Introduction 1. introduction

THE philosophical problems of identity have deep links with a number of the other areas of this book: universals, through the idea of sameness and difference of kind; being, through the idea of what are the most fundamental things which remain through change; mind and body, through the idea of the identity theory of mind and body; and time, through the idea of sameness and difference over t ime. Our concern in this chapter wil l largely be with the last of these; and with the problem of personal iden-tity as a special case of the problem of change. We will also look at the question of the relationship between a thing and its parts at a time.

These traditional problems only have a distant connection with questions about one's own 'identity' as these might arise in everyday life: that is, one's racial, social, or sexual identity. Questions of identity in this sense are really questions about one's most import-ant properties or relations, those properties (determining sexual orientation, for exam-ple) or relations (to a social group, for example) which are most important in determining who one is. Whatever philosophical problems arise with the notion of one's identity in this sense do not fall within the scope of tradit ional metaphysics.

2. The features of ident i ty In his Tractatus, Wittgenstein said, 'to say of two things that they are identical is nonsense, and to say of one thing that it is identical wi th itself is to say nothing at all' (Wittgenstein 1921: 5.5303). Some philososphers have seen this fact as indicating why there is no real problem of identity. David Lewis, for example, writes:

identity is utterly simple and unprobletnatic. Everything is identical to itself; nothing is ever identical to anything else except itself. There is never any problem about what makes something identical to itself; nothing can ever fail to be. And there is never any problem about what makes two things iden-tical; two things can never be identical. (Lewis 1986b: 192-3)

4 is true that identity is, in a certain way, a very simple concept. Identity statements c a|ms about the identity of thingsare often very easy to understand. Let's take a simple

ample. The original name of the English novelist George Eliot was 'Mary Ann Evans'. w since the English word 'is' is used to express other ideas than identity (as in The Pe is Catholic', where it is the 'is' of predication), we shall use the ' = ' sign to indicate

what is being expressed is identity. We can then express the true identity claim thu cha

s- Mary Ann Evans = George Eliot'. The relation expressed by the ' = ' sign can be scterized in terms of certain distinguishing logical features: symmetry, reflexivity, 'tivity, and being governed by what is known as Leibniz's Law of the Indiscernibility

fldenti

of ident

ra/s. Box VIII.2 describes the first three of these logical features of the relation

and | ty. What is of interest to us here is Leibniz's Law. This says that if any objects A a r e identical, then whatever is true of A is true of fi (that is, they are indiscernible).

528 IDENTITY

Box V I I I . 1 . The ship of Theseus

Theseus was a warrior of ancient Greek legend. Thomas Hobbes (1655) tells th story that after Theseus died, his ship was placed in the harbour in Athens to hon our his memory. As time went on, parts of the ship started to decay and had to be replaced by new parts. Eventually, all the original parts of the ship were replaced No problem, we might say: Theseus' ship still exists, and is lying in the harbour After all, we think that an object can survive gradual change in its parts. But sup-pose now that the old parts of the ship were collected together in a warehouse' and assembled (in their admittedly fragile state) in exactly the same arrangement as they had in the original ship. Now there are two ships. We seemed to have already agreed that Theseus' ship is in the harbour, on the grounds that an object can survive a gradual replacement of its parts. But we should also agree that an artefact can be dismantled and reassembled in another place. Applying this idea to the case of the ship, it seems we should conclude that the ship in the ware-house is Theseus' ship. So we seem to have a reason to think that each ship is Theseus' ship. But this cannot be right; how can two ships be the same as one ship? So which one is the real ship of Theseus? (See Lowe 2002: 25-40 for further discussion.)

This law must be distinguished from the thesis of the Identity of Indiscernibles, also defended by Leibniz and also sometimes called 'Leibniz's Law'. The Identity of Indiscernibles says that if objects A and B have all the same properties, then A and B are identical. Although this might seem like an obvious truth at first sight, it is actually quite controversial for reasons which we discuss elsewhere (see the discussion in Part III: Being, section 6).

