Plato, Aristotle and the question of self: what makes you 'you'?

I'm going to talk about what makes you ‘you’.

This question has been a central question in the Western philosophical tradition since the fourth century, BCE. Most ancient and medieval philosophers held that every human on earth consists of a body and a soul, but they differed over what a soul is. Plato and his later followers held that our soul is a separate thing from our body and the essential part of each of us – what makes us who we are. At our death, our body becomes a decaying corpse, but our soul might be capable of continuing to live with or without a new body. By contrast, Aristotle, like almost all contemporary philosophers, held that a human having a soul is simply that human's body behaving and thinking in a human way. It's our body, the essential part, that makes each of us who we are. And so, at death, when our body becomes a decaying corpse, necessarily, we cease to exist.

My own view, developed in a recent book of mine, Are We Bodies or Souls?, is that Plato was basically right. And in this short talk I shall seek to give you some idea about why I think this.

I approach this question by means of a similar question: what makes a human at one time the same person as some human at an earlier or later time? For example, what makes a certain person called ‘George’, currently accused of committing a certain murder in 1970, the same person as the person who committed that murder. We sometimes use the word ‘person’ in the same sense as ‘personality’. And we might say that, in this sense, George is not the same person as he was in 1970.

But our question is: What makes him the same person as the person whose personality has changed?

One apparently obvious answer to this question is that George is the same person as the person who had George’s body in 1970. Since most of the cells of our bodies are replaced gradually by new cells over the course of seven years, having the same body means having a body made of the same cells or ones resulting from gradual replacement of its earlier cells. But what if a large part of George’s body is removed at one time? Surely, in that case, if George has a heart transplant, he still continues to exist with a new heart. And similarly for the transplant of any other bodily organ or several bodily organs together at one time, with one exception – our brain.

What makes the brain special is that our conscious life depends on the operation of our brains, and without the capacity for being conscious, we would not exist. We move our limbs only by causing some brain event that results in their motion, and we see and hear what is happening in the world only because light and sound waves create changes in our eyes and ears that cause brain events, which in turn result in our perceptual experiences. So perhaps the answer to our question is that the later George is the same person as the earlier George, if and only if he has the same brain as that George.

But again, the same question arises – how much of the same brain does George need in order to be the same person as the earlier George? Surgeons often remove diseased parts of the brain, and although it is not yet possible for surgeons to replace a damaged brain part with a part transplanted from another body, this should become possible within the next century. So, how much of someone’s brain could be replaced before that person ceases to exist?

One important fact about the brain which highlights this issue is that the brain consists of two central hemispheres – the left hemisphere and the right hemisphere – and also a lower part, the cerebellum. The content of our conscious life – what we are thinking about and remembering, feeling and believing – depends on the upper part of the cerebral hemispheres, the cerebral cortex. Patients suffering from some otherwise incurable epilepsy sometimes have parts of one hemisphere removed and may even have the whole hemisphere removed in an operation called an ‘anatomical hemispherectomy.’ What happens then is that most of the functions of that hemisphere are taken over by the other hemisphere; and the patient thinks and behaves in much the same way as before, at least in the respect that their memories and personalities remain largely the same.

This is the case whichever of the two hemispheres is removed – showing that there is very considerable overlap between the brain states of the two hemispheres that give rise to conscious memories and personality. Let us suppose, when one of the hemispheres is removed from George, it is replaced by a hemisphere taken from the brain of a different person, James. Following this, it is to be expected that the resulting person will have some memories and aspects of personality deriving from those of James, as well as the ones that George had previously. However, now let us suppose that George’s remaining hemisphere is also removed (and, as we have already seen, a person only needs one hemisphere in order to function well). Is that person still George (because he has the same lower brain, the cerebellum), or is he James because his one remaining cerebral hemisphere, which gives rise to his memories and personality, is that of James?

One day, this may become a very important practical issue. Consider the case of a person whose cerebral hemispheres are diseased and must both be cut out. As we have seen, someone can function well with only one hemisphere, so the surgeon offers the patient an operation which will involve replacing his two diseased hemispheres with one hemisphere taken from another person. The surgeon might be able to tell the patient whether the operation is likely to be successful in the sense that after the operation, there will be a living conscious person, but what the surgeon will be unable to tell the patient is who that person will be – whether that surviving person (who will still have the other part of the patient’s original brain) will be the patient (with some false memories and perhaps new personality,) or someone else. Someone who undergoes a serious operation will want to know whether they themselves will survive that operation. The surgeon will be unable to tell them the answer, and no new scientific experiment could possibly help to answer that question.

Faced with difficulties of this kind, many contemporary philosophers claim that personal identity is a matter of degree; the degree to which someone has the same brain parts, and similar memories and personality to those of an earlier person, determines the degree to which that person is the same person.

Thus, on the personal identity view, if a later George has had 60 per cent of his brain parts replaced, then he would be 60 per cent identical to the original George. But in that case, the parts removed from George’s brain could be implanted in someone else, and that other person might then be 40 per cent identical to the original George. It would then seem to follow that George, as he exists before the operation, is going to have to 60 per cent degree the experiences of one subsequent person, and to 40 per cent degree the experiences of the other subsequent person.

If the experiences of the former person are very happy ones, and the experiences of the other person are very miserable ones, then George is going to have experiences which are partly happy and partly miserable. However, after the operation the two subsequent persons may live entirely different lives, and then no one future person will have both a happy life and a miserable life – one will have a happy life and the other one will have a miserable life.

This shows that personal identity theory, supposing personal identity to be a matter of degree, makes no sense; someone at one time either is or is not the same person as a person at an earlier time, and there is no intermediate possibility.

In that case, being George cannot solely consist of having a certain proportion of a physical substance – brain matter, and/or a certain proportion of George’s properties (his memories and personality). It must consist of something else which is indivisible. The only remaining possibility is that there is an individual non-physical substance, a particular soul which someone who is George has, and someone who is not George does not have. George needs his own soul in order to remain the same individual person.

Certainly, the more a subsequent person has George’s brain parts and memories and personality, the more probable it is that he is George; but this is not what constitutes being George – it is only evidence of who is George. And that gives us the answer to our original question, what makes you ‘you’ – the answer is ’your soul’. Plato was right.

Richard Swinburne FBA is Emeritus Professor of the Philosophy of Religion at the University of Oxford.