## 1. The Problems of Personal Identity



There is no single problem of personal identity, but rather a wide
range of questions that are at best loosely connected and not always
distinguished. Here are the most familiar:



**Characterization.** Outside of philosophy, the term
'personal identity' commonly refers to properties to which
we feel a special sense of attachment or ownership. My personal
identity in this sense consists of those properties I take to
"define me as a person" or "make me the person I
am". (The precise meaning of these phrases is hard to pin down.)
To have an "identity crisis" is to become unsure of what
my most characteristic properties are--of what sort of person I
am in some deep and fundamental sense. This individual personal
identity contrasts with my gender, ethnic, and national identity,
which consist roughly of the sex, ethnic group, or nation I take
myself to belong to and the importance I attach to this.



Someone's personal identity in this sense is contingent and
temporary: the way I define myself as a person might have been
different, and can vary from one time to another. It is a subset,
usually a small one, of the properties someone has: it could happen
that being a philosopher and a parent belong to my identity but not
being a man or a cyclist, while someone else has the same four
properties but feels differently towards them, so that being a man and
a cyclist belong to his identity but not being a philosopher or a
parent. Someone may not even need to *have* the properties
belonging to her identity: if I become convinced that I am Napoleon,
being an emperor could be one of the properties central to the way I
define myself and thus part of my identity, even though the belief is
false.



What determines someone's personal identity in this sense is
sometimes called the *characterization question* (Schechtman
1996: 1). It asks, in the expectation of a deep and revealing
psychological answer, Who am I? (Glover 1988: part 2 and Ludwig 1997
are useful discussions.)



**Personhood.** What is it to be a person, as opposed to
a nonperson? What have we people got that nonpeople haven't got?
The question often arises in connection with specific cases: we may
ask, for example, at what point in our development from a fertilized
egg there comes to be a person, or what it would take for a chimpanzee
or a Martian or a computer to be a person, if they could ever be. An
ideal account of personhood would be a definition of the word
'person', filling the blanks in the formula
'Necessarily, *x* is a person at time *t* if and
only if ... *x* ... *t* ...'.



The most common answer is that to be a person is to have certain
special mental properties. Locke, for instance, said that a person is
"a thinking intelligent being, that has reason and reflection,
and can consider itself as itself, the same thinking thing, in
different times and places" (1975: 335; Baker 2000: ch. 3 is a
detailed account of this sort). Others propose a less direct
connection between personhood and these special mental properties: for
example that to be a person is be capable of acquiring them (Chisholm
1976: 136f.), or to belong to a kind whose members typically have them
when healthy and mature (Wiggins 1980: ch. 6). (A very different
answer is mentioned in section 6.)



**Persistence.** What does it take for a person to
persist from one time to another--to continue existing rather
than cease to exist? What sorts of things is it possible, in the
broadest sense of the word 'possible', for you to survive,
and what sort of event would necessarily bring your existence to an
end? What determines which past or future being is you? Suppose you
point to a child in an old class photograph and say,
"That's me." What makes you that one rather than one
of the others? What is it about the way she relates then to you as you
are now that makes her you? For that matter, what makes it the case
that anyone at all existing back then is you? This is sometimes called
the question of *personal identity over time*, as it has to do
with whether the earlier and the later being are one thing or
two--that is, whether they are numerically identical. An answer
to it is an account of our *persistence conditions*.



Historically this question often arises from the thought that we might
continue existing after we die (as in Plato's *Phaedo*).
Whether this could happen depends on whether biological death
necessarily brings our existence to an end. Imagine that after your
death there really will be someone, in this world or the next, who
resembles you in certain ways. How would she have to relate to you as
you are now in order to *be* you, rather than someone else?
What would the Higher Powers have to do to keep you in existence after
your death? Or is it even possible? The answer to these questions
depends on the answer to the persistence question.



**Evidence.** How do we find out who is who? What
evidence bears on the question of whether the person here now is the
one who was here yesterday? One source of evidence is first-person
memory: if you remember doing some particular action (or seem to), and
someone really did do it, this supports the claim that that person is
you. Another source is physical resemblance: if the person who did it
looks just like you--or better, if she is in some way physically
or spatio-temporally continuous with you--that too supports her
being you. Which of these sources is more fundamental? Does
first-person memory count as evidence all by itself, or only insofar
as we can check it against physical facts? What should we do when
these considerations support opposing verdicts?



Suppose Charlie's memories are erased and replaced with accurate
memories (or apparent memories) of the life of someone long
dead--Guy Fawkes, say (Williams 1956-7). Ought we to
conclude on these grounds that the resulting person is not actually
Charlie, but Guy Fawkes brought back to life? Or should we instead
infer on the basis of physical continuity that he's just Charlie
with different memories? What principle would answer this
question?



The evidence question dominated the anglophone literature on personal
identity from the 1950s to the 1970s (Shoemaker 1963, 1970 and
Penelhum 1967 are good examples). It's important to distinguish
it from the persistence question. What it takes for you to persist
through time is one thing; how we ought to evaluate the relevant
evidence is another. If the criminal had fingerprints just like yours,
the courts may rightly conclude that he is you, but having your
fingerprints is not *what it is* for a past or future being to
be you: it's neither necessary (you could survive without any
fingers at all) nor sufficient (someone else could have fingerprints
just like yours).