What wefollowing most contemporary philosophersare calling 'Leibniz's Law' is something which should be quite uncontroversial. For if George Eliot really is the very same person as Mary Ann Evans, then how could something be true of Eliot which is not true of Evans? If George Eliot wrote Middlemarch then it follows from Leibniz's Law and the fact that Mary Ann Evans = George Eliot that Mary Ann Evans wrote Middlemarch. Of course it does: for 'they' are the same person.

It is helpful to distinguish Leibniz's Law, formulated in this way'if A = B then what-ever is true of A is true of B'from a related but distinct principle which says that if two names refer to the same object, then they can always be substituted for one another wi thout altering the truth or falsehood of sentences in which they occur. This principle (sometimes called 'the substitutivity of co-referring singular terms') does not seem to be generally t rue. Consider, for example, the fact that some people have read George Eliot's books under the false impression that she is a man. Such people, ignorant of the identity of George Eliot wi th Mary Ann Evans, would surely not thereby believe that

'H I i ' i ' i l INTRODUCTION 529

Mary Ann Evans is a man. So the sentence,

'Some people believe that George Eliot is a man.'

can be true, while the sentence,

'Some people believe that Mary Ann Evans is a man.'

is false. Yet the sentences differ only in containing the co-referring names 'George Eliot' and 'Mary Ann Evans'. So how can one be true and the other false? There seems to be a problem, then, w i th substituting co-referring names wi th in sentences containing (for example) words like 'believes t h a t . . . '. (This is sometimes called 'Frege's puzzle of identity': see Wiggins 1976; also Salmon 1986.)

But this problem does not touch Leibniz's Law, properly understood. For since Mary Ann Evans = George Eliot, it is true of Eliot that some people believed she was a man, and it is equally true of Evans. Leibniz's Law is not a controversial thesis, but surely one of the essential characteristics of the concept of identity. (As Frege said, 'this explana-tion of Leibniz's could be called an axiom that brings out the nature of the relation of identity; as such it is fundamentally important'. Quoted in Wiggins 2001: 27.) Identity, so characterized in terms of symmetry, reflexivity, transitivity, and Leibniz's Law, seems to be the paradigm of a fundamental logical concept. So what can anyone mean when they say that there are philosophical problems of identity?

3. Leibniz's Law and the problem of change The problem arises when we begin to reflect on the obvious fact that things change. Consider a simple case: a metal bar, call it 6, is hot yesterday, and cools down so that it is cold today. Intuitively, it is the very same (i.e. identical) th ing, 8, which is hot yester-day and cold today. But being hot and being cold are incompatible properties: nothing tan be both hot and cold. Yet Leibniz's Law implies that if 6 today = B yesterday, then | . | is cold today then B is cold yesterday; and if 8 is hot yesterday so is B today. Genuine identicals must be indiscernible. So how then can anything change and remain the same thing? (See Box VIII.1: The Ship of Theseus.)

Before considering the possible solutions to this ancient problem, let's just clarify what e mean here by change. When an object changes, a predicate (like 'is cold') is true of it ich was not previously true of it. But the mere idea of a predicate's coming to be true mething does not capture our idea of change, since a predicate can come to be true

gj ornething when it has not changed in any way. When you first discovered that umber pi was irrational, for example, the predicate 'x is thought by you to be irra-

tornes to be true of pi. But surely the number pi has not changed at all; indeed, if ers exist at all, they cannot chanqe. At best what we have here is what has been

ca||ecj 'm . nere Cambridge change' (see Part VII: Time and Space, section 3) of which real

cnariQp ic i .. a species. What real change involves is the gaining or losing of an object's Pro ProPerties, those properties it has in and of itself, as opposed to its relational

les, those properties it has because of its relationships to other things. An object's mxrinsic property, for instance, and a change in an object's mass is a paradigm

