Book Excerpt - Chapter 1, "Personal Identity" by Harold W.Noonan, 2003

Chapter 1
AN INITIAL SURVEY

1.1
Introduction
What am I? And what is my relationship to the thing I call 'my body'? Thus each 
of us can pose for himself the philosophical problems of the nature of the self 
and the relationship between a person and his 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. Man has always hoped to survive his 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; whilst 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. 

1.2
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.

1.3
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¡ªfor artefacts can be repaired 
and patched up and living things are necessarily involved in a constant exchange 
of matter with their environment¡ª 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 that view, to be discussed at more length in Chapter 11, is 
in fact called by its author (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 after-life. For it does not. What it 
does exclude, however, is any possibility of an after-life otherwise than by 
resurrection. But that can hardly be regarded as a conclusive objection to it. 

1.4
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¡ªthe brain¡ªseems 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 his body, but the resulting person 
would be paralysed. Even so, he 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 him to send messages in Morse
Code. Another device, attached to some sensory nerve, could enable him 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'.
But a fairly simple modification of the Bodily Criterion can accommodate the
Brown/Brownson case, whilst 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¡ªwhich, contingently, is the brain¡ªwhich 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.

1.5
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¡ªa 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 3 or 4 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¡ªthe 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.

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 which remove substantial parts of the brain are not infrequent. It 
might bepossible one day to remove a whole hemisphere without killing the 
patient, theother 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 his 
brain goes it cannot make any difference if his 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.

1.6
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 in these papers is directed
against those who accept this conclusion.

We can imagine, he says, the removal of the information from a brain into
some storage device (the device, that is, is put into a state 
informationtheoretically equivalent to the total state of the brain), whence it 
is then put back into the same or another brain. It seems clear, he says, and in 
this he seems to be correct, that if this were done to one man, information 
being removed from his brain (for purposes of brain repair, for instance) and 
then put back, then ¡ª supposing that he recovered all his dispositions, with 
regard to memory and so forth, that he had had before¡ªwe should not dream of 
saying that he did not, at the later stage, really remember. The passage of the 
information via the device would not count as the kind of causal route to his 
later knowledge which was incompatible with that later knowledge's being memory. 
As things are, the sorts of causal route that go outside the body do not count 
for memory: if a man learns anew of his past experiences by reading what he 
earlier wrote about them in his diary, then he does not remember his earlier 
experiences. But the imagined passage of the information via the device is 
obviously not a case which would fall under this ban: the replacement of the 
information is not as such 'learning again'; it is not, then, in itself 
incompatible with the later knowledge being memory.

Moreover, Williams says, it seems pretty clear that under these circumstances
a man should be counted the same if this were done to him, and in the process he
were given a new brain (the repairs, let us say, actually required a new part). 
But, if so, the Physical Criterion of personal identity must, of course, be 
abandoned.

Yet there seems to be nothing that a defender of that criterion, who has 
followed the route charted above to his position, could convincingly say against
Williams's claim. For it is implicit in his position that the reason why (part) 
brain identity should be preferred to bodily identity as a criterion of personal 
identity is that it is the brain and not the rest of the body that carries with 
it psychological identity¡ªidentity of memory, personality and character. It is 
this alone which justifies the privileged status that his criterion of personal 
identity assigns to that particular bodily organ. But in Williams's case the 
brain no longer performs this function: psychological identity is secured 
without identity of brain or part brain, just as in the Brown/Brownson case it 
is secured without identity of body. It thus seems quite unmotivated for the 
defender of the Physical Criterion of personal identity, if he has been led to 
it by anything like the path sketched above (and what other path could there 
be?), to resist Williams's conclusion. But to accept it is to abandon his 
position.

This argument is an argument against the Physical Criterion of personal
identity considered as providing a necessary condition of personal identity. 
Further development of Williams's story provides an equally powerful argument 
against the Physical Criterion qua sufficient condition of personal identity. 
For let us suppose that before Brown's brain is transplanted into Robinson's 
body, it is wiped clean of the information contained in it, and via the use of 
Williams's brain-state transfer device is put into a state information-
theoretically identical with that of Robinson's brain. The transplant then takes 
place, but Brownson now not only has Robinson's body, he also has Robinson's 
memories, character and personality traits. What he does not have is Robinson's 
brain, and so by the Physical Criterion of personal identity he is not Robinson 
but Brown, whose brain he has. But surely this must be wrong, at least if 
Brownson is Brown in the original version of the story, and the fact that 
Brownson has Brown's brain must in this case be regarded as being quite 
irrelevant to the question of his identity with Brown; just as irrelevant as was 
the fact, in the original version of the story, that he did not have Brown's 
left leg.