**Population.** If the persistence question is about
which of the characters introduced at the beginning of a story have
survived till the end of it, we may also ask how many are on the stage
at any one time. What determines how many of us there are right now?
If there are eight billion people on the earth at present, what
facts--biological, psychological, or what have you--make
that the right number?



You may think the number of people at any given time (or at least the
number of *human* people) is simply the number of human
organisms there are then. But this is disputed. Some say that cutting
the main connections between the cerebral hemispheres results in
radical disunity of consciousness so that two people share a single
organism (Nagel 1971; for skeptical views see Wilkes 1988: ch. 5 and
van Inwagen 1990: 188-212). Others say that a human being with
multiple personality could literally be the home of two or more
thinking beings (Wilkes 1988: 127f., Rovane 1998: 169ff.; see also
Olson 2003, Snowdon 2014: ch. 7). Still others argue that two people
can share an organism in cases of conjoined twinning (Campbell and
McMahan 2016; see also Olson 2014).



The population question is sometimes called the problem of
"synchronic identity", as opposed to the "diachronic
identity" of the persistence question; but these terms need
careful handling. They are apt to give the mistaken impression that
identity comes in two kinds, synchronic and diachronic. The truth is
simply that there are two kinds of situations where we can ask how
many people (or other things) there are: those involving just one
moment and those involving several. To make matters worse, the term
'synchronic identity' is sometimes used to express the
personhood question.



**Personal ontology.** What are we? What properties of
metaphysical importance do we human people have, in addition to the
mental properties that make us people? What, for instance, are we made
of? Are we composed entirely of matter, as stones are, or are we
partly or wholly immaterial? Where do our spatial boundaries lie, if
we are spatially extended at all? Do we extend all the way out to our
skin? If so, what fixes those boundaries? Do we have temporal as well
as spatial parts? Are we substances--metaphysically independent
beings--or is each of us a state or an activity of something
else?



Here are some of the main proposed accounts of what we are (Olson
2007):


* We are biological organisms ("animalism": van Inwagen
1990, Olson 1997, 2003a).
* We are material things "constituted by" organisms: a
person is made of the same matter as a certain animal, but they are
different things because what it takes for them to persist is
different (Baker 2000, Johnston 2007, Shoemaker 2011).
* We are temporal parts of animals: each of us stands to an organism
as your childhood stands to your life as a whole (Lewis 1976).
* We are spatial parts of animals: something like brains, perhaps
(Campbell and McMahan 2016, Parfit 2012), or temporal parts of brains
(Hudson 2001, 2007).
* We are partless immaterial substances--souls--as Plato,
Descartes, and Leibniz thought (Unger 2006: ch. 7), or compound things
made up of an immaterial soul and a biological organism (Swinburne
1984: 21).
* We are collections of mental states or events: "bundles of
perceptions", as Hume said (1978 [1739]: 252; see also Quinton
1962, Campbell 2006).
* There is nothing that we are: we don't really exist at all
(Russell 1985: 50, Unger 1979, Sider 2013).



There is no consensus or even a dominant view on this question.



**What matters in survival.** What is the practical
importance of facts about our persistence? Why does it
*matter*? If you had to choose between continuing to exist or
being annihilated and replaced by someone else exactly like you, what
reason would you have to prefer one over the other? And what reason do
you have to care about what will happen to you, as opposed to what
will happen to other people? Or is there any such reason? Imagine that
surgeons are going to put your brain into my head and that neither of
us has any choice about this. The resulting person will be in terrible
pain after the operation unless one of us pays a large sum in advance.
If we were both entirely selfish, which of us would have a reason to
pay? Will the resulting person--who will think he is you--be
responsible for your actions or for mine? (Or both, or neither?) These
questions are summarized in the phrase *what matters in
survival*.



The answer may seem to turn entirely on whether the resulting person
would *be* you or I. Only *I* can be responsible for my
actions. The fact that some person is me, by itself, gives me a reason
to care about him. Each person has a special, selfish interest in her
own future and no one else's. Identity itself (numerical
identity) is what matters in survival. But some say that I could have
an entirely selfish reason to care about someone else's future
for his own sake. Perhaps what gives me a reason to care about what
happens to the man people will call by my name tomorrow is not that he
*is* me, but that he is then psychologically continuous with me
as I am now (see Section 4). If someone else were psychologically
continuous tomorrow with me as I am now, *he* would have what
matters to me and I ought to transfer my selfish concern to him.
Likewise, someone else could be responsible for my actions, and not
for his own. Identity itself has no practical importance. (Sosa (1990)
and Merricks (2022) argue for the importance of identity; Parfit
(1971, 1984: 215, 1995) and Martin (1998) argue against.)



That completes our survey. Though some of these questions may bear on
others, they are to a large extent independent. Many discussions of
personal identity leave it unclear which one is at stake.