530 IDENTITY

case of real change. An object's position in time, by contrast, seems to be a n example of a relation or a relational property, and a change in an object's posit' . 'c does not seem to be a real change in the object (for more on the notion of i

ntrinsic Prop-es raise

taveFat

erties, see Part IV: Universal and Particulars). Relations and relational proD interesting questions of their own, and some changes in relations can be real r-h 1

cnanges but we have enough trouble dealing with change in intrinsic properties, so we will ' trate on this. Our problem here is about how an object can remain the same whil h ing its intrinsic properties, and for this reason David Lewis calls it 'the prohl temporary intrinsics' (Chapter 41). But we shall continue to call it 'the problem of ch

The problem of change involves three assumptions, which it will be convenient ton

1. Change: an object can have an intrinsic property F at one time and not ha another time.

2. Identity across time: the same identical object can exist in its entirety at differe times.

3. Leibniz's Law: if A B, whatever A and B are, then whatever is true of A is true of B and vice versa.

Our metal bar has the intrinsic property of being hot, at one time, and the intrinsic prop-erty of being not hot, at another time (Change); it is the same entire bar which exists at both times (Identity across time); given these two assumptions, the conflict with Leibniz's Law is obvious. If the bar at the earlier t ime is hot, and is identical with the bar at the later time, then according to Leibniz's Law the bar at the later time should be hot. But it is not, so how can it be identical wi th the bar at the earlier time? It looks as if one of (1)-(3) should be given up. But which?

Can Change be denied? Recent philosophy provides us wi th two radical ways of deny-ing it. In Chapter 40, Roderick Chisholm argues that, strictly speaking, material objects cannot survive change in their properties. The important phrase here is 'strictly speak-ing'. Chisholm follows the eighteenth century philosopher Bishop Butler in distinguish-ing identity in the 'loose and popular' sense from identity in the 'strict and philosophical' sense. In the strict and philosophical sense expressed by Leibniz's Law, no object can be F and not-F. Here Chisholm grasps the nettle and accepts that in this sense objects do not change; when an object loses a property, it ceases to exist. (Similarly, when an object loses a part, it ceases to exist. The theory of parts and wholes is called mereology. Hence Chisholm's view about an object and its parts is known as mereological essentialism: the view that an object's parts are essential to it.)

But although he denies that one object can really undergo change, Chisholm main-tains that one can carry on talking as if there is one object existing over time, since it is convenient to do so; and he gives an account of how 'successive entities' (entia succes-siva) may be related in a way that mimics genuine identity over time. Using a phrase of Hume's, Chisholm says that we 'may feign identity when what we are dealing with is in fact only a succession of related objects'.

The second radical solution is to give up Change by giving up its implicit assumption that objects exist at more than one time, whether they change or not. This is the presentist view that only the present is real, or exists (see Part VII: Time and Space, section 4). Since, according to presentism, the past and the future do not exist, neither do any objects

INTRODUCTION 531 I exist in the past and the future. All objects exist only in the present, and since no object has incompatible properties in the present, no object has incompatible properties when it exists. If presentism is true, then the problem disappears.

But presentism is a very radical view. It says the past and future have no reality at all. Yet there are clearly truths about the pastfor example, that we were bornand about the futurefor example, that we will die. What makes these truths true, if not facts about the past and the present? Another question which the presentist has to answer is when exactly the present is: how long does it last? It seems that as soon as we try and refer to the present, it ends up disappearing ' into ' the past.

Lewis's approach, in Chapter 41, is to deny Identity across time (as defined above). Instead of th inking of the whole metal bar as being the thing to which the properties of hotness and coldness belong, Lewis instead argues that these properties actually belong to parts of the bar. The bar yesterday is what is hot, and the bar today is what is cold; and on Lewis's view the bar yesterday and the bar today are parts of the whole bar, which is extended across its whole life in space and time. The bar yesterday and the bar today are temporal parts of the bar. And just as there is no problem with the bar being cold at one of its spatial parts (say, at one end) and hot at another, there is no problem with the idea that it might be cold at one of its temporal parts and hot at another. Since it is not being claimed that one thing is both hot and cold at the same t ime and place, the problem disappears.

