Personal identity deals with philosophical questions that arise aboutourselves by virtue of our being people (or as lawyers andphilosophers like to say,persons). This contrasts withquestions about ourselves that arise by virtue of our being livingthings, conscious beings, moral agents, or material objects. Many ofthese questions occur to nearly all of us now and again: What am I?When did I begin? What will happen to me when I die? Others are moreabstruse. They have been discussed since the origins of Westernphilosophy, and most major figures have had something to say aboutthem. (There is also a rich but challenging literature on the topic inEastern philosophy: see the entryMind in Indian Buddhist Philosophy.)

The topic is sometimes discussed under the problematic termself. This term is sometimes synonymous with‘person’, but often means something different: a sort ofunchanging, immaterial subject of consciousness, for instance (as inthe phrase ‘the myth of the self’). It is often usedwithout any clear meaning and will be avoided here.

After surveying the main questions of personal identity, the entrywill focus on our persistence through time.

• 1. The Problems of Personal Identity
• 2. Understanding the Persistence Question
• 3. Accounts of Our Persistence
• 4. Psychological-Continuity Views
• 5. Fission
• 6. The Too-Many-Thinkers Objection
• 7. Animalism and Brute-Physical Views
• 8. Wider Themes
• Bibliography
• Academic Tools
• Other Internet Resources
• Related Entries

1. The Problems of Personal Identity

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

Characterization. Outside of philosophy, the term‘personal identity’ commonly refers to properties to whichwe feel a special sense of attachment or ownership. My personalidentity in this sense consists of those properties I take to“define me as a person” or to “make me the person Iam”. (The precise meaning of these phrases is hard to pin down.)To have an “identity crisis” is to become unsure about mymost characteristic properties—about what sort of person I am insome deep sense. To ask about it is to ask, in the expectation of adeep and revealing psychological answer, Who am I? My individualpersonal identity contrasts with my gender, ethnic, and nationalidentity, which consist roughly of the sex, ethnic group, or nation Itake myself to belong to and the importance I attach to it.

This sort of personal identity is contingent and temporary: the way Idefine myself as a person might have been different, and can vary fromone time to another. It is a subset, usually a small one, ofsomeone’s properties. It could happen that being a philosopherand a parent belong to my identity but not being a man or a cyclist,while someone else has the same four properties but feels differentlytowards them, so that being a man and a cyclist belong to his identitybut not being a philosopher or a parent. Someone may not even need tohave the properties belonging to her identity: if I becomeconvinced that I am Napoleon, being an emperor could be one of theproperties central to the way I define myself and thus (perhaps) partof my identity, even though I’m deluded and have never been anemperor.

What determines someone’s personal identity in this sense issometimes called thecharacterization question (Schechtman1996: 1). (Glover 1988: part 2 and Ludwig 1997 are usefuldiscussions.)

Personhood. What is it to be a person, as opposed toa nonperson? What have we people got that nonpeople lack? The questionoften arises in connection with specific cases: we may ask, forexample, at what point in our development from a fertilized egg therecomes to be a person, or what it would take for a chimpanzee or aMartian or a computer to be a person, if they could ever be. An idealaccount of personhood would be a definition of the word‘person’, filling the blanks in the formula‘Necessarily,x is a person at timet if andonly if …x …t …’.

The most common answer is that to be a person is to have certainspecial 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, indifferent times and places” (1975: 335; Baker 2000: ch. 3 is adetailed account of this sort). Others propose a less directconnection between personhood and these special mental properties:that to be a person is be capable of acquiring them, for example(Chisholm 1976: 136f.), or to belong to a kind whose members typicallyhave them when healthy and mature (Wiggins 1980: ch. 6). (A verydifferent answer is mentioned in section 6 below.)

Persistence. What does it take for a person topersist from one time to another—to continue existing ratherthan cease to exist? What sorts of things is it possible, in thebroadest sense of the word ‘possible’, for you to survive,and what sort of event would necessarily bring your existence to anend? What determines which past or future being is you? Suppose youpoint to a child in an old class photograph and say,“That’s me.” What makes you that one rather than oneof the others? What is it about the way she relates then to you as youare now that makes her you? For that matter, what makes it the casethat anyone at all existing back then is you? This is sometimes calledthe question ofpersonal identity over time, as it has to dowith whether the earlier and the later being are one thing ortwo—that is, whether they are numerically identical. An answerto it is an account of ourpersistence conditions.

