Personal Identity (Stanford Encyclopedia of Philosophy)

There is no single problem of personal identity, but rather a wide
range of questions that are at best loosely connected. Discussions in
this area do not always make clear which one is at stake. Here are the
most familiar:

Who am I? Outside of philosophy, ‘personal
identity’ usually refers to properties to which we feel a
special sense of attachment or ownership. Someone’s personal
identity in this sense consists of those properties she takes to
“define her as a person” or “make her the person she
is”, and which distinguish her from others. (The precise meaning
of these phrases is hard to pin down.) To have an “identity
crisis” is to become unsure of what one’s most
characteristic properties are—of what sort of person, in some
deep and fundamental sense, one is. This “personal
identity” contrasts with ethnic or national identity, which
consists roughly of the ethnic group or nation one takes oneself to
belong to and the importance one attaches to this.

One’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 could happen that
being a philosopher and a parent belong to my identity, but not being
a man and living in Yorkshire, while someone else has the same four
properties but feels differently towards them, so that being a man and
living in Yorkshire belong to his identity but not being a philosopher
or a parent. And these attitudes are all subject to change.

Depending on how the term is defined, it may also be possible for a
property to belong to someone’s “identity” without
her actually having it: 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 an element of my identity, even though my
belief is false.

The Who am I? question—sometimes called the characterization
question (Schechtman 1996: 1)—is what determines
someone’s personal identity in this sense (Glover 1988: part 2,
Ludwig 1997).

Personhood. What is it to be a person, as opposed to
a nonperson? What have we people got that nonpeople haven’t got?
More specifically, we can ask 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 an electronic computer to be a person, if
they could ever be. An ideal account of personhood would be a
definition of the word person, taking the form
‘Necessarily, x is a person at time t if and
only if … x … t …’, with
the blanks appropriately filled in. The most common answer is that to
be a person at a time is to have certain special mental properties
then (e.g. Baker 2000: ch. 3). Others propose a less direct connection
between personhood and mental properties: for example that to be a
person is be capable of acquiring those properties (Chisholm 1976:
136f.), or to belong to a kind whose members typically have them when
healthy and mature (Wiggins 1980: ch. 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 adventures 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 who existed back then is you? This is
sometimes called the question of personal identity over time.
That’s because it’s about whether the earlier being and
the later being are one 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 out of the hope (or fear) that
we might continue to exist after we die (as in Plato’s
Phaedo). Whether this could happen depends on whether
biological death necessarily brings one’s 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 that
being 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 there anything they
could do? 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 continuity: if the person who did it
looks just like you, or even better if she is in some sense physically
or spatio-temporally continuous with you, that too is reason to think
she is you. Which of these sources is more fundamental? Does
first-person memory count as evidence all by itself, for instance, or
only insofar as we can check it against publicly available physical
facts? What should we do when they 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 the basis of memory evidence, that the resulting person
is not Charlie but Guy Fawkes brought back to life, or should we
instead infer on the basis of physical continuity that he is simply
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 (good examples include Shoemaker
1963, 1970 and Penelhum 1967, 1970). It is 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 conclude that he is you. But even if they are right to
do so, having your fingerprints is not what it is for a past
or future being to be you: it is 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 to become those at 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 now? If there are some seven billion people on the earth at
present, what facts—biological, psychological, or what have
you—make that the right number? The question is not what
causes there to be a certain number of people at a given
time, but what there being that number consists in. It’s like
asking what sort of configuration of pieces amounts to winning a game
of chess, rather than what sorts of moves typically lead to
winning.

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 (ignoring any that don’t count as
people). But this is disputed. Some say that cutting the main
connections between the cerebral hemispheres results in radical
disunity of consciousness, and that because of this, two people share
a single organism (see e.g. Nagel 1971; Puccetti 1973 argues that
there are two people within each normal human being; see also 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 2003b, Snowdon 2014: ch. 7). Still others argue that two people
can share an organism in cases of conjoined twinning (Campbell and
McMahan 2010; see also Olson 2014).

This is sometimes called the problem of “synchronic
identity”, as opposed to the “diachronic identity”
of the persistence question. These terms need careful handling,
however. 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.