## 2. Understanding the Persistence Question



Turn now to the persistence question. Few concepts have led to more
misunderstanding than identity over time. The persistence question is
often confused with others or stated in a tendentious way.



It asks roughly what is necessary and sufficient for a past or future
being to be someone existing now. If we point to you now, then
describe someone or something existing at another time, we can ask
whether we are referring twice to one thing or once to each of two
things. The persistence question is what determines the answer to such
queries. (And there are precisely analogous questions about the
persistence of other things, such as dogs or stones.)



Here are three common misunderstandings of this question. Some take it
to ask what it *means* to say that a past or future being is
you. This would imply that we can answer it simply by reflecting on
our linguistic knowledge--on what we mean by the word
'person', for example. The answer would be knowable a
priori. It would also imply that necessarily all people have the same
persistence conditions--that the answer to the question is the
same no matter what sort of people we considered. Though some endorse
these claims (Noonan 2019b: 84-93), they are disputed. What it takes
for us to persist might depend on whether we are biological organisms,
which we cannot know a priori. And if there could be immaterial
people--gods or angels, say--what it takes for them to
persist might differ from what it takes for a human person to persist.
In that case our persistence conditions could not be established by
linguistic or conceptual analysis.



Second, the persistence question is often confused with the question
of what it takes for someone to *remain the same person* (as in
this passage by Bertrand Russell (1957: 70): "Before we can
profitably discuss whether we shall continue to exist after death, it
is well to be clear as to the sense in which a man is the same person
as he was yesterday.") If Baffles were to change in certain
ways--if she lost much of her memory, say, or changed
dramatically in character, or became severely disabled--we might
ask whether she would still be the person she was before, or instead
become a different person. This is not a question about
persistence--about numerical identity over time. To ask whether
Baffles is the same person that *she* was before, or to say
that she is a different person from the one *she* used to be,
presupposes that she herself existed at the earlier time. The question
arises only when numerical identity is assumed. To ask about
Baffles' persistence, by contrast, is to ask not whether she has
continued to be the same person, but whether she has continued to
exist at all.



When we speak of someone's remaining the same person or becoming
a different one, we mean remaining or ceasing to be a certain
*sort* of person. For someone no longer to be the same person
is for her still to exist, but to have changed in some important way.
This typically has to do with her individual identity in the sense of
the characterization question--with changes in respect of those
properties that "define someone as a person."



Third, the persistence question is often taken to ask what it takes
for the same person to exist at two different times. The most common
formulation is something like this:


1. If a person *x* exists at one time and a person *y*
exists at another time, under what possible circumstances is it the
case that *x* is *y*?



This asks, in effect, what it takes for a past or future person to be
you, or for you to continue existing *as a person*. We have a
person existing at one time and a person existing at another, and the
question is what is necessary and sufficient for them to be one person
rather than two.



This is narrower than the persistence question. We may want to know
whether each of us was ever an embryo, or whether we could survive in
an irreversible vegetative state (where the resulting being is
biologically alive but has no mental properties). These are clearly
questions about what it takes for us to persist. But as personhood is
most commonly defined (recall Locke's definition quoted
earlier), something is a person at a given time only if it has certain
special mental properties at that time. Embryos and human beings in a
vegetative state, having no mental properties at all, are thus not
people when they're in that condition. And in that case we
cannot infer anything about whether you were once an embryo or could
exist in a vegetative state from a principle about what it takes for a
past or future *person* to be you.



We can illustrate the point by considering this answer to question
1:



> 
> Necessarily, a person *x* existing at one time is a person
> *y* existing at another time if and only if *x* can, at
> the first time, remember an experience *y* has at the second
> time, or vice versa.
> 



That is, a past or future person is you just if you (who are now a
person) can now remember an experience she had then, or she can then
remember an experience you're having now. Call this the
*memory criterion*. (It too is often attributed to Locke,
though it's uncertain whether he actually held it: see Behan
1979.)



The memory criterion may seem to imply that if you were to lapse into
an irreversible vegetative state, you would cease to exist (or perhaps
pass to the next world): the resulting being could not be you because
it would not remember anything. But no such conclusion follows.
Assuming that an organism in a vegetative state is not a person, this
is not a case involving a person existing at one time and a person
existing at another time. The memory criterion can only tell us which
past or future *person* you are, not which past or future being
generally. It says what it takes for someone to persist *as a
person*, but not what it takes for someone to persist without
qualification. So it implies nothing about whether you could exist in
a vegetative state or even as a corpse, or whether you were once an
embryo. As stated, it's compatible with your surviving with no
memory continuity at all, as long as this happens when you are not a
person (Olson 1997: 22-26, Mackie 1999: 224-228).



No advocate of the memory criterion would accept this. The view is
intended to imply that if a person *x* exists now and a being
*y* exists at another time--whether or not it's a
person then--they are one just if *x* can now remember an
experience *y* has at the other time or vice versa. But this
not an answer to Question 1: what it takes for a person existing at
one time and a person existing at another time to be one rather than
two. It's an answer to a more general question: what it takes
for something that is a person at one time to exist at another time as
well, whether or not it's a person then:


2. If a person *x* exists at one time and something *y*
exists at another time, under what possible circumstances is it the
case that *x* is *y*?