Lewis describes his approach in terminology which has now become standard (the term-inology comes from Johnston (1987b)). Say that things persist when they exist at more than one t ime. Then there are two ways for something to persist: either by existing in their entirety at each moment, which Lewis calls 'enduring'; or by being extended across space-time, and at each moment a spatio-temporal part of the object existing. This he calls 'perduring'. The endurance approach to the problem of persistence treats objects as three-dimensional entities, extended across the three spatial dimensions. The perdu-rance approach treats objects as four-dimensional entities, extended across space and time. Hence the perdurance approach is sometimes called 'four-dimensionalism' (see Sider 2001; the idea derives f rom Quine 1961b; see Part VII: Time and Space, for the idea of a dimension). Four-dimensionalism about objects is distinct from four-dimensionalism about space and time. One could believe in the four-dimensionalist view of space-time without believing that objects are four-dimensional entities (see Mellor 1981: chapter 8). The distinctive feature of four-dimensionalism about objects (or perdurance) is that objects have temporal parts.

These are the proposed solutions to the problem of change which involve giving up Change and Identity across time. But if we cannot be persuaded to give up these assumptions, what should we say? Can we give up Leibniz's Law? Surely not. To think that something could be true of A but not true of 8 when A just is B is mind-boggling. IT we gave up Leibniz's Law, essential as it is to the notion of identity, then it would cease to be clear what it is that we are holding on to when we say that Identity across time is true. But perhaps there are other options.

Perhaps we should take more seriously into account the time at which properties are Pad. One way to do this is to say that the kinds of properties we are talking about Properties that an object can have or lack throughout its lifeare 'time-indexed': that is,

532 IDENTITY

they are only ever had at a time, so specifying what properties a thing has should specifying when it has them. Another way of saying the same kind of thing is to

ln-volve

say that all (temporary) properties are really relations to times: to say that the bar is hot tori not really to ascribe to it an intrinsic property hotness, but to say that it stands in th tion being hot at to a time, namely yesterday. (This is discussed by Lewis in Chapter 41 also by Merricks 1994.) The essence of this solution is that the bar's being hot-at-tirrtet cold-at-time t + 7 are not really conflicting properties, so it is not the case that someth' is true of the bar yesterday which is not true of it today.

The diff iculty wi th this approach is that it while attempting to account for change, it actually seems to eliminate it. For we previously said that real change w something having a property at one time and not having it at another. However, if p ror i erties are essentially 'time-indexed' in this kind of way, then it is not true that objects which change come to lack the very same properties which they previously had. For if the heat of the bar is really being-hot-at-t, then the bar does not come to lack this pron-erty: for today (t + 7) it is equally true that the bar has the property of being-hot-at-t And it will always have this property. So it looks as if this approach to the problem ends up denying Change after all. (For an alternative way of relating identity to time, see Gallois 2003.)

4. The problem develops: parts and wholes In any case, whatever moves we make to defend this approach, it seems that bringing in a reference to the time at which a property is had will not solve all the problems of identity. In particular, it does not help with the problem of how an object is related to its parts, and how it can survive the loss of its parts. This problem has been vividly presented in an argument of Peter van Inwagen's (1981). (A similar kind of argument was originally raised by Geach 1962, in defence of his thesis of 'relative identity', and discussed inter alia by Wiggins 1980 and Lowe 1989.) We will brief ly expound this argu-ment here; it has connections with the discussion of substance in Part III: Being.