Historically this question often arises from the thought that we mightcontinue existing after we die (as in Plato’sPhaedo).Whether this could happen depends on whether biological deathnecessarily brings our existence to an end. Imagine that after yourdeath there really will be someone, in this world or the next, whoresembles you in certain ways. How would she have to relate to you asyou are now in order tobe you, rather than someone else?What would the Higher Powers have to do to keep you in existence afteryour death? Or is it even possible? The answer to these questionsdepends on the answer to the persistence question.

Evidence. How do we find out who is who? Whatevidence bears on the question of whether the person here now is theone who was here yesterday? One source of evidence is first-personmemory: if you remember doing some particular action (or seem to), andsomeone really did do it, this supports the claim that that person isyou. Another source is physical resemblance: if the person who did itlooks just like you—or better, if she is in some way physicallyor spatio-temporally continuous with you—that too supports herbeing you. Which of these sources is more fundamental? Doesfirst-person memory count as evidence all by itself, or only insofaras we can check it against physical facts? What should we do whenthese considerations support opposing verdicts?

Suppose Charlie’s memories are erased and replaced with accuratememories (or apparent memories) of the life of someone longdead—Guy Fawkes, say (Williams 1956–7). Ought we toconclude on these grounds that the resulting person is not actuallyCharlie, but Guy Fawkes brought back to life? Or should we insteadinfer on the basis of physical continuity that he’s just Charliewith new memories? What principle would answer this question?

The evidence question dominated the anglophone literature on personalidentity from the 1950s to the 1970s (Shoemaker 1963, 1970 andPenelhum 1967 are good examples). It’s important to distinguishit from the persistence question. What it takes for you to persistthrough time is one thing; how we ought to evaluate the relevantevidence is another. If the criminal had fingerprints just like yours,the court may rightly conclude that he is you, but having yourfingerprints is notwhat it is for a past or future being tobe you: it’s neither necessary (you could survive without anyfingers at all) nor sufficient (someone else could have fingerprintsjust like yours).

Population. The persistence question is about whichof the characters introduced at the beginning of a story have survivedtill the end of it. But we can also ask how many are on the stage atany one time. What determines how many of us there are right now? Ifthere are eight billion people on the earth at present, whatfacts—biological, psychological, or what have you—makethat the right number?

You may think the number of people at any given time (or at least thenumber ofhuman people) is simply the number of humanorganisms there are then. But this is disputed. Some say that cuttingthe main connections between the cerebral hemispheres results inradical disunity of consciousness so that two people share a singleorganism (Nagel 1971; for skeptical views see Wilkes 1988: ch. 5 andvan Inwagen 1990: 188–212). Others say that a human being withmultiple personality could literally be the home of two or morethinking beings (Wilkes 1988: 127f., Rovane 1998: 169ff.; see alsoOlson 2003, Snowdon 2014: ch. 7). Still others argue that two peoplecan share an organism in cases of partial twinning (Campbell andMcMahan 2016; see also Olson 2014).

The population question is sometimes called the problem of“synchronic identity”, as opposed to the “diachronicidentity” of the persistence question; but these terms needcareful handling. They are apt to give the mistaken impression thatidentity comes in two kinds, synchronic and diachronic. The truth issimply that there are two kinds of situations where we can ask howmany people (or other things) there are: those involving just onemoment and those involving an extended period. To make matters worse,the term ‘synchronic identity’ is sometimes used toexpress the personhood question.

Personal ontology. What are we? What properties ofmetaphysical importance do we human people have, in addition to themental properties that make us people? What, for instance, are we madeof? Are we made entirely of matter, as stones are, or are we partly orwholly immaterial? Where do our spatial boundaries lie, if we arespatially 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 asspatial parts? Are we substances—metaphysically independentbeings—or is each of us a state or an activity of somethingelse?

Here are some of the main proposed accounts of what we are (Olson2007):

• We are biological organisms (“animalism”: van Inwagen1990, Olson 1997, 2003a).
• We are material things “constituted by” organisms: aperson is made of the same matter as a certain animal, but they aredifferent things because what it takes for them to persist isdifferent (Baker 2000, Johnston 2007, Shoemaker 2011).
• We are temporal parts of animals: each of us stands to an organismas 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 thingsmade up of an immaterial soul and a biological organism (Swinburne1984: 21).
• We are collections of mental states or events: “bundles ofperceptions”, as Hume said (1978 [1739]: 252; see also Quinton1962, 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 practicalimportance of facts about our persistence? Why does itmatter? If you had to choose between continuing to exist orbeing annihilated and replaced by someone else exactly like you, whatreason would you have to prefer one over the other? And what reason doyou have to care about what will happen to you, as opposed to whatwill happen to other people? Or is there any such reason? Imagine thatsurgeons are going to put your brain into my head and that neither ofus has any choice about this. The resulting person will be in terriblepain 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 topay? Will the resulting person—who will think he is you—beresponsible for your actions or for mine? (Or both, or neither?) Thesequestions are summarized in the phrasewhat matters insurvival.