Those who ask Question 1 are commonly assuming that every person is a
person *essentially*: nothing that is in fact a person could
possibly exist without being a person. (By contrast, no student is a
student essentially: something that is in fact a student can exist
without being a student.) This claim, "person
essentialism," implies that whatever is a person at one time
must be a person at every time when she exists, making Questions 1 and
2 equivalent.



But person essentialism is controversial (Olson and Witt 2020).
Combined with a Lockean account of personhood, it implies that you
were never an embryo: at best you may have come into being when the
embryo that gave rise to you developed certain mental capacities. Nor
could you exist in a vegetative state. It rules out the brute-physical
view described in the next section. Whether we were once embryos or
could exist in a vegetative state, or whether we are people
essentially, would seem to be substantive questions that an account of
our persistence should answer, not matters to be presupposed in the
way we frame the debate.


## 3. Accounts of Our Persistence



Three main sorts of answers to the persistence question have been
proposed. *Psychological-continuity* views say that our
persistence consists in some psychological relation, the memory
criterion mentioned earlier being an example. You are that future
being that in some sense inherits its mental features from
you--beliefs, memories, preferences, the capacity for rational
thought, and so on--and you are that past being whose mental
features you have inherited in this way. There is dispute over what
sort of inheritance this has to be--whether it must be
underpinned by some kind of physical continuity, for instance, and
whether it requires a "non-branching"
restriction--and about what mental features need to be inherited.
(We will return to some of these points.) But most philosophers
writing on personal identity since the early 20th century have
endorsed some version of this view: e.g. Dainton 2008, Hudson (2001,
2007), Johnston (1987, 2016), Lewis (1976), Nagel (1986: 40), Parfit
(1971; 1984: 207; 2012), Shoemaker (1970; 1984: 90; 1997; 1999, 2008,
2011), Unger (1990: ch. 5; 2000).



A second answer is that our persistence consists in a physical
relation not involving psychology: you are that past or future being
that has your body, or that is the same biological organism as you
are, or the like. Call these *brute-physical* views. (Advocates
include Ayers (1990: 278-292), Carter (1989), Olson (1997),
Snowdon (2014), van Inwagen (1990: 142-188), and Williams
(1956-7, 1970).)



Some try to combine these views, saying that we need both mental and
physical continuity to survive, or that either would suffice without
the other (Nozick 1981: ch. 1, Langford 2014, Madden 2016, Noonan
2021).



Both views agree that there is something that it takes for us to
persist--that there are informative, nontrivial, necessary and
sufficient conditions for a person existing at one time to exist at
another time. A third view, *anticriterialism*, denies this.
Psychological and physical continuity are evidence for persistence, it
says, but do not always guarantee it and may not be required. The
clearest advocate of this view is Merricks (1998; see also Swinburne
1984, Lowe 1996: 41ff., 2012; Langford 2017; for criticism see
Zimmerman 1998, Shoemaker 2012). There is also debate about how
anticriterialism should be understood (Olson 2012, Noonan 2011,
2019a).


## 4. Psychological-Continuity Views



Most people--most Western philosophy teachers and students,
anyway--feel immediately drawn to psychological-continuity views.
If your brain were transplanted, and that organ carried with it your
memories and other mental features, the resulting person would be
convinced that he or she was you. This can make it easy to suppose
that she *would* be you, and this would be so because of her
psychological relation to you. But there is no easy path from this
thought to an attractive answer to the persistence question.



What psychological relation might it be? We have already mentioned
memory: a past or future being might be you just if you can now
remember an experience she had then or vice versa. This proposal faces
two historical objections, dating to Sergeant and Berkeley in the 18th
century (see Behan 1979) but more famously discussed by Reid and
Butler (see the snippets in Perry 1975).



First, suppose a young student is fined for overdue library books, and
as a middle-aged lawyer remembers paying the fine. In her dotage,
however, she remembers her law career but has entirely forgotten not
only paying the fine but all the other events of her youth. According
to the memory criterion the young student is the middle-aged lawyer,
the lawyer is the elderly woman, but the elderly woman is not the
young student. This is an impossible result: if *x* and
*y* are one and *y* and *z* are one, *x*
and *z* cannot be *two*. Identity is transitive; memory
continuity is not.



Second, it seems to belong to the very idea of remembering that you
can remember only your own experiences. To remember paying a fine (or
the experience of it) is to remember *yourself* paying. That
makes it uninformative to say that you are the person whose
experiences you can remember--that memory continuity is
sufficient for us to persist. It's uninformative because we
could not know whether someone genuinely remembers a past experience
without already knowing whether she is the one who had it. Suppose we
ask whether Blott, who exists now, is the same as Clott, whom we know
to have existed at some past time. The memory criterion tells us that
Blott is Clott just if Blott can now remember an experience Clott had
then. But Blott's seeming to remember one of Clott's
experiences counts as genuine memory only if Blott actually is Clott.
So we should already have to know whether Blott is Clott before we
could apply the principle that is supposed to tell us whether she is.
(There is, however, nothing uninformative about the claim that memory
connections are *necessary* for us to persist--that you
cannot survive in a condition in which you are unable to remember
anything, for example.)