What am I? What sort of things, metaphysically
speaking, are you and I and other human people? What are our
fundamental properties, in addition to those 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 and no further, for instance? If
so, what fixes those boundaries? Are we
substances—metaphysically independent beings—or is each of
us a state or aspect or activity of something else?

Here are some of the main proposed answers (Olson 2007):

• We are biological organisms (“animalism”: Snowdon
  1990, 2014, van Inwagen 1990, Olson 1997, 2003a).
• We are material things “constituted by” organisms: a
  person 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: brains perhaps (Campbell and
  McMahan 2010, Parfit 2012), or temporal parts of brains (Hudson 2001,
  2007).
• We are partless immaterial substances—souls—as Plato,
  Descartes, and Leibniz thought (see also Unger 2006: ch. 7), or
  compound things made up of an immaterial soul and a material body
  (Swinburne 1984: 21).
• We are collections of mental states or events: “bundles of
  perceptions”, as Hume said (1739 [1978: 252]; see also Quinton 1962,
  Campbell 2006).
• There is nothing that we are: we don’t really exist at all
  (Russell 1985: 50, Wittgenstein 1922: 5.631, Unger 1979, Sider
  2013).

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

What matters in identity? What is the practical
importance of facts about our persistence? Why does it
matter? What reason have you to care whether you yourself
continue to exist, rather than someone else just like you existing in
your place? Imagine that surgeons are going to put your brain into my
head and that neither of us has any choice about this. Suppose 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 presumably think he is you—be responsible
for your actions or for mine? (Or both, or neither?)

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 matters
practically. 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), or because he relates to me in some other way that does not imply
that we are the same person. If someone other than me 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. (See
Shoemaker 1970: 284; Parfit 1971, 1984: 215, 1995; Sosa 1990, Martin
1998.)

That completes our survey. Though some of these questions may bear on
others, they are to a large extent independent. It’s important
not to confuse them.

We turn now to the persistence question. Few concepts have been the
source of more misunderstanding than identity over time. The
Persistence Question is often confused with other questions or stated
in a tendentious way.

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

Some take the persistence question to ask what it means to
say that a past or future being is you. This would imply that we can
answer it by working out the meaning of terms such as
‘person’, or by analysing the concepts they express. The
answer would be knowable a priori if at all. 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 2003:
86–92), 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, such as gods or
angels, what it takes for them to persist might differ from what it
takes for a human person to persist.

We sometimes ask what it takes for someone to remain the same
person. The idea is that if you were to alter in certain
ways—if you lost much of your memory, say, or became badly
disabled, or had a dramatic change in character—you would no
longer be the person you were before. This is not the persistence
question. The two questions can have different answers. Suppose you
change in such a way as to “become a different person”:
the answer to the question of whether you are the same person is No.
The persistence question asks, in this case, whether you would still
exist. And the answer to that question is Yes: if you are a
different person, then you still exist, just as you do if you remain
the same person.

When we speak of remaining the same person or of 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 has to do
with her individual identity in the sense of the characterization
question—with what sorts of changes would count as losing the
properties that define someone as a person. It has nothing to do with
persistence. Asking what it takes for someone to “retain her
personal identity”, or to lose it, also appears to be about
characterization rather than persistence.

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. 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 a foetus, or whether someone
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 being a
person is most often defined as having 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). This implies that something is a person at
a given time only if it has those mental properties then. It follows
that early-term foetuses and human beings in a vegetative state,
having no mental properties at all, are not people at those times. In
that case we cannot infer anything about whether you were once an
embryo or could come to be a vegetable 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 are having now. Call this the memory
criterion. (It too is sometimes attributed to Locke, though
it’s doubtful 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 a human vegetable 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 purports to tell us which past or
future person you are, but 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 come to
be a vegetable or even a corpse, or whether you were ever an embryo.
As stated, the memory criterion is 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 one who thinks that we persist by virtue of memory continuity would
accept this. The memory criterion is intended to imply that if a
person x exists now and a being y exists at another
time–whether or not it is is 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 is 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 as a person or not:

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 state the persistence question as Question 1 generally do so
because they assume 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. It
makes Questions 1 and 2 equivalent.

But person essentialism is controversial. Combined with a Lockean
account of personhood, it implies that you could never have been 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 come to
be a human vegetable. For that matter, it rules out our being
biological organisms, since no organism is a person essentially: every
human organism starts out as an embryo and may end up in a vegetative
state. It rules out both animalism and the brute-physical view
described in the next section.

