Personal Identity

Harold Noonan

• Publisher: Routledge
• Year: 2019
• ISBN:
  9781351606509
• Edition: 3
• Topic: Philosophie
• Subtopic: Histoire et théorie de la philosophie

Who am I? What is a person? What does it take for a person to persist from one time to another? What is the relation between the mind and the body? These are just some of the questions that constitute the problem of personal identity, one of the oldest and most fundamental of philosophical questions. Personal Identity, Third Edition is a clear and comprehensive introduction to these questions and more. Harold Noonan places the problem of personal identity in the context of more general puzzles about identity, discussing the major historical theories and more recent debates. The book also includes essential historical and philosophical background to the problem of personal identity as found in the arguments of Locke, Reid and Hume among others.

The third edition of Personal Identity has been thoroughly reviewed in light of advances in the latest literature and research. This includes significant revision to the important problems of the simple and complex distinction and its relation to reductionism; temporal parts; and the distinction between perdurance and endurance theorists. Noonan also includes an up to date examination of personal identity and memory and personal identity and animalism, particularly the work of Shoemaker, Parfit, Olson and hybrid theorists.

Including helpful chapter summaries and annotated further reading at the end of each chapter, Personal Identity, Third Edition is essential reading for all students of philosophy of mind and metaphysics, as well as students interested in ethics.

1An initial survey

Introduction

What am I? And what is my relationship to the thing I call ‘my body’? Thus each of us can pose for themselves the philosophical problems of the nature of the self and the relationship between a person and their body. The nature of personal identity over time, and the link, if any, between personal identity and bodily identity are aspects of these problems and it is this, of course, that accounts for the immense philosophical interest in the concept of personal identity. But, perhaps unlike some other philosophical problems, the nature of personal identity is not merely of interest to professional philosophers but also a matter of great practical concern to all of us, philosophers and non-philosophers alike. Humans have always hoped to survive their bodily death, and it is a central tenet of many religions that such survival is a reality. But, of course, whether such survival is possible, and what forms, if any, it might take, are matters which depend crucially on the nature of personal identity over time. For to survive, in the sense that concerns us, means to continue to exist as persons identifiable as those here and now. Again, our concept of personal identity is intimately linked with our concept of responsibility for past actions and with our practices of praise and blame; while our own pasts and futures are the primary focus of many of our central emotions and attitudes. Were we to give up the idea of a person as a unitary continuing entity, it is hard to imagine the drastic impact this would have on our picture of the world and our emotional and moral responses to it.

In what follows we will be looking closely both at the history of the problem of personal identity and at the main solutions to the problem defended in contemporary debate. But it will be useful, before getting too involved in details, to begin with a survey of these solutions and a sketch of the main arguments put forward in their favour.

Constitutive and evidential criteria

The problem of personal identity over time is the problem of giving an account of the logically necessary and sufficient conditions for a person identified at one time being the same person as a person identified at another. Otherwise put, it is the problem of giving an account of what personal identity over time necessarily consists in, or, as many philosophers phrase it, the problem of specifying the criterion of personal identity over time. On an alternative use of the term ‘criterion’, to specify a criterion of personal identity over time would be to say something about what could count as evidence for personal identity. It is important to be aware at the outset that this is not what philosophers are interested in when they debate the problem of personal identity. Their concern is with the constitutive, the metaphysical-cum-semantic, not the evidential, criterion of personal identity. Of course, this is not to say that a philosophical account of personal identity can just put aside as a mere irrelevance what actually counts as evidence for personal identity. For both our own identity over time and that of others is, we ordinarily think, something of which we can have knowledge. Conceivably this common opinion may be mistaken, but the onus of proof must be on the philosopher who says so. In the absence of such proof then it must be regarded as a condition of adequacy on any account of what personal identity consists in that it not entail that personal identity is unknowable, or not knowable in the ways we ordinarily take it to be, or leave it completely mysterious how it can be known in these ways.

I shall have more to say later in elucidation of these points; but that will suffice for the moment as a specification of our problem. Let us now turn to its possible solutions.

The bodily criterion

The most natural theory of personal identity, which would be almost anyone’s first thought, is that personal identity is constituted by bodily identity: P2 at time t2 is the same person as P1 at time t1 if and only if P2 has the same body as P1 had. I shall call this the bodily criterion of personal identity. According to this view, personal identity is essentially no different from the identity of material objects in general. An artefact, like a ship, or a living thing, like an oak tree or a horse, persists through time. Its persistence does not consist in its retention of the same matter – for artefacts can be repaired and patched up and living things are necessarily involved in a constant exchange of matter with their environment – but in its retention of the same form as its matter undergoes gradual replacement. Likewise, according to the bodily criterion of personal identity, what is required for the identity of person P2 at time t2 and person P1 at time t1 is not that P2 and P1 are materially identical but merely that the matter constituting P2 has resulted from that constituting P1 by a series of more or less gradual replacements in such a way that it is correct to say that the body of P2 at t2 is identical with the body of P1 at t1.