The answer may seem to turn entirely on whether the resulting personwouldbe you or I. OnlyI can be responsible for myactions. The fact that some person is me, by itself, gives me a reasonto care about him. Each person has a special, selfish interest in herown future and no one else’s. Identity itself (numericalidentity) is what matters in survival. But some say that I could havean entirely selfish reason to care about someone else’s futurefor his own sake. Perhaps what gives me a reason to care about whathappens to the man people will call by my name tomorrow is not that heis me, but that he is then psychologically continuous with meas I am now (see Section 4). If someone else were psychologicallycontinuous tomorrow with me as I am now,he would have whatmatters to me and I ought to transfer my selfish concern to him.Likewise, someone else could be responsible for my actions, and notfor 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 onothers, they are largely independent. Many discussions of personalidentity leave it unclear which one is at stake.

2. Understanding the Persistence Question

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

It asks roughly what is necessary and sufficient for a past or futurebeing to be someone existing now. If we point to you now, and thendescribe someone or something existing at another time, we can askwhether we are referring to two different things or simply referringtwice to one thing. The persistence question is what determines theanswer to such queries. (And there are precisely analogous questionsabout the persistence of dogs, rocks, and other things.)

Here are three common misunderstandings of this question. Some take itto ask what itmeans to say that a past or future being isyou. This would imply that we can answer it simply by reflecting onour linguistic knowledge—on what we mean by the word‘person’, for example. The answer would be knowable apriori. It would also imply that all people must have the samepersistence conditions—that the answer to the question is thesame no matter what sort of people we considered. Though some endorsethese claims (Noonan 2019b: 84–93), they are disputed. What ittakes for us to persist might depend on whether we are biologicalorganisms, which we cannot know a priori. And if there could beimmaterial people—gods or angels, say—what it would takefor them to persist might differ from what it takes for a human personto persist. In that case our persistence conditions could not beestablished by linguistic or conceptual analysis.

Second, the persistence question is often confused with the questionof what it takes for someone toremain the sameperson—as in this passage from Bertrand Russell (1957: 70):“Before we can profitably discuss whether we shall continue toexist after death, it is well to be clear as to the sense in which aman is the same person as he was yesterday.” If Baffles were tochange in certain ways—if she lost much of her memory, say, orchanged dramatically in character, or became severelydisabled—we might ask whether she would still be the person shewas before, or instead become a different person. This is not aquestion about persistence—about numerical identity over time.To ask whether Baffles is the same person thatshe wasbefore, or to say that she is a different person from the oneshe used to be, presupposes that she herself existed at theearlier time. The question arises only when numerical identity isassumed. To ask about Baffles’ persistence, by contrast, is toask not whether she is still the same person, but whether she stillexists at all.

When we speak of someone’s remaining the same person or becominga different one, we mean remaining or ceasing to be a certainsort of person. For someone no longer to be the same personis 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 ofthe characterization question—with changes in respect of thoseproperties that “define someone as a person.”

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

1. If a personx exists at one time and a personyexists at another time, under what possible circumstances is it thecase thatx isy?

This asks, in effect, what it takes for a past or futureperson to be you, or for you to continue existingas aperson. We have a person existing at one time and a personexisting at another, and the question is what is necessary andsufficient for them to be one person rather than two.

This is narrower than the persistence question. We may want to knowwhether each of us was ever an embryo, or whether we could survive inan irreversible vegetative state (where the resulting being isbiologically alive but has no mental properties). These are clearlyquestions about what it takes for us to persist. But as personhood ismost commonly defined (recall Locke’s definition quotedearlier), something is a person at a given time only if it has certainspecial mental properties at that time. Embryos and human beings in avegetative state, having no mental properties at all, are thus notpeople when they’re in that condition. And in that case wecannot infer anything about whether you were once an embryo or couldexist in a vegetative state from a principle about what it takes for apast or futureperson to be you.

We can illustrate the point by considering this answer to question1:

Necessarily, a personx existing at one time is a persony existing at another time if and only ifx can, atthe first time, remember an experiencey has at the secondtime, or vice versa.

That is, a past or future person is you just if you (who are now aperson) can now remember an experience she had then, or she can thenremember an experience you’re having now. Call this thememory criterion. (It too is often attributed to Locke,though it’s uncertain whether he actually held it: see Behan1979.)