One response to the first problem (about transitivity) is to modify
the memory criterion by switching from direct to indirect memory
connections: the old woman is the young student because she can recall
experiences the lawyer had at a time when the lawyer remembered the
student's life. The second problem is commonly met by replacing
memory with "quasi-memory", which is just like memory but
without the identity requirement: even if it's impossible to
remember doing something you didn't do but someone else did, you
could still "quasi-remember" it (Penelhum 1970: 85ff.,
Shoemaker 1970; for criticism see McDowell 1997).



But there remains the obvious problem that there are many times in our
pasts that we cannot remember or quasi-remember at all, and to which
we are not linked even indirectly by an overlapping chain of memories.
There is no time when you could recall anything that happened to you
while you dreamlessly slept last night. The memory criterion has the
absurd implication that you have never existed at any time when you
were unconscious, and that the person sleeping in your bed last night
was someone else.



A better solution replaces memory with the more general notion of
causal dependence (Shoemaker 1984, 89ff.). We can define two notions,
psychological connectedness and psychological continuity. A being is
*psychologically connected*, at some future time, with you as
you are now just if she is in the psychological states she is in then
in large part *because of* the psychological states you are in
now (and this causal link is of the right sort: see Shoemaker 1979).
Having a current memory (or quasi-memory) of an earlier experience is
one sort of psychological connection--the experience causes the
memory of it--but there are others. The important point is that
our current mental states can be caused in part by mental states we
were in at times when we were unconscious. For example, most of your
current beliefs are the same ones you had while you slept last night:
they have caused themselves to continue existing. You are then
*psychologically continuous*, now, with a past or future being
just if some of your current mental states relate to those he or she
is in then by a chain of psychological connections.



That would enable us to say that a person *x* who exists at one
time is the same thing as something *y* existing at another
time just if *x* is, at the one time, psychologically
continuous with *y* as it is at the other time. This avoids the
most obvious objections to the memory criterion.



It still leaves important questions unanswered, however. Suppose we
could somehow copy all the mental contents of your brain to mine, much
as we can copy the contents of one computer drive to another, thereby
erasing the previous contents of both brains. Whether this would be a
case of psychological continuity depends on what sort of causal
dependence counts. The resulting being (with my brain and your mental
contents) would be mentally as you were before, and not as I was. He
would have inherited your mental properties in a way--but a funny
one. Is it the right way? Could you literally move from one organism
to another by "brain-state transfer"?
Psychological-continuity theorists disagree. (Shoemaker (1984:
108-111, 1997) says yes; Unger (1990: 67-71) says no; see
also van Inwagen 1997.)


## 5. Fission



A more serious worry for psychological-continuity views is that you
could be psychologically continuous with two past or future people at
once. If your cerebrum--the upper part of the brain largely
responsible for mental features--were transplanted, the recipient
would be psychologically continuous with you by anyone's lights,
and any psychological-continuity view will imply that she would be
you. If we destroyed one of your cerebral hemispheres, the resulting
being would also be psychologically continuous with you.
(Hemispherectomy--even the removal of the left hemisphere, which
controls speech--is sometimes carried out as a treatment for
severe epilepsy: see Shurtleff *et al*. 2021.) And it would be
the same if we did both at once, destroying one hemisphere and
transplanting the other: the recipient would be you on any
psychological-continuity view.



But now suppose that both hemispheres are transplanted, each into a
different empty head. (We needn't pretend that the hemispheres
are exactly alike.) The two recipients--call them Lefty and
Righty--will each be psychologically continuous with you. The
psychological-continuity view as we have stated it implies that any
being who is psychologically continuous with you must be you. It
follows that you are Lefty and also that you are Righty. But that
again is an impossible result: if you and Lefty are one and you and
Righty are one, Lefty and Righty cannot be two. And yet they are:
there are evidently two people after the operation. One thing cannot
be numerically identical with two different things.



Psychological-continuity theorists have proposed two different
solutions to this problem. One, sometimes called the
"multiple-occupancy view", says that if there is fission
in your future, then there are two of you, so to speak, even now. What
we think of as you is really two people, who are now exactly similar
and located in the same place, doing the same things and thinking the
same thoughts. The surgeons merely separate them (Lewis 1976, Perry
1972, Noonan 2019b: 141-144).



The multiple-occupancy view is usually combined with the general
metaphysical claim that people and other persisting things are
composed of temporal parts (often called
"four-dimensionalism"; see Hudson 2001, Sider 2001a, Olson
2007: ch. 5). For each person, there is, for example, such a thing as
her first half: an entity just like the person only briefer, like the
first half of a meeting. On this account, the multiple-occupancy view
is that Lefty and Righty coincide before the operation by sharing
their pre-operative temporal parts or "stages", then
diverge by having different temporal parts located afterwards. They
are like two roads that coincide for a stretch and then fork, sharing
some of their spatial parts but not others. Much as the roads are just
like one road where they overlap, Lefty and Righty are just like one
person before the operation when they share their temporal parts. Even
they themselves can't tell that they are two. There are two
coinciding people before the operation because of what happens later,
just as there may be coinciding two roads here because of what's
the case elsewhere. Whether we really are composed of temporal parts,
however, is disputed. (Its consequences are explored further in
section 8.)