Whether we were once embryos or could become vegetables, or whether we
are people essentially, are substantive questions that an account of
our persistence should answer, not matters to be unwittingly settled
in advance by the way we frame the debate. Question 1 is tendentious,
by presupposing that we can only survive as people. Question 2 is
neutral.

Four main sorts of answers to the persistence question have been
proposed. The most popular are psychological-continuity
views. They say that our persistence consists in some
psychological relation. 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. There is also disagreement
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. The memory criterion mentioned earlier is an example.
Advocates of psychological-continuity views include Garrett (1998),
Hudson (2001, 2007), Johnston (1987, 2016), Lewis (1976), Nagel (1986:
40), Noonan (2003), Parfit (1971; 1984: 207; 2012), Perry (1972),
Shoemaker (1970; 1984: 90; 1997; 1999, 2008, 2011), and Unger (1990:
ch. 5; 2000).

A second answer is that our persistence consists in some sort of brute
physical relation. You are that past or future being that has your
body, or that is the same biological organism as you are, or the like.
It has nothing to do with psychological facts. Call these
brute-physical views. (Don’t confuse them with the view
that physical evidence has some sort of priority over psychological
evidence in finding out who is who. That has to do with the evidence
question.) Their advocates include Ayers (1990: 278–292), Carter
(1989), Mackie (1999), Olson (1997), 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).

A different sort of proposal, narrativism, is that what it
takes for us to persist has to do with the stories we tell about
ourselves. We understand our lives in terms of narratives about the
momentous events in our past and their influence on our later
decisions and character. These narratives can be
“identity-constituting”. The thought is not just that they
bear on our “personal identity” in the sense of the
characterization question—what sort of people we are in some
fundamental sense. They literally determine when we begin and end.
Roughly speaking, a past being is you just if you now have narratives
of the right sort identifying you with her as she was then. A future
being is you just if the narratives she has then identify her with you
as you are now. Remembering a past event may be necessary for it to
figure in an identity-constituting narrative, but it’s not
sufficient, distinguishing narrativist from psychological-continuity
views. Narrativists about persistence include Schechtman (1996: esp.
ch. 5, 2001) and Schroer and Schroer (2014); critics include Strawson
(2008) and Olson and Witt (2019). DeGrazia 2005: ch. 3 is a helpful
overview.

All these 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 fourth 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 are anticriterialist views
about things other than people as well. And there is debate about how
anticriterialism should be understood (Olson 2012, Noonan 2011,
2019).

Most people—most Western philosophy teachers and students,
anyway—feel immediately drawn to psychological-continuity views
(Nichols and Bruno 2010 give experimental evidence for this). 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 the
person would be you, and that this would be so because she
was psychologically continuous with you. It is difficult, however, to
get from this thought to an attractive answer to the persistence
question.

What psychological relation might our persistence consist in? We have
already mentioned memory: a past or future being might be you if and
only if you can now remember an experience she had then, or vice
versa. This proposal faces two 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.
Later, as a middle-aged lawyer, she remembers paying the fine. Later
still, in her dotage, 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 trivial and 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 you
cannot know whether someone genuinely remembers a past experience
without already knowing whether she is the one who had it. Suppose we
want to know whether Blott, who exists now, is the same as Clott, whom
we know to have existed at some time in the past. The memory criterion
tells us that Blott is Clott just if Blott can now remember an
experience Clott had at that past time. 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
trivial or uninformative about the claim that memory connections are
necessary for us to persist. We can know that the corpse
resulting from my death cannot remember any events from my life
without knowing already whether it’s me.)

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 a new concept, “retrocognition” or
“quasi-memory”, which is just like memory but without the
identity requirement: even if it’s self-contradictory to say
that you 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).

Neither move gets us far, however, as both the original and the
modified memory criteria face a more obvious problem: 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. The person sleeping in your bed
last night must have been 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. We can then say that
you are 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.

Now suppose that a person x who exists at one time is the
same thing as something y existing at another time if and
only 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, and that
this erased 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). (Schechtman 2001 gives a different sort of
objection to the psychological-continuity strategy.)