The memory criterion may seem to imply that if you were to lapse intoan irreversible vegetative state, you would cease to exist (or perhapspass to the next world): the resulting being could not be you becauseit would not remember anything. But no such conclusion follows.Assuming that an organism in a vegetative state is not a person, thisis not a case involving a person existing at one time and a personexisting at another time. The memory criterion can only tell us whichpast or futureperson you are, not which past or future beinggenerally. It says what it takes for someone to persistas aperson, but not what it takes for someone to persist withoutqualification. So it implies nothing about whether you could exist ina vegetative state or even as a corpse, or whether you were once anembryo. As stated, it’s compatible with your surviving with nomemory continuity at all, as long as this happens when you are not aperson (Olson 1997: 22–26, Mackie 1999: 224–228).

No advocate of the memory criterion would accept this. The view isintended to imply that if a personx exists now and a beingy exists at another time—whether or not it’s aperson then—they are one just ifx can now remember anexperiencey has at the other time or vice versa. But thisnot an answer to Question 1: what it takes for a person existing atone time and a person existing at another time to be one rather thantwo. It’s an answer to a more general question: what it takesfor something that is a person at one time to exist at another time aswell, whether or not it’s a person then:

2. If a personx exists at one time and somethingyexists at another time, under what possible circumstances is it thecase thatx isy?

Those who ask Question 1 are commonly assuming that every person is apersonessentially: nothing that is in fact a person couldpossibly exist without being a person. (By contrast, no student is astudent essentially: something that is in fact a student can existwithout being a student.) This claim, “personessentialism,” implies that whatever is a person at one timemust be a person at every time when she exists, making Questions 1 and2 equivalent.

But person essentialism is controversial (Olson and Witt 2020).Combined with a Lockean account of personhood, it implies that youwere never an embryo: at best you may have come into being when theembryo that gave rise to you developed certain mental capacities. Norcould you exist in a vegetative state. It rules out the brute-physicalview described in the next section. Whether we were once embryos orcould exist in a vegetative state, or whether we are peopleessentially, would seem to be substantive questions that an account ofour persistence should answer, not matters to be presupposed in theway we frame the debate.

3. Accounts of Our Persistence

Three main sorts of answers to the persistence question have beenproposed.Psychological-continuity views say that ourpersistence consists in some psychological relation, the memorycriterion mentioned earlier being an example. You are that futurebeing that in some sense inherits its mental features fromyou—beliefs, memories, preferences, the capacity for rationalthought, and so on—and you are that past being whose mentalfeatures you have inherited in this way. There is dispute over whatsort of inheritance this has to be—whether it must beunderpinned by some kind of physical continuity, for instance, andwhether 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 philosopherswriting on personal identity since the early 20th century haveendorsed 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 physicalrelation not involving psychology: you are that past or future beingthat has your body, or that is the same biological organism as youare, or the like. Call thesebrute-physical views. (Advocatesinclude 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 andphysical continuity to survive, or that either would suffice withoutthe other (Nozick 1981: ch. 1, Langford 2014, Madden 2016, Noonan2021).

Both views agree that there is something that it takes for us topersist—that there are informative, nontrivial, necessary andsufficient conditions for a person existing at one time to exist atanother time. A third view,anticriterialism, denies this.Psychological and physical continuity are evidence for persistence, itsays, but do not always guarantee it and may not be required. Theclearest advocate of this view is Merricks (1998; see also Swinburne1984, Lowe 1996: 41ff., 2012; Langford 2017; for criticism seeZimmerman 1998, Shoemaker 2012). There is also debate about howanticriterialism 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 yourmemories and other mental features, the resulting person would beconvinced that he or she was you. This can make it easy to supposethat shewould be you, and this would be so because of herpsychological relation to you. But there is no easy path from thisthought to an attractive answer to the persistence question.

What psychological relation might it be? We have already mentionedmemory: a past or future being might be you just if you can nowremember an experience she had then or vice versa. This proposal facestwo historical objections, dating to Sergeant and Berkeley in the 18thcentury (see Behan 1979) but more famously discussed by Reid andButler (see the snippets in Perry 1975).

To see the first objection, imagine that a young student is fined foroverdue library books. As a middle-aged lawyer she remembers payingthe fine, but in her dotage she remembers her law career but hasentirely forgotten not only paying the fine but all the other eventsof her youth. According to the memory criterion the student is thelawyer and the lawyer is the elderly woman, but the elderly woman isnot the student. This is an impossible result: ifx andy are one andy andz are one,xandz cannot betwo. Identity is transitive; memorycontinuity is not.