There are further arguments against the Physical Criterion of personal identity.
One major difficulty its champion faces is to say something sensible about the
situation in which both brain hemispheres are transplanted, but into different
bodies. We shall have much to do with this possibility below. But at this point 
I wish to note a rather different difficulty for the Physical Criterion.
When philosophers speak of the problem of personal identity they do not use
'person' as a mere synonym of 'human being'. Rather they use it in the sense
introduced by Locke (1961): 'a thinking intelligent being with reason and
reflection that can consider itself as itself, the same thinking being, in 
different times and places'. In short, 'person' in the philosophical debate, is 
a functional term, a term that applies to something just in case it has certain 
capacities (like 'genius' or 'prophet'). The possibility of persons other than 
human beings is then something that cannot be denied, as Locke himself 
emphasized by reference to the possibility (which he was in fact rather inclined 
to accept as an actuality) of a very intelligent rational parrot. (It was in 
fact Locke's main concern to argue that such a parrot would not be a man, but it 
is implicit in his discussion that it would be a person.) But if there can be 
persons other than human beings there seems to be nothing inconceivable about 
the idea of persons other than human beings in whom no bodily organ occupies the 
role the brain occupies in the human organism. If so, however, the Physical 
Criterion of personal identity will be inapplicable to them. Consequently, 
unless one abandons the demand for a criterion of identity over time for persons 
as such, and settles for the view that there are different criteria of identity 
for different kinds of person (human persons, parrot persons, extra-terrestrial 
persons and so on) one must reject the Physical Criterion of personal identity 
even as an account of the logically necessary and sufficient conditions of 
identity over time for human persons.

I have now outlined, I believe, the main considerations which have been
influential in persuading many recent philosophers that personal identity cannot
be constituted by the persistence of any physical entity.

The memory criterion
An alternative view, for which many of these arguments seem to provide strong
support, is that personal identity is constituted by psychological factors. The
essence of this is the thought that, given the importance for our attitudes 
towards persons of their memories, character and personality traits, continuity 
in respect of these should be taken to constitute personal identity¡ªwhether or 
not this continuity is caused by the persistence of some bodily organ, such as 
the brain; and the absence of continuity in these respects involves the absence 
of personal identity, even if there is identity of body or identity of brain. I 
shall now explain what this means.

The simplest version of this view is suggested by John Locke's writings.
According to this version of the view, what is crucial to personal identity is
memory, and it is memory alone that needs to be appealed to in providing a
criterion of personal identity. This view is especially tempting because of the
fact that memory is crucially involved in our awareness, from the first-person
viewpoint, of our own identity over time. But the notion of memory is a very
wide one. I can be said to remember my 7 times table, or that Columbus sailed
the ocean blue in 1492, or that I am due to take my son to a football match next
Saturday. This is factual memory, retention of previously acquired knowledge. I
can also be said to remember how to pilot a plane or to do a handstand. This is
retention of previously acquired abilities. But there is also the memory of 
events witnessed or participated in, typically reported in the form: 'I remember 
X's Fing' (as opposed to the typical report of factual memory: 'I remember that 
X Fed'), and, as a special case of such event-memory, there is the memory of 
one's own experiences and actions, which one will report in first-person memory 
claims.

It is such experience-memory that Locke's writings suggest should provide the
criterion of personal identity over time. However, as numerous writers have
noted, this immediately leads into difficulties. I cannot now remember many of
the experiences I underwent yesterday, yet it can hardly be denied that I am the
same person as the one who underwent those experiences. Again, the account of
personal identity in terms of experience-memory appears to conflict with one of
the logical properties of identity, namely transitivity. A relation R is 
transitive just in case, if x is R-related to y and y is R-related to z then x 
must be R-related to z. Thus if x is taller than y and y is taller than z then x 
must be taller than z. But it seems that on Locke's account I, as I am now, 
might be the same person as the 19 year old who went up to Cambridge in 1968, 
but not the same person as the 11 year old who first went to grammar school in 
1961, even though the account certifies that the 19 year old is the same person 
as the 11 year old. For I might now have vivid recollections of my first day in 
Cambridge, but have forgotten all about my first day at grammar school, though 
the 19 year old I was in 1968 still had recollections of that day These 
objections to Locke's account, first made by his great eighteenth-century 
opponents Butler and Reid, are ones we will be right. But they have led most 
philosophers basically sympathetic to Locke to a
distinction between psychological connectedness and psychological continuity,
and to an explicit restatement of the Lockean account in terms of the latter
notion.

Let us say that, between P today and P twenty years ago, there are direct
memory connections if P can now remember having some of the experiences
that P had twenty years ago. Even if there are no such direct memory
connections between P now and P twenty years ago, there may still be continuity
of memory. This will be so if between P now and P twenty years ago there has
been an overlapping chain of direct memories, i.e. if P now remembers some of
his experiences of the previous year¡­and nineteen years ago remembered some
of his experiences of the year before. The Lockean account of personal identity
can then be revised to read: P2 at t2 is the same person as P1 at t1 just in 
case P2 at t2 is linked by continuity of experience-memory to P1 at t1. Let us 
refer to this as the Memory Criterion of personal identity.

1.8
The psychological continuity criterion
But although this reformulation of the Lockean idea avoids the most obvious
objections it still involves the claim that personal identity is to be accounted 
for solely in terms of experience-memory. But many modern philosophers who are
otherwise sympathetic to the Lockean approach think that not only 
experiencememory, but other psychological facts, should be taken into account in 
defining personal identity. For there is no reason to think that our concept of 
ourselves as reidentifiable individuals is so tied up with the notion of memory 
as to exclude the relevance of any other types of psychological continuity. 
Besides direct memories, there are several other kinds of direct psychological 
connection. One
such connection is that which holds between an intention and the later act in
which this intention is carried out. Other such direct psychological connections
are those which hold when a belief, or a desire, or any other psychological
feature, persists. These direct psychological connections are accessible to
consciousness, but others need not be. Thus we can count as direct psychological
connections the links between childhood experiences and adult character traits,
fears and prejudices. In general any causal links between past factors and 
present psychological traits can be subsumed under the notion of psychological
connectedness.