The solution more commonly proposed by psychological-continuity
theorists abandons the claim that psychological continuity by itself
suffices for us to persist, and says that a past or future being is
you only if she is then psychologically continuous with you and *no
other being* then is. (There is no circularity in this. We need
not know the answer to the persistence question in order to know how
many people there are at any one time: that comes under the population
question.) So neither Lefty nor Righty is you: they both come into
existence when your cerebrum is divided. If both your cerebral
hemispheres are transplanted, you cease to exist--though you
would survive if only one were transplanted and the other destroyed.
Fission is death. (Shoemaker 1984: 85, Parfit 1984: 207; 2012: 6f.,
Unger 1990: 265).



This proposal, the "non-branching view", has the
surprising consequence that if your brain is divided, you will survive
if only one half is preserved, but you will die if both halves are.
That looks like the opposite of what we should expect: if your
survival depends on the functioning of your brain (because
that's what underlies psychological continuity), then the more
of that organ we preserve, the greater ought to be your chance of
surviving.



In fact the non-branching view implies that transplanting one
hemisphere and leaving the other in place would be fatal. Its
consequences are especially surprising if brain-state transfer counts
as psychological continuity: in that case, even copying your total
brain state to another brain without doing you any physical harm would
kill you.



These consequences are not only hard to believe, but mysterious as
well. Keeping half your brain functioning is normally sufficient for
your survival, according to psychological-continuity views. Why then
would you not survive if the other half too were kept functioning,
separate from the first? How could an event that would normally ensure
your survival destroy you if accompanied by a second such event having
no causal effect on the first (Noonan 2019b: 128-141)?



The non-branching view is largely responsible for the interest in the
question of what matters in identity. Faced with the prospect of
having one of your hemispheres transplanted, there is no evident
reason to want the other one to be destroyed. Most of us, it seems,
would rather have both preserved, even if they go into different
heads. Yet on the non-branching view that is to prefer death over
continued existence. This leads Parfit and others to say that that is
precisely what we *ought* to prefer. We have no reason to want
to continue existing, at least for its own sake. What you have reason
to want (assuming that your life is going well) is that there be
someone in the future who is psychologically continuous with you,
whether or not she is you. The usual way to achieve this is to
continue existing yourself, but on the non-branching view it's
not necessary.



Likewise, even the most selfish person may have a reason to care about
the welfare of the beings who would result from her undergoing
fission, whether or not either of them would be her. The non-branching
view suggests that the sorts of practical concerns you ordinarily have
for yourself can apply to someone other than you. More generally,
facts about numerical identity--about who is who--have no
practical importance. All that matters is who is psychologically
continuous with whom. Psychological-continuity views are often said to
be superior to brute-physical views in accounting for what matters in
identity. Fission cases threaten this claim. (Lewis 1976 and Parfit
1976 debate whether the multiple-occupancy view can preserve the
conviction that identity is what matters practically.)


## 6. The Too-Many-Thinkers Objection



Another objection to psychological-continuity views is that they rule
out our being biological organisms (Carter 1989, Ayers 1990:
278-292, Snowdon 1990, Olson 1997: 80f., 100-109). This is
because no sort of psychological continuity appears to be either
necessary or sufficient for a human organism to persist.



We can see that it's not necessary by noting that each human
organism persists as an embryo without psychological continuity. And
we can see that it's not sufficient by imagining that your brain
is transplanted. In that case the recipient would be uniquely
psychologically continuous with you, and this continuity would be
continuously physically realized. Any psychological-continuity view
implies that she would be you. More generally, any person would go
with her transplanted brain. But it does not appear as if any organism
would go with its transplanted brain. It looks as if the operation
would simply move an organ from one organism to another, like
transplanting a liver. It follows that if you were an organism, you
would stay behind with an empty head, contrary to
psychological-continuity views.



Psychological-continuity views do not merely rule out our being
essentially or "fundamentally" organisms, but our being
organisms at all. They say that each person has the property of
persisting by virtue of psychological continuity: of being such that
psychological continuity (perhaps with a non-branching restriction) is
both necessary and sufficient for it to continue existing. But no
organism has this property. (Or at least no human organism does, and
we are clearly not non-human organisms.) Or again: every person would
go with her transplanted brain, but no organism would do so. And if
every person has a property that no organism has, then no person is an
organism.



That is said to be a problem for psychological-continuity views
because healthy, adult human organisms appear to be conscious and
intelligent. Suppose they are. And suppose, as
psychological-continuity views appear to imply, that we ourselves are
not organisms. Three awkward consequences follow.



First, you are one of two intelligent beings sitting there and reading
this entry: there is, in addition to you, an organism reading it. More
generally, there are two thinking beings wherever we thought there was
just one, a person and an organism distinct from it.