The second objection is that it seems to belong to the very idea ofremembering that you can remember only your own experiences. Toremember paying a fine (or the experience of it) is to rememberyourself paying. That makes it uninformative to say that youare the person whose experiences you can remember—that memorycontinuity is sufficient for us to persist. It’s uninformativebecause we could not know whether someone genuinely remembers a pastexperience 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 in the past. The memory criterion tellsus that Blott is Clott just if Blott can now remember an experienceClott had then. But Blott’s seeming to remember one ofClott’s experiences counts as genuine memory (the objectiongoes) only if Blott actually is Clott. So we would already have toknow whether Blott is Clott before we could apply the principle thatis supposed to tell us whether she is. (There is, however, nothinguninformative about the claim that memory connections arenecessary for us to persist—that you could not survivewithout being able to remember anything, for example.)

One response to the first objection (about transitivity) is to modifythe memory criterion by switching from direct to indirect memoryconnections: the old woman is the young student because she can recallexperiences the lawyer had at a time when the lawyer remembered thestudent’s life. The second problem is commonly met by replacingmemory with “quasi-memory”, which is just like memory butwithout the identity requirement: even if it’s impossible toremember doing something you didn’t do but someone else did, youcould 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 ourpasts that we cannot remember or quasi-remember at all, and to whichwe are not linked even indirectly by an overlapping chain of memories.There is no time when you could recall anything that happened to youwhile you dreamlessly slept last night. The memory criterion has theabsurd implication that you have never existed at any time when youwere unconscious, and that the person sleeping in your bed last nightwas someone else.

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

That would enable us to say that a personx who exists at onetime is the same thing as somethingy existing at anothertime just ifx is, at the one time, psychologicallycontinuous withy as it is at the other time. This avoids themost obvious objections to the memory criterion.

It still leaves important questions unanswered, however. Suppose wecould somehow copy all the mental contents of your brain to mine, muchas we can copy the contents of one computer drive to another, therebyerasing the previous contents of both brains. Whether this would be acase of psychological continuity depends on what sort of causaldependence counts. The resulting being (with my brain and your mentalcontents) would be mentally as you were before, and not as I was. Hewould have inherited your mental properties in a way—but a funnyway. Is it the right way? Could you literally move from one organismto another by “brain-state transfer”?Psychological-continuity theorists disagree. (Shoemaker (1984:108–111, 1997) says yes; Unger (1990: 67–71) says no; seealso van Inwagen 1997.)

5. Fission

A more serious worry for psychological-continuity views is that youcould be psychologically continuous with two past or future people atonce. If your cerebrum—the upper part of the brain largelyresponsible for mental features—were transplanted, the recipientwould be psychologically continuous with you by anyone’s lights,and any psychological-continuity view will imply that she would beyou. If we destroyed one of your cerebral hemispheres, the resultingbeing would also be psychologically continuous with you.(Hemispherectomy—even the removal of the left hemisphere, whichcontrols speech—is sometimes carried out as a treatment forsevere epilepsy: see Shurtleffet al. 2021.) And it would bethe same if we did both at once, destroying one hemisphere andtransplanting the other: the recipient would be you on anypsychological-continuity view.

But now suppose that both hemispheres are transplanted, each into adifferent empty head. (We needn’t pretend that the hemispheresare exactly alike.) The two recipients—call them Lefty andRighty—will each be psychologically continuous with you. Thepsychological-continuity view as we have stated it says that any beingwho is psychologically continuous with you must be you. It followsthat you are Lefty and also that you are Righty. But that again isimpossible: it cannot be that you and Lefty are one and you and Rightyare one, but Lefty and Righty are two. Yet they are: there are clearlytwo people after the operation. One thing cannot be numericallyidentical with two different things. We can see this in another way byasking how many people there are in the whole story, from start tofinish. If you are both Lefty and Righty, the answer is one: the onlyperson in the story is you. Yet seeing as Lefty is not Righty, theanswer must be at least two. Your being both Lefty and Righty wouldimply that the number of people in the story is both one and more thanone.

Psychological-continuity theorists have proposed two differentsolutions to this problem. One, sometimes called the“multiple-occupancy view”, says that if there is fissionin your future, then there are two of you, so to speak, even now. Whatwe think of as you is really two people, who are now exactly similarand located in the same place, doing the same things and thinking thesame thoughts. The transplant operation merely separates them (Lewis1976, Perry 1972, Noonan 2019b: 141–144).