We can now define psychological continuity generally in the way we
previously defined continuity of experience-memory, namely as the holding of
overlapping chains of such direct psychological connections; and then define
personal identity over time by saying that P2 at t2 is the same person as P1 at 
t1 if and only if P2 at t2 is psychologically continuous with P1 at t1. Let us 
call this the Psychological Continuity Criterion of personal identity. However, 
like the other accounts of personal identity already considered, this proposal 
is not without its difficulties. There are two main lines of objection.

1.9
The circularity objection
The first, originally brought against Locke by Butler, is that the Criterion is
viciously circular. Memory cannot occur as an ingredient in a definition of
personal identity because memory already presupposes personal identity¡ªas
knowledge in general presupposes truth. It is not absolutely clear, when 
Butler's words are read in context, exactly what point he had in mind. But the 
argument his words have suggested to the opponents of the Psychological 
Continuity Criterion is the following. We distinguish between veridical and 
apparent memory and accept without difficulty that people can seem (to 
themselves) to remember
doing things which they did not do, which were in fact done by other people (the
standard example of this given by Flew 1951, is that of George IV, who in his
declining years 'remembered' his dashing leadership at the Battle of Waterloo¡ª
though he was not even present on that field). But how is this distinction to be
made if not by an appeal to personal identity? If so, however, memory not only
entails but presupposes personal identity: in the sense that the conclusive
verification of the proposition that someone genuinely remembers F-ing must
involve the conclusive verification of the proposition that he, that same 
person, did indeed F. To know that someone genuinely remembers F-ing one must 
know that he F-ed. Consequently personal identity cannot be defined in terms of 
memory since one must already be in possession of the concept of personal 
identity, and be able to determine that it applies, in order to be in a position 
to operate with the concept of memory at all.

The customary reply to this objection by modern defenders of the Psychological 
Continuity Criterion, originally given by Sydney Shoemaker, is that while this 
may be true of the concept of memory one can define a more general
concept of quasi-memory, of which it is not true, but which is in all other
essential respects identical with our ordinary concept of memory. In particular,
quasi-memory, like memory, is capable of yielding knowledge of the past which
is based neither on evidence nor testimony. Psychological continuity can then be
redefined in terms of quasi-memory (and the other types of direct psychological
connections mentioned above, generalized where necessary in an analogous
fashion) and the Psychological Continuity Criterion of personal identity cleared
of the accusation of circularity.

1.10
The reduplication argument
We shall be considering the circularity objection to the Psychological 
Continuity Criterion of personal identity and the Shoemaker response to it in 
more detail objection to the Psychological Continuity Criterion remains.
This is the famous Reduplication Argument originally proposed by Bernard
Williams (1956¨C7). Williams imagines the case of a man he calls Charles who
turns up in the twentieth century claiming to be Guy Fawkes:

All the events he claims to have witnessed and all the actions he claims to
have done point unanimously to the life of some one person in the past¡­Guy
Fawkes. Not only do all Charles'memory-claims that can be checked fit the
pattern of Fawkes' life as known by historians, but others that cannot be
checked are plausible, provide explanations of unknown facts and so on.
(1956¨C7:332)

It is tempting in this case to identify Charles, as he now is, with Guy Fawkes, 
in other words to regard the case as one of reincarnation. For what Williams is 
in effect supposing is that the evidence available in the case is everything for 
which believers in reincarnation could possibly wish. But, Williams argues, one 
is not obliged to do so, and in fact so to describe the case would be vacuous. 
For if this were to happen to Charles it could also happen simultaneously to his 
brother Robert There would then be two equally good candidates for identity with 
Guy Fawkes, and since two people cannot be one person neither could be Guy
Fawkes. Hence, Williams concludes, neither should one identify Charles with
Guy Fawkes in the original case where there is no reduplication, for the absence
of Robert from that case has nothing to do with the intrinsic relations between
Charles and Guy Fawkes¡ªthe relations that obtain between them independently
of what is true of other people¡ªbut it is absurd to suppose that whether a later
person P2 is identical with an earlier person P1 can depend upon facts about
people other than P1 and P2.

This objection does not apply only to putative cases of reincarnation, where a
present-day defender of the Psychological Continuity Criterion of personal
identity might claim that his Criterion is anyway not satisfied (i.e. that the 
later person merely seems to, but does not actually have, genuine quasi-memories 
of the earlier person's experiences). It applies also in cases which the 
defender of the Psychological Continuity Criterion of personal identity must 
regard as providing undeniable examples of personal identity.

Consider, for example, that variant of the Brown/Brownson case, suggested
earlier as motivating a move from the Brain Criterion of personal identity to 
the
Physical Criterion, in which only half of Brown's brain is transplanted into
Robinson's body, with consequent transference of psychological states. The
defender of the Psychological Continuity Criterion ought to regard this as a
paradigm case of personal identity, but consider again the case¡ªhereafter to be
referred to as the fission case¡ªin which both of Brown's brain hemispheres are
transplanted, but into different bodies (and let us suppose for the sake of the
example, what is conceivably but not actually the case, that the two hemispheres 
are equipollent in their linguistic abilities, etc.). Williams's Reduplication
Argument can now be brought to bear just as in the case of Charles and Guy
Fawkes.