Second, we would expect the organism not just to be intelligent, but
to be psychologically indistinguishable from you. That would make it a
person, if being a person amounts to having mental special properties
(as on Locke's definition)--a second person in addition to
you. In that case it cannot be true that *all* people (or even
all human people) persist by virtue of psychological continuity,
contrary to psychological-continuity views. Some--those who are
organisms--would have brute-physical persistence conditions.



Third, it's hard to see how you could know whether you yourself
were the nonanimal person with psychological persistence conditions or
the animal person with brute-physical ones. If you thought you were
the nonanimal, the organism would use the same reasoning to conclude
that *it* was too. For all you could ever know, it seems, you
might be the one making this mistake.



We can illustrate the nature of this epistemic problem by imagining a
three-dimensional duplicating machine. When you step into the
"in" box, it reads off your complete physical (and mental)
condition and uses this information to assemble a perfect duplicate of
you in the "out" box. The process causes momentary
unconsciousness but is otherwise harmless. Two beings wake up, one in
each box. The boxes are indistinguishable. Because each being will
have the same apparent memories and perceive identical surroundings,
each will think, for the same reasons, that he or she is you--but
only one will be right. If this happened to you, it's hard to
see how you could know, afterwards, whether you were the original or
the duplicate. (Suppose the technicians who work the machine are sworn
to secrecy and immune to bribes.) You would think, "Who am I?
Did I do the things I seem to remember doing, or did I come into being
only a moment ago, complete with false memories of someone
else's life?" And you would have no way of answering these
questions. In the same way, psychological-continuity views raise the
questions, "What am I? Am I a nonanimal that would go with its
brain if that organ were transplanted, or an animal that would stay
behind with an empty head?" And here too there seem to be no
grounds on which to answer them.



This is the "too-many-thinkers" or
"thinking-animal" objection to psychological-continuity
views. The most popular defense against it is to say that, despite
sharing our brains and showing all the outward signs of consciousness
and intelligence, human organisms do not think and are not conscious.
There simply *are* no thinking animals to create problems for
psychological-continuity views (Shoemaker 1984: 92-97, Lowe
1996: 1, Johnston 2007: 55; Baker 2000 offers a more complex
variant).



But although this is easy to say, it's hard to defend. If human
organisms cannot be conscious and intelligent, it would seem to follow
that no biological organism could have any mental properties at all.
This threatens to imply that human organisms are "zombies"
in the philosophical sense: beings physically identical to conscious
beings, with the same behavior, but lacking consciousness (Olson
2018). And it leaves us wondering *why* organisms cannot be
conscious. The best proposed answer is given by Shoemaker (1999, 2008,
2011), who argues that it is because organisms have the wrong
persistence conditions, but it's highly controversial.



A second option is to concede that human organisms are psychologically
indistinguishable from us, but try to explain how we can still know
that we are not those organisms. The best-known proposal of this sort
focuses on personhood and first-person reference. It says that not
just any being with mental properties of the sort that you and I
have--rationality and self-consciousness, for
instance--counts as a person (contrary to anything like
Locke's definition). A person must also persist by virtue of
psychological continuity. It follows that human animals are not people
(thus avoiding the second awkward consequence, about personhood).



Further, personal pronouns such as 'I', and the thoughts
they express, refer only to people in this sense. So when your animal
body says or thinks 'I', it refers not to itself but to
you, the person. The organism's statement 'I am a
person' does not express the false belief that *it* is a
person, but the true belief that you are. So the organism is not
mistaken about which thing it is: it has no first-person beliefs about
itself at all. And you're not mistaken either. You can infer
that you are a person from the linguistic facts that you are whatever
you refer to when you say 'I', and that 'I'
never refers to anything but a person. You can know that you are not
the animal thinking your thoughts because it is not a person and
personal pronouns never refer to nonpeople (thus avoiding the third,
epistemic consequence; see Noonan 1998, 2010, Olson 2002; for a
different approach see Brueckner and Buford 2009).



The too-many-thinkers problem arises on the assumption that we are not
organisms, which appears to follow from psychological-continuity views
because organisms don't seem to persist by virtue of
psychological continuity. But some say that human organisms do persist
by virtue of psychological continuity. Although you are an organism,
the transplant operation would not move your brain from one organism
to another, but would cut an organism down to the size of a brain,
move it across the room, and then give it new parts to replace the
ones it lost. The view is sometimes called "new animalism"
(Madden 2016, Noonan 2021; see also Langford 2014, Olson 2015:
102-106).


## 7. Animalism and Brute-Physical Views



Animalism says that we human people are organisms. This does not imply
that all organisms, or even all human organisms, are people: as we saw
earlier, human embryos and animals in a vegetative state are not
people on the most common definitions of that term. Being a person may
be only a temporary property of us, like being a student. Nor does it
imply that all people are organisms: it is consistent with there being
wholly inorganic people such as gods or intelligent robots. Animalism
is not an answer to the personhood question. (It is consistent, for
instance, with Locke's definition of 'person'.)