Consider a particular human bodywhich we shall simply call 'Body'on a particular day, say Monday. Body has as one of its parts its left hand. Now consider the rest of Body on Monday, considered apart from its left hand. We can call this 'Body-minus-its-left-hand' or 'Body-minus' for short. Body-minus is that piece of matter consisting of Body minus its left hand. It is plain that

(1) Body on Monday Y= Body-minus since Body has a hand and Body-minus does not. Now suppose that on Tuesday, Body's left hand is amputated. Body still exists, we want to say; it has only lost its left hand. It is natural to insist that Body is still the numerically same object after the amputation; that is,

(2) Body on Monday = Body on Tuesday

But it seems tha t Body on Tuesday is the same th ing as Body-minus, since that was defined as Body minus its left hand, and that is what Body on Tuesday seems to be,

KiiaiMiidK

INTRODUCTION 533

Box V I I I . 2 . The logical propert ies o f iden t i t y

Identity is normally considered to be a relation (see Part IV: Universals and Particulars). If it is a relation, then it is different from many other relations. For many relations (e.g. being taller) hold between at least two things, whereas we know that identity only holds between a thing and itself. If A and B are two objects, then 'A = B' is false. But there are other relations which can hold between a thing and itself: a vain man can admire himself, for example.

Symmetry, reflexivity, and transitivity are among the most important logical properties of relations. Suppose R is a relation, a, b, and c are related objects, and a sentence of the form 'aRb' says that a bears relation R to b. Then we can describe these three important properties as follows:

Reflexivity: aRa Symmetry: if aRb then bRa Transitivity: if aRb and bRc then aRc

Some relations have some of these properties, some have them all, some have none. The relation of loving, for example, is neither symmetrical, reflexive, nor transitive. If Anthony loves Cleopatra, then it does not fo / /owthat Cleopatra loves Anthony, even if it is true (loving is not symmetrical); if Anthony loves Cleopatra, then it does not fo l low tha t Anthony loves Anthony or that Cleopatra loves Cleopatra (loving is not reflexive); and if Anthony loves Cleopatra and Cleopatra loves Caesar, then it does not fo l low that Anthony loves Caesar (loving is not trans-itive). Other relations have some but not all of these properties: being taller, for example, is transitive, but not reflexive or symmetrical.

Identity is reflexive, transitive, and symmetrical. Any relation wi th all these three properties is called an equivalence relation. There are other equivalence relations apart from identity: having the same biological father as someone else s an equivalence relation, for example.

too. So:

>0 Body on Tuesday = Body-minus.

ut(2). (3), and the transitivity of identity (see Box VIII.2) imply (4) Body on Monday = Body-minus,

which contradicts our assumption (1).

on what argument is very worrying for someone who believes in endurance, since it is based

seem to be (at first sight) very innocuous assumptions. And notice that it would not say that properties are 'time-indexed'even if we originally found a solution of that

P ausible. There is no room for this solution here. Some other solution is needed. th . nwa9en's own solution is that Body-minus does not exist. But Body-minus is just

implement ' of Body's left hand; so if Body's left hand exists, then isn't it arbitrary

534 IDENTITY

Bod

K

to say that Body-minus does not exist? Van Inwagen agrees, and concludes th left hand does not exist either! Van Inwagen later developed the radical view , Da" ' s

only material beings which exist are organisms and fundamental particles I Inwagen 1990 and see Part III: Being). In the context of this view, it is easy to unri ^ why he can say that Body exists, because Body is an organism (for a somewhat ' view to Van Inwagen's see Merricks 2001). ar

Another solution is to deny (2): Body on Monday is not identical to Body on T One could deny (2) because one thought, for Chisholm's reasons, that no mated I ities can maintain their identity (in the 'strict and philosophical sense') if thev ch Or one could deny it for Lewis's 'perdurantist' reasons: Body on Monday is not idp r to Body on Tuesday because they are distinct temporal parts of the four-dimensin entity which Body actually is. (For critical discussion of temporal parts, see Thomson 19S and Lowe 2002: chapter 4.)

The assumption (2) which is challenged here embodies our commonsensical assumption Change or Identity across time. Is there a response to the puzzle which can retain (2) and therefore these common-sense beliefs? Since the unacceptable conclusion, (4), follows from the transitivity of identity and (2) and (3), then it looks as if the only option is to give up (3). (Denying transitivity should really be a last resort.) In other words, Body on Tuesday is not identical wi th Body-minus. At first sight, this seems crazy. After all, Body on Tuesday occupies exactly the same region of space as Body-minus, it is made of the same matter, and it has all the same parts. So what can it mean to say they are distinct?

