer, D.E.. Parfit, the Reductionist View, and Moral Commitment. Twentieth World Congress of Philosophy. Boston, Massachusetts, U.S.A. (1998)
Pyle, Andrew. Key Philosophers in Conversation. Routledge.(1999). Chapter 16. pp179-195.

Scruton, Roger. Modern Philosophy — An Introduction and Survey. Mandarin.(1994). Chapter 22. pp.304-307.

Torriani,T. Continuity without Identity. Rootless Self-Images (Recovering Ethnic Identity) (1998), Section 1.3.
Warburton, Nigel. Philosophy — The Classics. Routledge.(1998). Chapter 5. pp 55-56.

Westphal, Jonathan. Philosophical Propositions. Routledge.(1998). Chapter 7. pp. 89-106.

Wilkes, Kathleen. The Systematic Elusiveness of ' I '. The Philosophers' Magazine 12, Autumn 2000. pp. 46-47.