Or consider the case in which, via Williams's brain-state transfer device,
Robinson's brain is put into a state information-theoretically equivalent to
Brown's. Again the defender of the Psychological Continuity Criterion must
regard the case as a clear example of personal identity. But if this could 
happen to Robinson it could also happen simultaneously to his friend Smith. Once 
again, then, the Reduplication Argument can be brought to bear. For its 
happening simultaneously to Smith would leave the intrinsic relations between 
Brown and Robinson wholly unaffected.

Consequently the defender of the Psychological Continuity Criterion of
personal identity cannot afford just to ignore Williams's Reduplication
Argument. He must respond to it.

1.11
The revised psychological continuity criterion
There are two main replies to this argument. One reply, adopted by many
defenders of the Psychological Continuity Criterion, is simply to take the bull 
by the horns and to reject the principle underlying Williams's argument. This is 
the principle that whether a later individual x is identical with an earlier 
individual y can depend only on facts about x and y and the relationships 
between them: no facts about any other individual can be relevant to whether x 
is y. I shall call this principle the Only x and y principle. Applied to the 
special case of personal identity, it asserts that whether a certain later 
person P2 is identical with a certain earlier person P1 can depend only on facts 
about P2 and P1 and the intrinsic relationships between them; no facts about 
individuals other than P2 and P1 can be relevant to whether P2 is the same 
person as P1. If this principle is rejected the Reduplication Argument can be 
side-stepped very easily by revising the
Psychological Continuity Criterion to make psychological continuity a sufficient
condition of personal identity only in the absence of a 'rival candidate'. That 
is, we say: P2 at t2 is the same person as P1 at t1 just in case P2 at t2 is
psychologically continuous with P1 at t1 and there is no 'rival candidate' P2*
also psychologically continuous with P1.

But most philosophers who reply to Williams's argument by rejecting the
Only x and y principle also wish to allow that P2 can be the same person as P1
even if rival candidates exist, so long as P2's claim to identity with P1 is
stronger than those of its rivals. In other words, they prefer a 'best 
candidate' theory of personal identity to a 'no rival candidate' theory. Such a 
theory is put forward by Sydney Shoemaker (1970) and by Robert Nozick (1981). 
Nozick's version of the theory is the most sophisticated in the literature and 
we shall be examining it in detail later. He refers to it as 'the closest 
continuer' theory of personal identity. It asserts that P2 at t2 is the same 
person as P1 at t1 just in case P2 at t2 is (sufficiently) psychologically 
continuous with P1 at t1 and there is no
other continuer of Pl existing at t2 who is psychologically continuous with P1 
to an equal or greater degree. (Actually this statement would need to be further
qualified to deal with cases of 'fusion' as well as 'fission', i.e. merging as 
well as branching of links of psychological continuity, and also to deal with 
the existence of continuers existing at times between t1 and t2, but for now we 
can pass over these details.) We can call this the Revised Psychological 
Continuity Criterion.

Whether this line of reply to the Reduplication Argument can be sustained is a
matter of current controversy and we shall be looking at the matter in great 
detail in what follows. The intuitive objection can be brought out by reflection 
on the split-brain transplant case. Suppose that I am told that my brain is to 
be divided into two and the two halves transplanted into different bodies. Then 
according to the Revised Psychological Continuity Criterion I know that I will 
not survive and that two new people will be created by the fission. However, if 
I can persuade someone to destroy the right brain hemisphere before it is 
transplanted, thus eliminating the plurality of candidates, I will survive and 
be identical with the recipient of the left-brain hemisphere. Thus according to 
the Revised Psychological Continuity Criterion in this case my survival is 
logically dependent upon the non-existence of someone¡ªthe person resulting from 
the right-brain hemisphere transplant¡ªwho would not be me even if he were to 
exist. But how can my survival be thus logically dependent on the non-existence 
of someone else?

1.12
The multiple occupancy thesis
A second way a supporter of the Psychological Continuity Criterion can defend
himself against Williams's Reduplication Argument is to question the logic of
that argument. According to Williams, in a reduplication situation the rival
candidates for identity with the original person must be new existents, 
identical neither with him nor with one another. But it is possible, or so it 
has been argued by several recent writers (among them John Perry and David 
Lewis), to retain the Only x and y principle while rejecting this description of 
the reduplication situation. It must, of course, be accepted that the post-
fission rivals are distinct people, but it is possible, according to these 
philosophers, to reject the view that they are new existents; rather they have 
existed all along, but have only become
spatially distinct with the fission. There are various versions of this view. 
Their common element I will refer to, following Robinson (1985), as the Multiple
Occupancy Thesis. The essence of this thesis is that what makes it the case that
two people existing at a certain time are two may be facts about what is the 
case at other times, i.e. their distinction at the time in question may obtain 
only in virtue of facts extrinsic to that time, so that at the time, in David 
Lewis's words (1983, postscript to 'Survival and identity'), they comprise 'two 
minds with but asingle thought', not merely, to quote Robinson, 'as alike as two 
peas in a pod', but 'as alike as one pea in a pod'. 