According to this view, as I said, personal identity is essentially no different from the identity of such other living things as oak trees or horses. (A version of this view, to be discussed at length in Chapter 11, is the very popular view now called ‘animalism’ and called by one of its most prominent defenders [Olson 1997] ‘the biological approach’.) And this conforms to our ordinary experience. We do not in the normal run of things in fact ever regard it as an open question whether someone who, by the bodily criterion of personal identity, is identical with some earlier person, is that person, or whether someone who, by the bodily criterion of personal identity, is not identical with some earlier person, is not that person. Personal identity, as we know it in our everyday lives, outside psychopathological or other medical contexts, is in fact constituted by bodily identity.

Nor can it be made an objection against the bodily criterion of personal identity that it excludes any hope of an afterlife. For it does not. What it does exclude, however, is any possibility of an afterlife otherwise than by resurrection. But that can hardly be regarded as a conclusive objection to it.

The brain criterion

Nevertheless, the bodily criterion of personal identity has not proved popular with philosophers. For though it is undeniable that in our everyday experience personal identity is constituted by bodily identity, it seems all too easy to imagine possible cases in which this is not so. But if such cases are indeed possible then personal identity cannot, as a matter of logical or conceptual necessity, consist in bodily identity.

The sort of case which has led most modern philosophers to think that the bodily criterion of personal identity must be rejected is the following.

One part of the body – the brain and, in particular, the upper brain, the cerebrum – seems to be of crucial importance in determining the psychology of the person whose body it inhabits. Damage to someone’s brain can cause amnesia or radical changes in personality or character. Not so for damage to, say, one’s left knee. Imagine, then, that in the twenty-first century it is possible to transplant brains, as it is now possible to transplant hearts, and let us suppose that the brain of a Mr Brown is transplanted into the skull of a Mr Robinson. This could be done even with existing techniques. Just as my brain could be extracted, and kept alive by a connection with an artificial heart-lung machine, it could be kept alive by a connection with the heart and lungs in someone else’s body. The drawback, today, is that the nerves from my brain could not be connected with the nerves in the other’s body. My brain could survive if transplanted into their body, but the resulting person would be paralysed. Even so, they could be enabled to communicate with others. One crude method would be some device, attached to the nerve that would have controlled this person’s right thumb, enabling them to send messages in Morse Code. Another device, attached to some sensory nerve, could enable them to receive messages. Many people would welcome surviving, even if totally paralysed, if they could still communicate with others.

Let us suppose, however, that the surgeons in the twenty-first century are able to connect the nerves from Brown’s brain to the nerves in Robinson’s body. The result of the operation, call him Brownson, will then be a completely healthy person, without any paralysis, with Robinson’s body, but in character, memories and personality quite indistinguishable from Brown, and this not as a consequence of some freak accident but because of his possession of Brown’s brain. (There might be a problem about how Brown’s personality can express itself in the Robinson body if we imagine that the two bodies are very dissimilar in appearance, so, for the sake of the example, let us imagine that this is not so; let us imagine in fact that Robinson is Brown’s double.) Now who will this person be?

Most modern philosophers who have reflected on this case (which I have taken from Shoemaker 1963, with elaborations due to Parfit 1984) have not found this a difficult question to answer. They have found that they could not honestly deny that Brownson, in the case imagined, was Brown, and so they have been led to reject the bodily criterion of personal identity. As Parfit puts it (1984: 253), they have been led to accept that ‘receiving a new skull and a new body is just the limiting case of receiving a new heart, new lungs, and so on’. (See also Thomas Mann’s novella The Transposed Heads which has a similar presupposition.)

But a fairly simple modification of the bodily criterion can accommodate the Brown/Brownson case, while retaining the assumption that personal identity consists in nothing other than the persistence of a certain physical entity. The obvious response to the case is to say that it shows only that what is required for personal identity is not identity of the whole body but, merely, identity of that part of the body – which, contingently, is the brain – which is the central organ controlling memory, character and personality. According to this suggestion, P2 at t2 will be the same person as P1 at t1 just in case P2 at t2 has the same brain as P1 at t1. Let us call this the brain criterion of personal identity.

The physical criterion

But in fact this modification of the original bodily criterion of personal identity is not sufficiently radical. For if one accepts the Brown/Brownson case as a case of personal identity, one is bound to find compelling also other cases in which identity of brain is not preserved but the later person is psychologically identical with the earlier person, as Brownson is with Brown, in a way that is quite as scientifically comprehensible as in the Brown/Brownson case.