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
(even if there would also be important psychological differences). 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 considered a drastic but acceptable treatment
for otherwise-inoperable brain tumors: see Rigterink 1980.) What if we
did both at once, destroying one hemisphere and transplanting the
other? Then too, the one who got the transplanted hemisphere would be
psychologically continuous with you, and would be you according to the
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
future being who is psychologically continuous with you must be you.
It follows that you are Lefty and also that you are Righty. But that
cannot be: if you and Lefty are one and you and Righty are one, Lefty
and Righty cannot be two. And yet they are: there are indisputably two
people after the operation. One thing cannot be numerically identical
with two things that are distinct from each other. Suppose Lefty is
hungry at a time when Righty isn’t. If you are Lefty, you are
hungry at that time. If you are Righty, you aren’t. If you are
Lefty and Righty, you are both hungry and not hungry at once: a
straight contradiction.

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, Noonan
2003: 139–42; Perry 1972 offers a more complex variant).

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 Heller 1990: ch. 1, Hudson
2001, Sider 2001a, Olson 2007: ch. 5). For each person, there is 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”, and diverge later 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. At the places where the roads overlap, they are just like one
road. Likewise, the idea goes, at the times before the operation when
Lefty and Righty share their temporal parts, they are just like one
person. Even they themselves can’t tell that they are two.
Whether we really are composed of temporal parts, however, is
disputed. (Its consequences are explored further in section 8.)

The other solution to the fission problem abandons the intuitive claim
that psychological continuity by itself suffices for us to persist. It
says, rather, that a past or future being is you only if she is then
psychologically continuous with you and no other being 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.) This means that
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,
Garrett 1998: ch. 4).

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 is
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 you would perish if one of your
hemispheres were transplanted and the other left in place: you can
survive hemispherectomy only if the hemisphere to be removed is first
destroyed. This seems mysterious. Why should an event that would
normally preserve your existence bring it to an end if accompanied by
a second such event—one having no causal effect on the first? If
your brain is to be divided, why do we need to destroy half of it in
order to save you? (For discussion, see Noonan 2003: 12–15 and
ch. 7.)

The problem is especially acute 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 or psychological
harm would kill you. (“Best-candidate” theories such as
Nozick 1981: ch. 1 attempt to avoid this.)

The non-branching view makes the What matters? question especially
acute. Faced with the prospect of having one of your hemispheres
transplanted, there is no evident reason to prefer having the other
destroyed. Most of us 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 is that there be someone in the future who is
psychologically continuous with you, whether or not she actually is
you.

The usual way to achieve this is to continue existing yourself, but
the fission story shows that this is not necessary. Likewise, even the
most selfish person has a reason to care about the welfare of the
beings who would result from her undergoing fission, even if, as the
non-branching view implies, neither would be her. In the fission case,
the sorts of practical concerns you ordinarily have for yourself apply
to someone other than you. This suggests more generally that facts
about who is who have no practical importance. All that matters
practically is who is psychologically continuous with whom. (Lewis
1976 and Parfit 1976 debate whether the multiple-occupancy view can
preserve the conviction that identity is what matters
practically.)

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, 2003a).
This is because no sort of psychological continuity appears to be
either necessary or sufficient for a human organism to persist. Human
organisms have brute-physical persistence conditions. If your brain
were transplanted, the one who ended up with that organ would be
uniquely psychologically continuous with you (and this continuity
would be continuously physically realized). On any
psychological-continuity view, she would be you: the person would go
with her transplanted brain. But no organism would go with its
transplanted brain. The operation would simply move an organ from one
organism to another. So it seems, anyway. It follows that if you were
an organism, you would stay behind with an empty head. Even though
this is never going to happen, it shows that according to
psychological-continuity views we have a property that no organism
has, namely possibly moving from one organism to another by brain
transplant.

Again, a human organism could continue existing in an irreversible
vegetative state with no psychological continuity. If you were an
organism, you could too. But according to psychological-continuity
views you could not. It follows that human animals have a property
that we lack, namely possibly surviving as a vegetable.

This does not merely rule out our being essentially or
“fundamentally” organisms, but our being organisms at all:
nothing that is even contingently an organism would go with its
transplanted brain, or would cease to exist just by lapsing into an
irreversible vegetative state.