The defender of this view will respond that part of what it means is that Body and Body-minus have different persistence conditions: that is, what it takes for Body to continue to exist is different from what it takes for Body-minus to continue to exist. Suppose we cut off Body-minus's right hand. Then, on this view, Body continues to exist, but Body-minus ceases to exist, since Body-minus was defined as that piece of matter consisting of Body minus its left hand (and this includes the right hand). So something is true of Bodyit can survive the loss of its right handwhich is not true of Body-minus, so by Leibniz's Law they are distinct. The idea behind this approach is that entities of cer-tain kinds have persistence conditions appropriate to things of those kinds (see Lowe 1989, Wiggins 1980). It is in the nature of organisms, like Body, to persist throughout the loss of their parts; whereas made-up (or 'gerrymandered') entities like Body-minus do not have such conditions. This view involves a return to an Aristotelian view of secondary substances, natural kinds which contain within themselves the principles of their own persistence (see Part III: Being, section 5). However, the inescapable con-sequence of the view is that two distinct objects (albeit objects of different kinds) can be in the same place at the same time. And some f ind this hard to accept (see Burke 1994, Gibbard 1975).

5. Personal ident i ty Chisholm's solution to the problem of change, as we saw, was to deny that in the 'strict and philosophical sense' any material object can change its properties. We might oe happy wi th this solution'OK, nothing really is the same from day to day, but the

INTRODUCTION 535

fid be

the

entia successiva are similar enough to be grouped together as the same thing for all oractical purposes'except when we come to consider our own identity. The problem of identity over time applies equally to us; how can we persist through change? Yet i t seems unacceptable to treat persons as conglomerations of entia successiva. Surely there is a real unity which underlies our continued existence?

Chisholm agrees. He argues elsewhere that persons or people persist through change by retaining their identity (Chisholm 1976). A consequence of this, for Chisholm, is that persons are not material beings. But our ordinary conception of persons treats them as material beings. That is, we normally think of ourselves as embodied creatures w i th phys-ical and biological properties (call them bodily properties) as well as thoughts, feelings, and emotions. Which of these kinds of propertiesthe bodily or the psychologicalare most important in determining the identity of persons over time?

The problem of personal identity has revolved around this question. For it seems that there can be casesreal or imaginarywhere one's bodily properties can change rad-ically and one's psychological properties can stay the same, or where one's psychological properties can change radically and one's bodily properties stay the same (see below for some examples). Which criteriabodily or psychologicalare the criteria which determine a person's identity? Notice that the issue here is not about materialism versus dualism (see Part IX; Mind and Body). Of course, one could have a dualist conception of the person (see Swinburne 1984) which would imply the priority of the psychological crite-rion; but this does not mean tha t a materialist view of persons would have to choose the bodily criterion. For the materialist has to decide whether what determines a person's identity over t ime is the sameness of their body or the psychological characteristics embodied in their brain. Hence the choice between bodily and psychological criteria is not settled by choosing materialism; and this is illustrated by the fact that most discus-sions of personal identity in the late twent ieth century assumed materialism, but did not assume that materialism by itself solves the problem.

One materialist theory which takes the bodily criterion of personal identity very seri-ously is the animalist theory defended by David Wiggins (1980, 2001) and Paul Snowdon (Chapter 43). The heart of the animalist theory is the apparently obvious claim that we tan call express by saying 'I am an animal, of the species homo sapiens'. If this is true, then the conditions for our continued existence (our persistence conditions) are the con-ations for the continued existence of these particular animals. Snowdon recognizes that the biggest challenge to this view come from a famous style of thought-experiment hvolving imaginary brain transfers. A simple version goes as follows; suppose that your