The human brain has two very similar hemispheres: a left hemisphere and a right hemisphere. The left hemisphere plays a major role in the control of the limbs on the right side of the body and in the processing of information from the right side of the body and the right sides of the eyes. The right hemisphere plays a major role in the control of the limbs on the left side of the body and in the processing of information from the left side of the body and the left sides of the eyes. The left hemisphere typically has the linguistic and mathematical abilities of an adult, while the right hemisphere has these abilities at the level of a young child. But the right hemisphere, though less advanced in these respects, has greater abilities of other kinds, such as those involved in pattern recognition or musicality. After the age of three or four, the two hemispheres follow a ‘division of labour’ with each developing certain abilities. The lesser linguistic abilities of the right hemisphere are not intrinsic or permanent. People who have had strokes in the left hemisphere often regress to the linguistic ability of a young child, but with their remaining right hemisphere many can relearn adult speech. It is also believed that, in a minority of people, there may be no difference between the abilities of the two hemispheres.

In a normal adult, the two hemispheres are connected and communicate by a bundle of fibres: the corpus callosum. But in the treatment of some epileptics these fibres were cut. It was this that led to the discovery of the independent functioning and (typically) different roles of the two hemispheres. For when these patients were tested in specially designed experimental situations, the effect, in the words of one surgeon (Sperry 1968b: 724), was to appear to reveal ‘two independent spheres of conscious awareness, one in each hemisphere, each of which is cut off from the mental experience of the other . . . each hemisphere seems to have its own sensations, perceptions, concepts, impulses to act. Following the surgery each hemisphere has its own memories.’ The facts which prompted this description are set out in Nagel (1971). For example, in the case of these patients,

what is flashed to the right half of the visual field, or felt unseen by the right hand can be reported verbally. What is flashed to the left half field or felt by the left hand cannot be reported, though if the word ‘hat’ is flashed on the left the left hand will retrieve a hat from a group of concealed objects if the person is told to pick out what he has seen. At the same time he will insist verbally that he saw nothing. Or, if two different words are flashed to the two half fields (e.g., ‘pencil’ and ‘toothbrush’) and the individual is told to retrieve the corresponding object from beneath a screen with both hands, then the hands will search the collection of objects independently, the right hand picking up the pencil and discarding it while the left hand searches for it, and the left hand similarly rejecting the toothbrush which the right hand lights upon with satisfaction.

(Nagel 1971: 400)

Now, as indicated above, both hemispheres are not in fact necessary for survival. People have survived when one hemisphere has been put out of action by a stroke or injury, the other hemisphere then combining the functions of both. And if parts of a hemisphere are removed, at any rate early in life, the roles of these parts are often taken over by parts of the other hemisphere. Brain operations that remove substantial parts of the brain are not infrequent. It might be possible one day to remove a whole hemisphere without killing the patient, the other hemisphere taking over its functions as sometimes happens when one hemisphere is incapacitated by a stroke. But then we must reject the brain criterion of personal identity, for in such a case there will be personal identity without brain identity, the survivor only having part of the brain of the original person.

Admittedly in this case we have the rest of the body to hang on to, so we could appeal to the original bodily criterion of personal identity to justify our judgement. But an obvious extension of the case shows that this manoeuvre gets us nowhere.

Let us suppose that half of a man’s brain is destroyed and then the remaining half transplanted into another body with consequent transference of memories, personality and character traits. Here we can neither appeal to the original bodily criterion of personal identity nor to the brain criterion to justify the judgement that the surviving person is the brain hemisphere donor. Yet it seems quite clear that if we accept that Brownson is Brown in the original Brown/Brownson case we cannot deny that in this case also the survivor is the original brain-hemisphere donor. For if we accept that a person goes where their brain goes, it cannot make any difference if their brain in fact consists of only one brain hemisphere combining the functions usually divided between two.

This line of thought thus leads us away from both the original bodily criterion of personal identity and its too-simple modification, the brain criterion. But it does not yet force us to accept that personal identity does not consist in the persistence of any physical entity. Rather, we are led to what I shall call the physical criterion of personal identity, a version of which is put forward in Wiggins (1967) and discussed in Parfit (1984). The simplest formulation of this suggestion is that what is necessary for personal identity is not identity of the whole of the brain but identity of enough of the brain to be the brain of a living person: person P2 at t2 is the same person as person P1 at t1 if and only if enough of the brain of P1 at t1 survives in P2 at t2 to be the brain of a living person.

Objections to the physical criterion

The physical criterion of personal identity does not provide an easy stopping place, however, for someone who has been persuaded by the Brown/Brownson case and the brain-hemisphere transplant case to reject the bodily and brain criteria of personal identity. For if one is persuaded by these cases, yet another piece of science fiction leaves one with no convincing defence of the physical criterion.

The piece of science fiction in question is one employed by Bernard Williams (see Williams 1970 and ‘Are Persons Bodies?’ in Williams 1973). Williams is in fact one of the few modern writers on personal identity who have resisted the conclusion that Brownson is Brown, and his argument ...