This is another line of argument to be pursued further and to be examined in
much more detail. But if it is acceptable it allows us to retain the original 
version of the Psychological Continuity Criterion and avoid conflict with the 
Only x and y principle.

1.13
The simple view
Williams himself appears to take his Reduplication Argument as providing
support for some version of the view that personal identity requires some form 
of physical persistence, indeed for the Bodily Criterion of personal identity. 
But, as defenders of the Psychological Continuity Criterion were not slow to 
point out, and as is implicit in the discussion above, there is reason to 
suppose that if the Reduplication Argument has any cogency at all, then it 
applies equally to any plausible version of this view. Even if we insist on 
identity of the whole body as a necessary condition of personal identity, which 
it is very hard to do when one thinks of cases like the Brown/ Brownson case, it 
does not appear to be impossible to imagine a situation in which we were 
confronted by two bodies, either of which, but for the existence of the other, 
we would be happy to identity with a certain body (it seems to be possible to 
conceive, that is, a situation which
we would be tempted to describe as 'a man walking off in two directions'). And
if we pass on to versions of the view that personal identity requires physical
persistence which allow the identification of Brownson with Brown, it appears to
be impossible to find a plausible stopping point before we reach a version which
is clearly open to the Reduplication Argument. For if Brownson is Brown in
Shoemaker's original case it is surely impossible to deny that he is Brown when
he has only half of Brown's brain which nevertheless carries with it full
psychological continuity. But any version of the view that personal identity
requires physical persistence which licenses the identification in this case is 
wide open to the Reduplication Argument.

This has led some philosophers to the view that none of the proposals so far
considered can be a correct account of personal identity. Persistence of body 
and brain or psychological continuity and connectedness are criteria of personal
identity only in the sense of evidence: they are not what personal identity
consists in. Indeed, there is nothing (else) that personal identity consists in:
personal identity is an ultimate unanalysable fact, distinct from everything
observable or experienceable that might be evidence for it. Persons are 
separately existing entities, distinct from their brains, bodies and 
experiences. On the best known version of this view, a person is a purely mental 
entity: a Cartesian pure ego, or spiritual substance. This is in fact the form 
in which the view is adopted by its contemporary defenders, amongst whom the 
most prominent are Chisholm and Swinburne. Following Parfit I shall call this 
the Simple View. The view that there is something (else) that personal identity 
consists in. I shall refer to as the Complex View.

Defenders of the Simple View have pointed out that this view is to be found in
the writings of Butler (1736) and Reid (1941). Both Butler and Reid insist that
personal identity is identity in a stricter sense than the identity of material
objects. Butler, for example, maintains that the word 'same' is used in a 
'strict and philosophical' sense when applied to persons, but in a 'loose and 
popular' sense when applied to bodies. In a similar vein, Thomas Reid asserts 
that 'the identity¡­which we ascribe to bodies, whether natural or artificial, is 
not perfect identity; it is rather something which for convenience of speech we 
call identity.' Identity, he says, has no fixed nature when applied to bodies, 
and very often questions about it are questions about words. But identity when 
applied to persons has no ambiguity and admits not of degrees or of more or 
less. It is the foundation of all rights and obligations and of all 
accountableness, and the notion of it is fixed and precise. (1941)

The contemporary defenders of the Simple View of personal identity endorse
this, and regard it as a great merit of their view that it does set personal 
identity apart from the identity of other things.

As I have said, one of the main considerations in persuading its modern
defenders that the Simple View must be accepted has been the thought that no
criterion of personal identity in terms of any observable or experienceable 
facts can be sustained in the face of Williams's Reduplication Argument. That 
is, as Swinburne expresses it, no 'empiricist' theory of personal identity is 
tenable in the light of this objection. The Simple View is thought to resist 
this objection quite easily, for as Swinburne puts it:

The Simple View claims explicitly that personal identity is one thing, and
the extent of similarity in matter and apparent memory another. There is no
contradiction in supposing that the one should occur without the other.
Strong similarity of matter and apparent memory is powerful evidence of
personal identity¡­. Where there are two later persons P2 and P2' each of
whom has some continuity with the earlier person P1, the evidence
supports to some extent each of two hypotheses¡ªthat P2 is the same
person as P1, and that P2' is the same person as P1. It may give more
support to one hypothesis than to the other, but the less well supported
hypothesis might be the true one, or maybe neither hypothesis is true.
Perhaps Pl has ceased to exist, and two different persons have come into
existence. So the Simple View fully accepts that mere logic cannot
determine which experiences will be mine, but it allows that continuity of
memory and apparent memory and brain provides fallible evidence about this. 
And, of course, the duplication objection¡­has no force against the
Simple Theory. For although there can be equally good evidence that each
of two later persons is the same person as the earlier person, that evidence
is fallible, and since clearly only one person at one time can be strictly the
same person as one person at an earlier time, it follows that in one case the
evidence is misleading¡ªalthough we may not know in which case.
(Shoemaker and Swinburne 1984:20)