The multiple-occupancy view is usually combined with the generalmetaphysical claim that people and other persisting things arecomposed of temporal parts (often called“four-dimensionalism” or “perdurantism”; seeHudson 2001, Sider 2001a, Olson 2007: ch. 5). The idea is that foreach part of a person’s life, there is a thing just like theperson except that it exists only at that time. That thing is atemporal part of the person: it stands to the person as the first halfof a football match stands to the match. On this account, themultiple-occupancy view is that Lefty and Righty coincide before theoperation by sharing their pre-operative temporal parts or“stages”, then diverge by having different temporal partslocated afterwards. They are like two roads that coincide for astretch and then fork, sharing some of their spatial parts but notothers. Much as the roads are just like one road where they overlap,Lefty and Righty are just like one person before the operation whenthey share their temporal parts. Even they themselves can’t tellthat they are two. There are two coinciding people before theoperation because of what happens later, just as there may becoinciding two roads here because of what’s the case elsewhere.Whether we really are composed of temporal parts, however, isdisputed. (Its consequences are explored further in section 8.)

The second and more commonly proposed solution abandons the claim thatpsychological continuity by itself suffices for us to persist, andsays that a past or future being is you only if she is thenpsychologically continuous with you andno other being thenis. (There is no circularity in this. We need not know the answer tothe persistence question in order to know whether people existingsimultaneously are two or one: that comes under the populationquestion.) So neither Lefty nor Righty is you: they both come intoexistence when your cerebrum is divided. If both your cerebralhemispheres are transplanted, you cease to exist—though youwould 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 thesurprising consequence that if your brain is divided, you will surviveif only one half is preserved, but you will die if both halves are.That looks like the opposite of what we should expect: if yoursurvival depends on the functioning of your brain (becausethat’s what underlies psychological continuity), then the moreof that organ we preserve, the greater ought to be your chance ofsurviving.

In fact the non-branching view implies that transplanting onehemisphere and leaving the other in place would also be a fatal caseof fission. Its consequences are especially surprising if brain-statetransfer counts as psychological continuity: in that case, evencopying your total brain state to another brain without doing you anyphysical harm would kill you.

These consequences are not just hard to believe, but also mysterious.Keeping half your brain functioning is normally sufficient for yoursurvival, on a psychological-continuity view. Why then would you notsurvive if the other half too were kept functioning, separate from thefirst? How could an event that would normally ensure your survivaldestroy you if accompanied by a second such event (Noonan 2019b:128–141)?

The non-branching view is largely responsible for the interest in thequestion of what matters in survival. Faced with the prospect ofhaving one of your hemispheres transplanted, there is no evidentreason to want the other one to be destroyed. Most of us, it seems,would rather have both preserved, even if they go into differentheads. Yet on the non-branching view that is to prefer death overcontinued existence. This leads Parfit and others to say that that isprecisely what weought to prefer. We have no reason to wantto continue existing, at least for its own sake. What you have reasonto want (assuming that your life is going well) is that there besomeone in the future who is psychologically connected or continuouswith you, whether or not she is you. The usual way to achieve this isto continue existing yourself, but on the non-branching viewit’s not necessary.

Likewise, even the most selfish person may have a reason to care aboutthe welfare of the beings who would result from her undergoingfission, whether or not either of them would be her. The non-branchingview suggests that the sorts of practical concerns you ordinarily havefor yourself can apply to someone other than you. More generally,facts about numerical identity—about who is who—have nopractical importance. All that matters is who is psychologicallyconnected or continuous with whom. Psychological-continuity views areoften said to be superior to brute-physical views in accounting forwhat matters in survival. Fission cases threaten this claim. (Lewis1976 and Parfit 1976 debate whether the multiple-occupancy view canpreserve the conviction that identity is what matterspractically.)

6. The Too-Many-Thinkers Objection

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

We can see that it’s not necessary by noting that each humanorganism persists as an embryo without psychological continuity. Andwe can see that it’s not sufficient by imagining that your brainis transplanted. In that case the recipient would be uniquelypsychologically continuous with you, and this continuity would becontinuously physically realized. Psychological-continuity views implythat she would be you. A person would go with her transplanted brain.But it doesn’t seem that any organism would go with itstransplanted brain. It looks as if the operation would simply move anorgan from one organism to another, like transplanting a liver. Itfollows that if you were an organism, you would stay behind with anempty head, contrary to psychological-continuity views.

Psychological-continuity views do not merely rule out our beingessentially or “fundamentally” organisms, but our beingorganisms at all. They say that each person has the property ofpersisting by virtue of psychological continuity: of being such thatpsychological continuity (perhaps with a non-branching restriction) isboth necessary and sufficient for it to continue existing. But noorganism has this property. (Or at least no human organism does, andwe are clearly not non-human organisms.) Or again: every person wouldgo with her transplanted brain, but no organism would do so. And ifevery person has a property that no organism has, then no person is anorganism.

