- Locke defined 'person' as; 'a thinking intelligent being, that has reason and reflection, and can consider itself as itself, the same thinking thing in different times and places: which it does only by that consciousness, which is inseparable from thinking, and it seems to me essential to it' (Essay concerning Human Understanding, II.xxvii.9). As Locke saw, this definition gives 'person' a potentially wider application than the term 'human being', where the latter is understood as referring to members of a particular biological species. Locke implies that a 'rational parrot' he describes would count as a person, though not as a 'man' (i.e. human being), and he could have cited the fact that God is described by believers as a divine person.
- Locke's definition suggests that what marks off persons from other subjects of mental states is rationality and the possession of 'reflection', or self-consciousness1. It has also been held to be distinctive of persons that they are capable of language use, that they are social creatures, and that they view each other and themselves in terms of the concepts of morality (thus Locke's observation that 'person' is a 'forensic2 term, appropriating actions and their merit' (Essay, n.xxvil.26)). All of this requires that the mental states of persons include 'higher-order' beliefs, desires and intentions, whose contents include reference to other mental states, either those of the creature itself, as in self-consciousness3, or those of other creatures, or both, as in language use and reciprocal social relations (see "Dennett (Daniel) - Conditions of Personhood", 1976). It has also been held that it is distinctive of persons, and a requirement of the free will that has been thought the special province of persons, that they be the subject of a certain sort of second-order desires and intentions - desires or intentions to have (or not to have) certain first-order desires (see "Frankfurt (Harry) - Freedom of the Will and the Concept of a Person", 1971).
- The use of the term 'person' commits one to no particular view on the mind/body problem. But for most philosophers who have used the term, it is constitutive of the concept of a person that persons are (at least normally) embodied, and the subject of bodily as well as mental properties - as "Strawson (Peter) - Individuals: An Essay in Descriptive Metaphysics" (1959) has put it, they are equally subjects of 'P-predicates', which imply the possession of consciousness, and 'M-predicates', which can be shared vrith inanimate objects. Some see embodiment as required for the very existence of mental states, not (at least directly) because they are committed to materialism4 (see physicalism5, materialism)6, but because they think that the individuation7 of mental states requires their embodiment. This will be true on any views on which mental states are partly constituted by their relations to behaviour, and it will be true on 'externalist' views about mental content, according to which the intentional (representational) content of mental states (see intentionality) is determined in part by the nature of the environment with which the subject is in perceptual contact. Despite Locke's official adherence to a Cartesian view (he thinks that there is in each of us an immaterial substance which is that which thinks in us), his conception of persons is similar to Strawson's. (See Descartes.) A person is a 'thinking intelligent being', and while it is unclear how the thinking of a person is supposed to be related to the thinking of the associated immaterial substance, it seems clear that Lockean persons are subjects of both P-predicates and M-predicates. And, strikingly, Locke denied that the identity over time of a person consists in, or requires, the identity of an immaterial substance.
- It is true in general that saying what sort of thing an F is involves indicating what the identity conditions for Fs are - what count as parts of the same F, and what counts as events, phases, or stages in the history of one and the same F. And so a good reason for inquiring into the nature of personal identity, into the identity conditions of persons, is that this can be expected to throw light on what persons are. Another good reason for inquiring into this is that it provides a way of addressing the metaphysical puzzles - about change, substance, etc. - that arise whenever the identity over time of 'continuants' (see continuant) is addressed. But neither of these good reasons can account for the amount of attention which the topic of personal identity has attracted since Locke's Essay pushed it into prominence as a philosophical topic. Part of the explanation of this must be that we care about the identity of persons in a way in which we do not care about the identity of other things. Other things being equal, the destruction-cum-replacement-with-an-exact-duplicate8 of a chair or refrigerator will be regarded as 'as good as' its continued existence. But we do not feel this way about our family and friends. And in particular we do not feel this way about ourselves. A central fact about persons is that each of them has a 'special concern' for his or her future well-being; and because of this, each of them also has derivative special concern for the future well-being of the persons with whom he or she identifies because of ties of love and friendship. It is partly this that makes intelligible the involvement of the notion of personal identity in moral and legal practices which lies behind Locke's observation that 'person' is a 'forensic9' term. The various sanctions that enforce morality depend on the existence of this special concern, as do such ideas as that a person can justly be held accountable for past actions, and that goods bestowed on a person can compensate for injuries inflicted on that person at an earlier time. The existence of this special concern contributes to interest in personal identity in two different ways.
- First, because part of this special concern is a desire to exist in the future, a desire for 'survival', persons have an interest in whether the nature of personal identity is such as to allow them to survive bodily death - and discussions of personal identity have often been linked with discussions of the prospects of personal immortality.