1.14
The determinacy thesis
Another argument in its favour that modern defenders of the Simple View
emphasize is that acceptance of it enables one to endorse what I shall call the
Determinacy Thesis concerning personal identity Derek Parfit, who rejects it, 
has formulated this as the thesis that questions about personal identity must 
have answers even in cases in which 'though we know the answer to every other
question, we have no idea how to answer a question of personal identity' (1971).
One initially tempting thing to say about some of the puzzle cases described in
the literature on personal identity is that to ask whether it is right or wrong 
to identify the original person in the case with the candidate for identity with 
him that the case presents is to ask an empty question. That is, because of the
vagueness inherent in our concept of personal identity, the statement of 
identity in question is neither true nor false and consequently it is neither 
true nor false that the original person in the case still exists after the 
various events described in it
have occurred. (This assumes just one candidate for identity with the original
person. If there is more than one in the case it is tempting to say that the
indeterminacy may be greater still: it may be indeterminate both whether the
original person exists, and, if so, who he is.)

It is uncontroversial that it is possible to construct puzzle cases concerning 
the identity of material objects about which this would be the correct thing to 
say. Events can be imagined, indeed events sometimes really occur, which in 
Bernard Williams's (1970) nice phrase 'leave a conceptual shadow' over the 
identity of a material object.

One such case is described by Shoemaker (1963). In 1944 the Germans
destroyed the four centuries old bridge of Santa Trinita in Florence. Six years
later it was decided that it (?)should be rebuilt. On the original site there 
now
stands a bridge of a design exactly like that of the original, constructed by
Renaissance techniques and built in part with the original stones (each standing
in its original place), in part with new stones taken from the original quarry
These facts are all clear, but how are we to answer the question 'Is the present
bridge of Santa Trinita the very bridge that spanned the Arno 400 years ago?'
It seems clear that in a case like this to persist in arguing about the correct
answer to the question would be absurd. Things can be said in favour of the
identity and things can be said against it, but there is no right or wrong 
answer. 

Rather, what we have is a borderline case of identity, as we can have a
borderline case of baldness or tallness or fatness. And consequently, if 
anything practically important turns on whether or not we say that the bridge is 
the same one, there is room for a decision to be made by the law courts about 
the matter. Such a decision may be reasonable or unreasonable in the light of 
the facts and legal precedents, but it cannot be true or false, since it will 
not be a statement made employing our present concept of sameness of bridge but 
a recommendation that the concept be made more determinate in a particular
direction.

But should we accept in the same way that cases can be imagined in which a
conceptual shadow would be cast over the identity of a person? In particular, 
can I suppose that in certain circumstances it would be indeterminate whether I 
still existed, and if so, with whom I was then identical? And can I imagine
circumstances in which the only sensible thing for me (?)to do if I was 
concerned about this question would be to seek a decision from the law courts 
about the matter? The defenders of the Simple View of personal identity argue 
that these questions must be answered negatively. That is, that precisely 
because personal identity is something that can be known from the first-person 
viewpoint, the possibility of borderline cases must be rejected. In this 
respect, at least, they
argue, personal identity must be radically different from the identity of other
things. But, they insist, cases are certainly possible in which all the relevant
evidence leaves the question of personal identity unsettled. And so, they argue,
the radical difference between personal identity and the identity of other 
things which they claim¡ªnamely, the impossibility of borderline cases of 
personal identity¡ªcannot be secured if personal identity is held to consist, as 
it is
according to their empiricist opponents, in the obtaining of any observable or
experienceable state of affairs. For any such criterion must be stated in a way
that allows borderline cases of personal identity, or, absurdly, it will make 
the question whether personal iden- tity obtains turn, in some conceivable case, 
on a question which is of total insignificance in comparison with the importance 
of the matter of life or death which depends on it. It is Richard Swinburne who 
has been most vigorous in pressing this argument. He illustrates it (1973¨C4) by
supposing that we are drawn to a theory of personal identity which makes 
personal survival depend on the survival of exactly half the brain of the 
original person:
one molecule less, according to the theory, and we no longer have personal
identity. But, he says, it is absurd that such a small difference can make all 
the difference when the issue is one of life and death. If, on the other hand, 
this theory of personal identity is revised to allow persistence of 
approximately half of the brain to suffice for personal identity, then, since 
'approximately' is a vague term, personal identity becomes something allowing of 
borderline cases.

And it appears that exactly the same dilemma must confront any 'empiricist'
theory of personal identity. This, then, is the second important argument used 
by modern defenders of the Simple View of personal identity. We shall be
examining it more closely later on, when we will have to look in detail both at 
the nature of the indeterminacy which is possible for statements about the
identity of things other than persons and at the arguments that for persons,
peculiarly, such indeterminacy cannot arise.

1.15
What matters in survival
As well as the question of what personal identity consists in, there is also the
question of what its significance is, and what the nature is of the special 
interest we have in our own survival and well-being. This question was intro 
duced into the contemporary debate by Shoemaker (1970), but it is associated 
particularly with the name of Derek Parfit. Parfit sums up his own view in the 
slogan 'identity is not what matters in survival'. This slogan might seem 
baffling. But what Parfit means is that personal identity as such is of no 
significance and our own continued existence and well-being is, as such, of no 
special interest to us. This needs some explaining.