But a healthy, adult human organism seems a paradigm case of a
thinking being. If human organisms can think, yet (as
psychological-continuity views imply) we are not organisms, three
difficulties arise. First, you are one of two intelligent
beings sitting there and reading this entry. More generally, there are
two thinking beings wherever we thought there was just one.

Second, the organism would not merely think in some way or other, but
would presumably be psychologically indistinguishable from you. That
would make it a person, if being a person amounts to having certain
mental or behavioral 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. Some—those that are
organisms—would have brute-physical persistence conditions.

Third, it becomes hard to see how you could know whether you
were a nonanimal person with psychological persistence conditions or
an 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 make this epistemic problem more vivid 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 temporary
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 transplanted
brain, 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.

These three objections have been called the
“too-many-thinkers” or “thinking-animal”
problem.

The most popular defense of the psychological-continuity view against
this objection 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. Thinking animals are not
a problem for psychological-continuity views for the simple reason
that there are none (Shoemaker 1984: 92–97, Lowe 1996: 1,
Johnston 2007: 55; Baker 2000 is a subtle variant). If human organisms
cannot be conscious, it would seem to follow that no biological
organism of any sort could have any mental properties at all.
Shoemaker argues that this follows from the functionalist theory of
mind (1999, 2008, 2011). 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)

Another 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. A person must also persist by
virtue of psychological continuity. It follows that human animals are
not people (thus solving the second problem, about personhood).

Further, personal pronouns such as ‘I’, and the thoughts
they express, refer only to people. 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 are 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 solving the third, epistemic problem). (See Noonan
1998, 2010, Olson 2002; for a different approach based on epistemic
principles see Brueckner and Buford 2009.)

Or one could say that human organisms have psychological persistence
conditions. Despite appearances, 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—presumably
destroying the animal into which the brain is implanted. (This may be
the view of Wiggins (1980: 160, 180) and McDowell (1997: 237), and is
unequivocally endorsed by Madden (2016); see also Langford 2014, Olson
2015: 102–106.)

None of these objections arise on animalism, the view that we 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 persistent vegetative state may not count as people. Being a person
may be only a temporary property of you, like being a student. Nor
does animalism imply that all people are organisms. It is consistent
with the existence of wholly inorganic people: gods or angels or
conscious robots. It does not say that being an animal is part of what
it is to be a person (a view defended in Wiggins 1980: 171 and
Wollheim 1984: ch. 1 and criticized in Snowdon 1996). Animalism leaves
the answer to the personhood question entirely open. (It is
consistent, for instance, with Locke’s definition quoted in
section 2.)

Assuming that organisms persist by virtue of some sort of
brute-physical continuity, animalism implies a version of the
brute-physical view. Some endorse a brute-physical view without saying
that we are animals. They say that we are our bodies (Thomson 1997),
or that our identity through time consists in the identity of our
bodies (Ayer 1936: 194). This has been called the bodily criterion of
personal identity. It is obscure, and its relation to animalism is
uncertain.

Most versions of the brute-physical view imply that human people have
the same persistence conditions as certain nonpeople, such as dogs.
And it implies that our persistence conditions differ from those of
immaterial people, if they are possible. It follows that there are no
persistence conditions for people as such. (Baker (2000: 124) objects
strenuously to this.)

The most common objection to brute-physical views is the repugnance of
their implication that you would stay behind if your brain were
transplanted (e.g. Unger 2000; for an important related objection see
Johnston 2007, 2016). In other words, brute-physical views are
unattractive in just the way that psychological-continuity views are
attractive.

Animalists generally concede the force of this, but take it to be
outweighed by other considerations. First, animalism avoids the
too-many-thinkers problem. Second, it 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 are
seeing something that will, if all goes well, be born, learn to speak,
and eventually become an adult human person. Yet none of us is in any
way psychologically continuous with a 12-week-old foetus.

And the “transplant argument” may be less compelling than
it 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 such a thing. But it’s not obvious
that you could not survive it either. Maybe the operation could save
your life, though at great cost. We cannot confidently rule this out
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. The claim is not that this is obviously true, but just
that it’s not obviously false. And in that case itd’s not obvious
that a person must go with her transplanted brain.

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, they must explain 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 a human organism, which persists by virtue
of brute-physical continuity, and a part of many bizarre and
gerrymandered objects, such as “contacti persons” (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).