- Second, given that we have this special concern, there is a natural interest in finding an account of personal identity that makes our having it intelligible and rational.
- A different source of the interest philosophers have in personal identity - over and above the interest they have in the identity over time of continuants generally - is its distinctive epistemology. At first look, our judgements about the identity over time of persons other than ourselves seems to be grounded in much the same way as our judgements about the identity over time of other things; we go on such things as similarity of observable properties, or spatio-temporal continuity and continuity with respect to observable properties. But the memory-based judgements we make about our own past histories are not so grounded; when I say on the basis of memory that it was I who mowed the lawn yesterday, my judgement will not be based on remembered information about the person who did the mowing (I was not watching myself in a mirror as I mowed), or on observed continuities linking that person with my present self. And of course this direct memory-based knowledge of personal identity can be conveyed to others; so the epistemology of third-person judgements of personal identity, as well as of first-person judgements, is importantly different from that of our judgements about the identity of other sorts of things. This raises the question of what personal identity can be, that it can be known in this distinctive way.
- If one approaches the problem of personal identity from the mind/body problem, it may seem at first as if there should be just two possible solutions to it, one that will be favoured by Cartesian dualists and one that will be favoured by materialists10. The first asserts that the identity of a person over time consists in the identity over time of an immaterial mind or soul, while the second asserts that the identity of a person over time consists in the identity over time of a living human body. In the influential chapter of the Essay that began the modern history of the topic of personal identity, John Locke rejected both of these solutions. His own solution may seem to have the same form as those he rejects; he says that personal identity consists, not in sameness of immaterial substance (soul) or sameness of material substance (body), but in sameness of 'consciousness'. But Locke did not think that associated with each person there is a single entity, a 'consciousness', that necessarily exists just as long as the person exists (in the way the other views hold that associated with each person there is a soul, or a body, which necessarily exists as long as the person exists). By 'consciousness' he mainly means memory, and his view was that it is memory that links together, and unites into the history of a single person, the different parts of a person's life. On this view the distinctive epistemology of personal identity, the immediate access each person has in memory to his or her past reflects its metaphysical essence.
- Much of the support for the memory theory comes from thought experiments11 similar to one presented by Locke: 'Should the soul of a prince, carrying with it the consciousness of the prince's past life, enter and inform the body of a cobbler, as soon deserted by his own soul, everyone sees he would be the same person as the prince, accountable only for the prince's actions' (Essay, II.xxvii.l5). A similar story, which avoids the dualistic overtones of Locke's, involves the brain of one person being transplanted12 into the body of another, with the result that the brain recipient remembers the past life of the brain donor. The dominant intuition about such cases is that the person 'changes body'; this seems to count both against the view that personal identity consists in bodily identity and in favour of the memory theory. (The intuition about Locke's example is compatible with the view that personal identity consists in soul identity, while that about the brain transplant13 example is compatible with the view that personal identity consists in brain identity. Locke, however, thought that the 'consciousness' of a person, and with it the person, could be transferred from one immaterial substance to another; and as we shall see, neo-Lockeans have made similar claims about brains.)
- A famous counterexample to Locke's theory is Reid's 'brave officer' example, in which as a young officer a man remembers being punished as a small boy for robbing an orchard, and much later, as an old general, remembers the brave deeds of the young officer but has no recollection of the childhood incident. This refutes any version of the memory theory that implies that one's past includes only what one remembers. But it does not refute the more sophisticated 'memory continuity' theory, according to which two different 'person stages' (temporal slices of personal histories) belong to the history of the same person just in case they are members of a series of such stages, each member of which contains memories ('from the inside') of actions or experiences belonging to the preceding member of the series (see Temporal Parts, Stages). A common objection to the memory theory, first raised by Butler, is that it is circular. This has two versions.
- One is that the notion of personal identity enters into the contents of the memories the having of which is supposed to constitute personal identity. This assumes that these memories are irreducibly first-personal in content - memories expressible by saying 'I remember that I did so and so.' But the memory theorist can claim that the relevant memory facts can be expressed without using the notion of personal identity and without invoking such contents, namely by speaking simply of memories that are of particular experiences or actions.
- A different version of the circularity objection claims that the notion of memory itself must be defined in terms of the notion of personal identity, the idea being that in order to distinguish genuine remembering from mere seeming to remember we must impose the condition that in order to remember a past event (action, experience) one must be, i.e. be the same person as, someone who witnessed the event (did the action, had the experience). Memory theorists have attempted to meet this objection by invoking the concept of causality14. There are independent reasons for thinking that the remembering of a past event involves there being an appropriate causal connection between the past event remembered and the subsequent memory impression of it. By using this requirement to distinguish genuine remembering from mere seeming to remember, we can avoid having to use the notion of personal identity in making this distinction, and thus avoid the threatened circularity in the definition of personal identity in terms of memory - or rather, we can do this if the 'appropriate causal connection' can be characterized without use of the notion of personal identity.