As I understand it, Parfit's thesis is the claim that, contrary to what we are 
all naturally inclined to believe, we do not have a basic and non-derivative 
concern for our own future existence and well-being. What is of fundamental 
importance to us is that there be in the future people related by certain links 
of psychological continuity and connectedness to ourselves as we are now (let us 
call such future people 'Parfitian survivor'). Now, in the actual world, at the 
present time, the only way that I can secure that I have a Parfitian survivor 
tomorrow is to ensure that I myself am around tomorrow¡ªthat is, that I still 
exist tomorrow, that one of the people alive tomorrow is identical with me. 
Nevertheless, according to Parfit, my having a Parfitian survivor tomorrow does 
not entail that I exist tomorrow, and in certain conceivable, or possibly actual 
but merely future, circumstances, in which brain transplants, Star Trek 
technology, etc., are available, it will be possible to ensure a Parfitian 
survivor for oneself without
ensuring one's own future existence (one way to do this, according to Parfit,
would be to ensure for oneself a multiplicity of (equally good) Parfitian
survivors). Parfit's thesis that identity is not what matters in survival is 
then the thesis that, given one's fundamental desires and concerns, one would 
have no reason, in such a situation, for preferring a future in which one was 
present oneself to one in which one merely had Parfitian survivors. Now 
intuitively this is very implausible. Our interest in personal identity, the 
kind of importance it has for us, seems totally different from the type of 
interest we have in the identity of
other things. We value the people we care about as tokens rather than as types, 
as individuals rather than as instantiations of useful or desirable or 
attractive properties. We do not regard them as replaceable, and we certainly do 
not regard ourselves as replaceable. We can imagine without difficulty a society 
in which
'teletransportation' of the Star Trek variety is in general use as a means of
'transportation' of inanimate objects and food animals¡ªeven though it is
generally acknowledged in the society that what the process really involves is 
the destruction of one object and the creation elsewhere of a mere replica
numerically distinct from the original (of course, in the Star Trek series 
Captain Kirk and his colleagues do not regard teletransportation in this light¡ª
they regard it as a genuine form of transportation). The willingness of the 
people in such a society to employ teletransportation in this way would not 
strike us as in any way odd. But if we try to imagine that the people in this 
society, whilst continuing to acknowledge that what the process really involves 
is the destruction of one object and the creation elsewhere of a numerically 
distinct replica, none the less allow themselves and their loved ones to be 
teletransported, and in fact appear to treat teletransportation as a convenient 
alternative to travel, we run into immediate difficulties. And at first sight it 
seems as if we have succeeded only in imagining a society of madmen.

This illustrates the strength of the intuition that Parfit is opposing when he
claims that identity is not what matters in survival. For according to Parfit 
these men are not mad at all. On the contrary, they are acting exactly as it 
would be rational for us to act if we lived in their society and shared their 
beliefs about the nature of the teletransportation process. It would be quite 
irrational for us, according to Parfit, if we were in such circumstances, to 
refuse to step into the teletransporter because we believed that the people who 
would step out at the other end would not be ourselves but merely numerically 
distinct replicas. For this could only be relevant if our basic desires and 
concerns included a desire for our own continued existence and well-being. But, 
Parfit's thesis is, they do not.
Parfit's thesis that identity is not what matters in survival is, then, a quite
remarkable one. But he has an argument for it which is well worth considering.

1.16
Parfit's argument
This argument starts, as so much of the recent work on personal identity does,
from consideration of the problem of fission. To understand it, however, one
needs to make a distinction between two types of opinion we have with regard to
such puzzle cases about personal identity. First, we have opinions about how the
language of personal identity is to be applied to the case: that is, about what 
the correct answer to the question of personal identity the case poses actually 
is (or what the correct answers are to the questions posed when the case 
involves more than one later (earlier) candidate for identity with one earlier 
(later) person). These opinions reflect our mastery of our language, and in 
particular, our mastery of those parts of our language expressive of the concept 
of personal identity. In short, they reflect what I will call our 'semantic 
intuitions', and they are on a par with the opinions we have about the puzzle 
cases that can be constructed about the identity of things other than persons.
But when we consider puzzle cases about personal identity we often find
ourselves with opinions of a second sort. These are opinions about how it is
rational for the participants involved in the case¡ªthe people whose identity is 
at issue in it¡ªto behave, given the beliefs they are described as holding. 
Opinions of this second sort do not merely reflect our semantic intuitions; 
rather they reflect our fundamental desires and concerns. For we arrive at such 
opinions by imagining ourselves involved in the puzzle case as one of its 
participants and asking how we should then behave.