This is said to be a problem for psychological-continuity viewsbecause healthy, adult human organisms appear to be conscious andintelligent. Suppose they are. And suppose, aspsychological-continuity views seem to imply, that we ourselves arenot organisms. Three awkward consequences follow.

First, you are one of two intelligent beings sitting there and readingthis entry: there is, in addition to you, an organism reading it. Moregenerally, there are two thinking beings wherever we thought there wasjust one, a person and an organism distinct from it.

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

Third, it’s hard to see how you could know whether you yourselfwere the nonanimal person with psychological persistence conditions orthe animal person with brute-physical ones. If you thought you werethe nonanimal, the organism would use the same reasoning to concludethatit was too. For all you could ever know, it seems, youyourself might be the one making this mistake.

We can illustrate this epistemic problem by imagining athree-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 ofyou in the “out” box. The process causes momentaryunconsciousness but is otherwise harmless. Two beings wake up, one ineach box. The boxes are indistinguishable. Because each being willhave the same apparent memories and perceive identical surroundings,each will think, for the same reasons, that he or she is you—butonly one will be right. If this happened to you, it’s hard tosee how you could know, afterwards, whether you were the original orthe duplicate. (Suppose the technicians who work the machine are swornto 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 beingonly a moment ago, complete with false memories of someoneelse’s life?” And you would have no way of answering thesequestions. In the same way, psychological-continuity views raise thequestions, “What am I? Am I a nonanimal that would go with itsbrain if that organ were transplanted, or an animal that would staybehind with an empty head?” And here too there seem to be nogrounds on which to answer them.

This is the “too-many-thinkers” or“thinking-animal” objection to psychological-continuityviews. The most common defense against it is to say that, despitesharing our brains and showing all the outward signs of consciousnessand intelligence, human organisms do not think and are not conscious.There simplyare no thinking animals (Shoemaker 1984:92–97, Lowe 1996: 1, Johnston 2007: 55; Baker 2000 offers a morecomplex variant).

But although this is easy to say, it’s hard to defend. If humanorganisms cannot be conscious and intelligent, it would seem to followthat 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 consciousbeings, with the same behavior, but lacking consciousness (Olson2018). And it leaves us wonderingwhy organisms cannot beconscious. The best proposed answer is given by Shoemaker (1999, 2008,2011), who argues that it is because organisms have the wrongpersistence conditions, but it’s highly controversial.

A second option is to concede that human organisms are psychologicallyindistinguishable from us, but try to explain how we can still knowthat we are not those organisms. The best-known proposal of this sortfocuses on personhood and first-person reference. It says that notjust any being with mental properties of the sort that you and Ihave—rationality and self-consciousness, forinstance—counts as a person (contrary to anything likeLocke’s definition). A person must also persist by virtue ofpsychological 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 thoughtsthey express, refer only to people in this sense. So when your animalbody says or thinks ‘I’, it refers not to itself, but toyou, the person. The organism’s statement ‘I am aperson’ does not express the false belief thatit is aperson, but the true belief that you are. So the organism is notmistaken about which thing it is: it has no first-person beliefs aboutitself at all. And you’re not mistaken either. You can inferthat you are a person from the linguistic facts that you are whateveryou refer to when you say or think ‘I’, and that‘I’ (in its typical uses, at least) never refers toanything but a person. You can know that you are not the animalthinking your thoughts because it’s not a person, and thus whatyou refer to when you say or think ‘I’. This avoids thethird, epistemic version of the too-many-thinkers problem. (See Noonan1998, 2010, Olson 2002; for a different approach see Brueckner andBuford 2009.)

The too-many-thinkers objection is based on the assumption thatpsychological-continuity views rule out our being organisms. Somequestion this assumption: they suggest that human organismsdo persist by virtue of psychological continuity. Even if youare an organism, the transplant operation would not move your brainfrom one organism to another. Rather, it would cut an organism down tothe size of a brain, move it across the room, and then give it newparts to replace the ones it lost. This view is sometimes called“new animalism” (Madden 2016, Noonan 2021; see alsoLangford 2014, Olson 2015: 102–106).

7. Animalism and Brute-Physical Views

Animalism says that we human people are organisms. This does not implythat all organisms, or even all human organisms, are people: as we sawearlier, human embryos and animals in a vegetative state are notpeople on the most common definitions of that term. Being a person maybe only a temporary property of us, like being a student. Nor does itimply that all people are organisms: it is consistent with there beingwholly inorganic people such as gods or intelligent robots. Animalismis not an answer to the personhood question. (It is consistent, forinstance, with Locke’s definition of ‘person’.)