The dominant view among both animalists and their opponents is that
organisms persist by virtue of some sort of brute-physical continuity
with no psychological element. So most animalists accept a
brute-physical account of our persistence. And most advocates of
brute-physical views take us to be organisms. This suggests that we
have the same persistence conditions as certain nonpeople, such as
oysters. And our persistence conditions differ from those of
immaterial people, if there could be such things, so that there are no
persistence conditions for people as such. Though some object to this
(Baker 2000: 124), many psychological-continuity theorists say that
all beings with mental properties have the same persistence conditions
(Shoemaker 2008, Unger 2000), which has the same implication.



The most common objection to brute-physical views (and, by extension,
to animalism) focuses on their implication that transplanting your
brain into my head would not give you a new body, but would give me a
new brain. You would stay behind with an empty head (e.g. Unger 2000;
for an important related objection see Johnston 2007, 2016).
Animalists generally concede that this is counterintuitive, but take
this fact to be outweighed by other considerations: that we appear to
be organisms, for example, that it's hard to say what sort of
*non*organisms we might be, and that our being organisms would
avoid the too-many-thinkers problem. And animalism is compatible with
our beliefs about who is who in real life: every actual case in which
we take someone to survive or perish is a case where a human organism
does so. Psychological-continuity views, by contrast, conflict with
the appearance that each of us was once a foetus. When we see an
ultrasound picture of a 12-week-old foetus, we ordinarily think
we're seeing something that will, if all goes well, be born,
learn to speak, and eventually become an adult human person, yet no
person is in any way psychologically continuous with a 12-week-old
foetus.



And the "transplant objection" may be less compelling than
it first appears (Snowdon 2014: 234). Suppose you had a tumor that
would kill you unless your brain were replaced with a healthy donated
organ. This would have grave side-effects: it would destroy your
memories, plans, preferences, and other mental properties. It may not
be clear whether you could survive it, even if the operation were
successful. But is it really obvious that you could *not*
survive it? Maybe it could save your life, though at great cost. And
this might be so, the argument goes, even if the new brain gave you
memories, plans, and preferences from the donor. But if it's not
obvious that the brain recipient would not be you, then it's not
obvious that it would be the donor. A brain transplant might be
metaphysically analogous to a liver transplant. Again, the claim is
not that this is obviously true, but only that it's not
obviously false. And in that case it's not obvious that a person
must go with her transplanted brain. (Williams 1970 argues in a
similar way.)


## 8. Wider Themes



The debate between psychological-continuity and brute-physical views
cannot be settled without considering more general matters outside of
personal identity. For instance, psychological-continuity theorists
need to explain why human organisms are unable to think as we do. This
will require an account of the nature of mental properties. Or if
human organisms *can* think, psychological-continuity theorists
will want an account of how we can know that we are not those
organisms. This will turn on how the reference of personal pronouns
and proper names works, or on the nature of knowledge.



Some general metaphysical views suggest that there is no unique right
answer to the persistence question. The best-known example is the
ontology of temporal parts mentioned in section 5. It says that for
every period of time when you exist, short or long, there is a
temporal part of you that exists only then. This gives us many likely
candidates for being you--that is, many different beings now
sitting there and reading this. Suppose you are a material thing, and
that we know what determines your spatial boundaries. That should tell
us what counts as your current temporal part or
"stage"--the temporal part of you located now and at
no other time. But that stage is a part of a vast number of temporally
extended objects (Hudson 2001: ch. 4).



For instance, it is a part of a being whose temporal boundaries are
determined by relations of psychological continuity (Section 4) among
its stages. That is, one of the beings thinking your current thoughts
is an aggregate of person-stages, each of which is psychologically
continuous with each of the others and with no other stage. If this is
what you are, then you persist by virtue of psychological continuity.
Your current stage is also a part of a being whose temporal boundaries
are determined by relations of psychological *connectedness*.
That is, one of the beings now thinking your thoughts is an aggregate
of person-stages, each of which is psychologically connected with each
of the others and to no other stage. This may not be the same as the
first being, as some stages may be psychologically continuous with
your current stage but not psychologically connected with it. If this
is what you are, then psychological connectedness is necessary and
sufficient for you to persist (Lewis 1976). What's more, your
current stage is a part of an organism, which persists by virtue of
brute-physical continuity, and a part of many bizarre and
gerrymandered objects (Hirsch 1982, ch. 10). Some even say that you
are your current stage itself (Sider 2001a, 188-208). And there
would be many other candidates.



The temporal-parts ontology implies that each of us shares our current
thoughts with countless beings that diverge from one another in the
past or future. If this were true, which of these things should
*we* be? Of course, we are the things we refer to when we say
'I', or more generally the referents of our personal
pronouns and proper names. But these words would be unlikely to
succeed in referring to just one sort of thing--to only one of
the many candidates on each occasion of utterance. There would
probably be some indeterminacy of reference, so that each such
utterance referred ambiguously to many different candidates. That
would make it indeterminate what things, and even what sort of things,
we are. And insofar as the candidates have different histories and
different persistence conditions, it would be indeterminate when we
came into being and what it takes for us to persist (Sider 2001b).