Now the way Parfit argues for his claim that identity does not matter in
survival is as follows. First he describes a fission case, i.e. a case in which 
each of two later people is related to an earlier person in a way in which, but 
for the existence of the other 'candidate', we would be very strongly inclined 
to regard as constituting identity (in fact the fission case he considers is the 
split-brain transplant case described earlier). Next he argues (a) that the 
correct description of the case is that given by the Revised Psychological 
Continuity Criterion of personal identity, namely that the original person 
ceases to exist, but would not have done so if only one of the fission products 
had existed, but (b) that it would be quite irrational, if you were the original 
in the case, for you to be concerned about the impending fission in the same way 
as you would be about your impending death, or to think that you could gain 
anything by preventing the fission, e.g. by bribing a nurse to destroy one of 
the brain hemispheres before the transplant¡ªeven though by doing so you would 
ensure your own future existence (I owe this way of putting Parfit's point to 
Nozick 1981). Here (a) is an opinion of the first of the two types just 
distinguished, and (b) an opinion of the second type. Finally, Parfit concludes 
that these opinions (assuming them to be
shared by his readers) can only be reconciled by accepting that our fundamental
desires and concerns are not the ones we think we have, and do not include a 
nonderivative desire for our own continued existence and well-being. In short, 
he argues that the only explanation of our apparently conflicting intuitions 
about this case is that we do not in fact regard identity as what matters in 
survival.

Parfit regards this conclusion as having many corollaries for our views on
ethics and the nature of rationality. He also regards it as providing a means of
defending the Revised Psychological Continuity Criterion of personal identity
against Williams's Reduplication Argument. The principle underlying that
argument, it will be remembered, was the Only x and y principle, which, applied
to persons, states that whether a later person P2 is identical with an earlier 
person P1 can depend only on facts about P2 and P1 and the intrinsic relations 
between them: it cannot depend on facts about individuals other than P2 and P1. 
To accept the Revised Psychological Continuity Criterion of personal identity is 
to reject this principle. But, Parfit argues, the plausibility of the principle 
depends on the assumption that personal identity is what matters in survival: 
for it is only implausible to suppose that my identity or otherwise with a 
future person can depend on anything other than the intrinsic features of my 
relationship to him if it is thought to be a matter of great importance whether 
such identity obtains. If it is a trivial matter then there is no implausibility 
in supposing it to be something determined extrinsically. Consequently, he 
argues, once one accepts that identity is not what matters in survival there is 
no difficulty in accepting also that it has the 'best candidate' structure 
implied by the Revised Psychological Continuity Criterion of personal identity.

Parfit's arguments present a considerable challenge to those who wish to
combine preservation of the common-sense idea that what matters in survival is
identity with rejection of the Simple View of personal identity. Consequently
they have been the focus of much attention in recent years and have been
challenged at several points.

One influential line of argument has been that Parfit is wrong to suppose that
the fission case must be described in the way implied by claim (a) above, namely
as a case in which the original person ceases to exist when the fission takes 
place. A version of this criticism is presented in Lewis (1976). According to 
the theory of personal identity Lewis there states, which entails the Multiple 
Occupancy Thesis, no one ceases to exist when the fission takes place. Rather, 
two people who have been spatially coincident (and the subjects of the same 
mental states) now become spatially distinct. But if no one ceases to exist when 
the fission takes place, of course, it must be absurd to view it as death.

Another way of responding to Parfit's argument is simply to dig in one's heels
and deny his claim (b), that it would be quite irrational to regard one's 
impending fission as one would regard one's impending death, or to think that 
one could gain anything by ensuring that only one of the brain hemispheres was
transplanted. This line of reply to Parfit has attracted few, but one who does
adopt it is Jerome Shaffer, who writes:

Psychological continuity is important where there is identity, but not
otherwise¡­returning to our case of the man who splits, we would¡­say
that since identity is not preserved even though psychological continuity is
preserved, the man should feel quite differently about it from the way he
should feel about single transplantation. (1977:157)

However, the difficulty with this is just that Parfit's claim (b) is so 
immensely plausible, especially when defended by the argument from Nozick cited 
above.

The final possibility is to challenge Parfit's claim that if our intuitions 
about the fission case are as he supposes, namely (a) that the original person 
ceases to exist, but (b) that it would none the less be quite irrational for him 
to regard the fission as he would regard death, then the only explanation of 
this can be that identity is not what matters in survival, i.e. that our own 
continued existence and well-being is not something for which we have a basic 
and non-derivative concern. It might seem at this point as if this option is a 
non-starter; but in fact it is the one I favour, and I shall be elaborating it 
later.

These arguments against Parfit are ones we will be returning to later. But
Parfit's arguments have also attracted the attention of the contemporary
defenders of the Simple View of personal identity. It is, of course, part and
parcel of the Simple View that identity is what matters in survival. But, unlike 
the defenders of the Complex View, the proponent of the Simple View is not
directly challenged by Parfit's argument. As we have seen, since it is essential 
to his view that nothing observable or experienceable constitutes personal 
identity, he can maintain that in the fission case as described by Parfit it is 
simply indeterminate whether the original person ceases to exist and if not, 
with which of the fission products he is identical. For, from his point of view, 
the case is simply underdescribed. Thus the apparent rationality of the original 
person's indifference to the fact of his imminent fission is not, for the 
defender of the Simple View, just tantamount, as Parfit takes it to be, to the 
apparent rationality of his indifference to the fact of his imminent demise. For 
the Simple theorist is not committed to saying that the original person in a 
fission case does cease to exist when the fission takes place (in this respect, 
despite huge differences, his position is akin to that of David Lewis). Whether 
it is possible for the proponent of the Simple View of personal identity to 
develop this into a convincing reply to Parfit is another matter.

I have, I believe, now surveyed the main views and arguments concerning
personal identity which have excited the interest of philosophers in recent 
years.

I turn in the next chapter to the history of the problem.