For the most part, both animalists and their opponents say thatorganisms persist by virtue of some sort of brute-physical continuitywith no psychological element. So most animalists accept abrute-physical account of our persistence. And most advocates ofbrute-physical views take us to be organisms.

The most common objection to brute-physical views (and, by extension,to animalism) focuses on their implication that transplanting yourbrain into my head would not give you a new body, but would give me anew 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 the force of this, but take it to beoutweighed by other considerations: that we appear to be organisms,for example, that it’s hard to say what sort ofnonorganisms we might be, and that our being organisms wouldavoid the too-many-thinkers problem. And animalism is compatible withour beliefs about who is who in real life: every actual case in whichwe take someone to survive or perish is a case where an organism doesso. Psychological-continuity views, by contrast, conflict with theappearance that each of us was once a foetus. When we see anultrasound picture of a 12-week-old foetus, we ordinarily thinkwe’re seeing something that will, if all goes well, be born,learn to speak, and eventually become an adult human person, yet noperson is in any way psychologically continuous with a 12-week-oldfoetus.

And the objection may be less compelling than it first appears(Snowdon 2014: 234). Suppose you had a tumor that would kill youunless your brain were replaced with a healthy donated organ. Thiswould have grave side-effects: it would destroy your memories, plans,preferences, and other mental properties. It may not be clear whetheryou could survive it, even if the operation were successful. But is itreally obvious that you couldnot survive it? Maybe it couldsave your life, though at great cost. And this might be so, theargument goes, even if the new brain gave you memories, plans, andpreferences from the donor. But if it’s not obvious that thebrain recipient would not be you, then it’s not obvious that itwould be the donor. A brain transplant might be metaphysicallyanalogous to a liver transplant. Again, the claim is not that this isobviously true, but only that it’s not obviously false. And inthat case it’s not obvious that a person must go with hertransplanted brain. (Williams 1970 argues in a similar way.)

8. Wider Themes

The debate between psychological-continuity and brute-physical viewscannot be settled without considering more general matters outside ofpersonal identity. For instance, psychological-continuity theoristsneed to explain why human organisms are unable to think as we do. Thiswill require an account of the nature of mental properties. Or ifhuman organismscan think, psychological-continuity theoristswill want an account of how we can know that we are not thoseorganisms. This will turn on how the reference of personal pronounsand proper names works, or on the nature of knowledge.

Some general metaphysical views suggest that there is no unique rightanswer to the persistence question. The best-known example is theontology of temporal parts mentioned in section 5. It says that forevery period of time when you exist, short or long, there is atemporal part of you that exists only then. This gives us many likelycandidates for being you—that is, many different intelligentbeings now sitting there and reading this. Suppose you are a materialthing, and that we know what determines your spatial boundaries. Thatshould tell us what counts as your current temporal part or“stage”—the temporal part of you located now and atno other time. But that stage is a part of a vast number of temporallyextended objects (Hudson 2001: ch. 4).

For instance, it’s a part of a being whose temporal boundariesare determined by relations of psychological continuity (Section 4)among its stages. That is, one of the beings thinking your currentthoughts is an aggregate of person-stages, each of which ispsychologically continuous with each of the others and with no otherstage. If this is what you are, then you persist by virtue ofpsychological continuity. Your current stage is also a part of a beingwhose temporal boundaries are determined by relations of psychologicalconnectedness. That is, one of the beings now thinking yourthoughts is an aggregate of person-stages, each of which ispsychologically connected with each of the others and to no otherstage. This may not be the same as the first being, as some stages maybe psychologically continuous with your current stage but notpsychologically connected with it. If this is what you are, thenpsychological connectedness is necessary and sufficient for you topersist (Lewis 1976). What’s more, your current stage is a partof 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 (Sider2001a, 188–208). And there would be many other candidates.

The temporal-parts ontology implies that each of us shares our currentthoughts with countless beings that diverge from one another in thepast or future. In that case it’s not evident which of thesethings are us. Of course, we are the things we refer to when we say‘I’, or more generally the referents of our personalpronouns and proper names. But these words would be unlikely tosucceed in referring to just one sort of thing—to only one ofthe many candidates on each occasion of utterance. There wouldprobably be some indeterminacy of reference, so that each suchutterance referred ambiguously to many different candidates. Thatwould make it indeterminate what things, and even what sort of things,we are. And insofar as the candidates have different histories anddifferent persistence conditions, it would be indeterminate when wecame into being and what it takes for us to persist (Sider 2001b).

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Acknowledgments

Some material in this entry appeared previously in E. Olson,‘Personal Identity’, inThe Blackwell Guide to thePhilosophy of Mind, edited by S. Stich and T. Warfield, Oxford:Blackwell, 2003.