- But once we have seen that the memory theory must invoke the concept of causality15 in its account of personal identity, it becomes apparent that there is no reason to hold that the only causal connections that are constitutive of personal identity are those involved in memory. If we reflect on the brain-transfer case mentioned earlier, it seems plausible that the identity of the brain recipient with the brain donor consists as much in the fact that his personality, interests, skills, etc., are causally linked to those in the donor's past history (via a distinctive causal chain carried in the brain) as it does in the fact that his memories are causally linked to past episodes in the donor's life which they represent. This suggests a refinement of Locke's view according to which personal identity consists in a sort of psychological continuity16 and connectedness17, 'psychological C&C' ("Parfit (Derek) - Reasons and Persons", 1984), that is, in there being such causal links between successive phases of a person's mental life (between successive 'person stages'), and that memory continuity is just a special case of this.
- Such a view needs to be refined to deal with the possibility of 'fission' of persons, for example, with the version of the brain-transplant18 case in which the two hemispheres of someone's brain are transplanted19 into the (vacant) heads of two different bodies, with the result that there is psychological C&C between the state of one person before the operation and the states of two different persons after it. Clearly, the two offshoots cannot both be identical to the one person of whom they are both psychological duplicates20, since they are not identical to each other. Such a case can be handled by saying that what constitutes personal identity is not psychological C&C simpliciter, but rather non-branching psychological C&C. Alternatively, one could say that psychological C&C is the relation that 'continues' persons in existence, and that it is the 'closest continuer21' of a person who is identical with him: in the fission case the two continuers are equally close, so neither is the original person (see "Nozick (Robert) - Philosophical Explanations", 1981).
- As noted earlier, while the standard intuition about the brain-transfer example goes against the view that personal identity consists in the identity of human bodies, it does not go against the view that it consists in the identity of human brains. If it is psychological C&C as such that constitutes personal identity, it seems that there ought to be possible cases in which a person at one time is the same as a person at another time in virtue of a psychological C&C series of mental states that is not carried by any single physical object. A putative case of this is the 'teleportation22' of science fiction. We can think of this as involving a process whereby a brain is scanned, and at the same time destroyed, and the information obtained from the scanning is used to create a physical duplicate23 of it (either by the restructuring of an already existing brain, or by the building of a new one). Opinions differ sharply about whether such a procedure could be 'person-preserving'. A difficulty for those who think it could be is the 'branch-line case', in which the scanning procedure fails to destroy the original brain. Here there are, after the procedure, two persons (the owner of the original brain, and the owner of the duplicate)24 whose mental states are psychologically C&C with those of the original person. The dominant intuition about this case is that the owner of the original brain is identical to the original person and that the other person is a mere psychological duplicate25 of him. A common - although not uncontested - intuition is that if this is so, then in the case where the original brain is destroyed it is likewise true that the owner of the duplicate26 brain is a mere psychological duplicate27 of the original person.
- Recent debate over the nature of personal identity has mainly focused on the memory theory and its descendant, the psychological C&C theory. Opponents of this view have been divided among those who hold that bodily identity, or at any rate some sort of physical continuity28, is needed for personal identity (see "Unger (Peter) - Identity, Consciousness and Value", 1990; "Williams (Bernard) - The Self and the Future", 1970), and those who, following in the tradition of Butler and Reid, hold that no reductive analysis, or constitutive account, of personal identity is possible (see "Chisholm (Roderick) - Person and Object", 1975, "Swinburne (Richard) - Personal Identity: The Dualist Theory", 1984). In recent discussion of personal identity the emphasis has shifted somewhat from the question of what it consists in to the question of why it matters. Here too the cases of fission, teleportation29, etc., have played a prominent role. It seems plausible that someone who knows that she is about to undergo fission might naturally, and rationally, have the same sort of concern about the future well-being of both offshoots as one normally has about one's own future well-being, even though she realizes that, strictly speaking, she will be neither of them. This has been used to argue that what we really care about, in our desire to survive and our 'special concern' for the future, is not identity as such, but rather the psychological C&C that normally constitutes it. And it has been urged that recognition that identity as such does not matter would have a beneficial effect on our attitudes towards ourselves and others (see Parflt, 1984). But the natural view that it is identity that matters has staunch and resourceful defenders (see "Sosa (Ernest) - Surviving Matters", 1990; Unger, 1990
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