r a i n ls Put into the living body of another person, X, and X's brain goes into your body,

here would you be then? Many philosophers have argued that in a case like this, you 0 X would have 'swapped bodies'; you would now be where X's body is, and X wou ld where your body is. The reason behind this judgement is that your identity resides in

u r Psychological characteristics, and your psychological characteristics are based in

your bra Snowd'

m. Hence, where your brain goes, there you go, too. on responds by arguing that despite the intuitive pull of this conclusion, there are

fong arguments in favour of the alternative that I am an animal. Therefore, at the very what we have here are two conflicting opinions, creating an antinomy or paradox, and

knock-down argument in favour of the psychological view of personal identity.

536 IDENTITY

Snowdon then goes on to defend the animalist line, and draws some aen yeneral corylt

about the nature of this debate; though he recognizes that there is no i ,. lQris ' 'D IG QlCrn"

the psychological response to the brain-transplant thought-experiment (see n\ 'SSal of c

'-''Son 19Q7t a different defence of animalism). 3/ 'or

Like many philosophers (e.g. Johnston 1987a), Snowdon is attempting to terms of what has become an intractable debate. A similar attempt was mad 6 ^ e

inf luential paper by Derek Parfit (Chapter 42). Parfit argued that th or u, er" . . . . . . .

Db!ems of will

personal identity can be resolved by shifting our attention away from the aupct-I h o I r l n n t l n l \ , , I + U A o r D T o r. ol o o + o o o o + k o r o . . o r + ; o o ' , , , r . r+ tr U ,.! be identical wi th A or 8?', and onto another question, 'what is it that matter

most t0 thought. us in our continued existence?' Parfit uses a version of the brain-transplant

experiment where a person apparently divides, rather than just having its brain into another body. (This version is called a 'fission' case, and is in many ways anal to the case of the Ship of Theseus: see Box VIII.1.)

Suppose a person A divides into two others, 8 and C (see Parfit's paper for a desr' t ion of how such division might come about). 8 and C are, at the time of the divisi indistinguishable. The question then is: is A identical wi th either of the resulting peon| ' 8 and C? Because identity is a one-one relation (one thing cannot be identical with twol A cannot be identical wi th both. But since 8 and C are indistinguishable in their qual-ities, there is no basis on which to say that A is identical wi th one rather than the other So it looks as if A is identical wi th neither. Or at the very least, it looks as if we cannot determine whether A is identical with 8 or C. Maybe there is some fact of the matter about A's continued identity which we can never know. But it seems as if we have already taken into account any facts which could determine A's identity when we told the story; so A's identity on this conception would be a very mysterious 'further fact'. To say that A is identical wi th 8 or C, but we do not know which, sounds rather implaus-ible (though it does have its defenders: see Chisholm 1976).

So suppose we do agree with Parfit that A is identical with neither 8 nor C. Does this mean that A fails to survive the fission in any sense? Only if, Parfit argues, survival pre-supposes identity. He then makes a case that it does not: A can survive as 8 and as C. Survival, unlike identity, can be a one-many relation (one thing can survive as many others), and Parfit says that what should matter to us in a case like this is the continuity of our psychological traits, our thoughts and intentions and so on. The continuity of these does not presuppose identity. So identity should not matter to us.

One way to see the plausibility of Parfit's case is to ask yourself whether fission would be as bad as death for you. Suppose you had the choice between painless fission and painless obl i terat ion. Would you be indifferent between these two options? If you would prefer fission, then it seems that you think fission is not as bad for you as death. And if it is not as bad as death, then maybe this is because it is not death: something of you is surviving. However, it should be noted that it is controversial to draw substantial metaphysical conclusions from these imaginary cases; we should also consider the ways in which the conclusions we draw depend on how the stories are told (Williams 1973. chapter 4). And the defender of personal identity over time will resist the suggestion that the only reason that fission might be preferable to death is that one person can survive as two; for on their view, fission, unlike death, at least gives you a 50 per cent chance of surviving as the same identical person.