Personal Identity (Stanford Encyclopedia of Philosophy) Cite this entry Search the SEP • Advanced Search • Tools • RSS FeedTable of Contents• What's New• Archives• Projected ContentsEditorial Information• About the SEP• Editorial Board• How to Cite the SEP• Special CharactersSupport the SEPContact the SEP ©Metaphysics Research Lab,CSLI,Stanford University  Open access to the SEP is made possible by a world-wide funding initiative. Please Read How You Can Help Keep the Encyclopedia FreePersonal IdentityFirst published Tue Aug 20, 2002; substantive revision Tue Feb 20, 2007Personal identity deals with questions about ourselves quapeople (or persons). Many of these questions are familiar ones thatoccur to everyone at some time: What am I? When did I begin? What willhappen to me when I die? Discussions of personal identity go rightback to the origins of Western philosophy, and most major figures havehad something to say about it. (There is also a rich literature onpersonal identity in Eastern philosophy, which I am not competent todiscuss. Collins 1982 is a good source.)I will first survey the main questions that go under the heading ofpersonal identity. Most of the entry will then focus on the one thathas received most attention in recent times, that of personal identityover time. I will discuss what the question means, and the mainproposed answers. I will also try to show how these answers relate tosome of the other questions about personal identity, and to moregeneral questions in metaphysics and the philosophy of mind.1. The Problems of Personal Identity2. Understanding the Persistence Question3. Accounts of Our Identity Through Time4. The Psychological Approach5. Fission6. The Problem of the Thinking Animal7. The Somatic Approach8. Wider IssuesBibliographyOther Internet ResourcesRelated Entries1. The Problems of Personal IdentityThere is no single problem of personal identity, but rather a widerange of loosely connected questions. Here are the main ones:Who am I? We often speak of one's "personal identity"as what makes one the person one is. Your identity in this senseconsists roughly of those properties that make you unique as anindividual and different from others. Or it is the way you see ordefine yourself. Or it may be the network of values and convictionsthat structure your life. We might call it your individualpsychological identity. Your individual psychological identity is aproperty (or set of properties), and presumably one that you have onlycontingently: you might have had a different identity from the one youin fact have. Likewise, it is a property that you might have for awhile and then lose: you could acquire a new individual identity, orperhaps even carry on without one. (Ludwig 1997 is a typicaldiscussion of the Who am I? question.)Personhood. What is it to be a person? What isnecessary, and what is sufficient, for something to count as a person,as opposed to a non-person? What have people got that non-peoplehaven't got? This is more or less equivalent to asking for thedefinition of the word person. An answer would take the form“Necessarily, x is a person if and only if …x …”, with the blanks appropriately filledin. More specifically, we can ask at what point in one's developmentfrom a fertilized egg there comes to be a person, and what it wouldtake for a chimpanzee or a Martian or an electronic computer to be aperson, if they could ever be. (See e.g. Chisholm 1976: 136f., Baker2000: ch. 3.)Persistence. What does it take for a person topersist from one time to another — that is, for thesame person to exist at different times? What sorts ofadventures could you possibly survive, in the broadest sense of theword 'possible'? What sort of thing would necessarily bring yourexistence to an end? What determines which past or future being isyou? Suppose you point to a little girl in an old class photograph andsay, "That's me." What makes you that girl, 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 you existed at all back then? This is the question of personalidentity over time. An answer to it is an account of our persistenceconditions, or a criterion of personal identity over time (a"constitutive" rather than an evidential criterion: see the EvidenceQuestion below).Historically this question often arises out of the hope (or fear) thatwe might continue to exist after we die. Whether this is in any sensepossible depends on whether biological death is the sort of thing thatone could survive. Imagine that after your death there reallywill be someone, in the next world or in this one, who is rather likeyou. How would that being have to relate to you as you are now inorder to be you — rather than me, say, or a new person whodidn't exist before? What would the Higher Powers have to do in orderto keep you in existence (or bring you back into existence) after yourdeath? Or is there anything they could do? The answer to thesequestions depends 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? What ought we to do when different kindsof evidence support opposing verdicts? One source of evidence isfirst-person memory: if you remember doing something, or at least seemto remember, it was probably you who did it. Another source isphysical continuity: if the person who did it looks just like you, oreven better if she is in some sense physically or spatio-temporallycontinuous with you, that is reason to think she is you. Which ofthese sources is more fundamental? Does first-person memory supplyevidence all by itself, for instance, or does it count as evidenceonly insofar as we can check it against publicly available physicalevidence?The Evidence Question dominated the philosophical literature onpersonal identity from the 1950s to the 1970s (Shoemaker 1963 andPenelhum 1970 are good examples). Though it is sometimes confusedwith the Persistence Question, the two are not the same. What it takesfor you to persist through time is one thing; how we find out whetheryou have is another. If the criminal had fingerprints just like yours,the courts may conclude that he is you. But even if that isconclusive evidence, having your fingerprints is not what itis for a past or future being to be you: it is neither necessary(you could survive without any fingers at all) nor sufficient (someoneelse could have fingerprints just like yours).Population. If we think of the Persistence Questionas asking which of the characters introduced at the beginning of astory have survived to become the characters at the end of it, we canalso ask how many characters are on the stage at any one time. Whatdetermines how many of us there are now? If there are some sixbillion people on the earth at present, what facts — biological,psychological, or what have you — make it the case that that is howmany there are? The question is not what causes there to be acertain number of people at a given time, but what there being thatnumber of people consists in. It is like asking what sort ofconfiguration of pieces amounts to black's winning a game of chess,and not like asking what sorts of moves lead to its winning.You may think that the number of people at any given time is simplythe number of human organisms there are then (perhaps discountingthose in a defective state that don't count as people, and ignoringnon-human people, if there are any). But this is disputed. Surgeonssometimes cut the nerve bands connecting one's cerebral hemispheres(commissurotomy), resulting in such peculiar behavior assimultaneously pulling one's trousers up with one hand and pullingthem down with the other. You might think that this gives us twopeople sharing one organism. (See e.g. Nagel 1971. Puccetti 1973argues that there are two people within the skin of every normal humanbeing.) Or maybe a human being with split personality could literallybe the home of two or more different thinking beings (Wilkes 1988:127f., Rovane 1998; see also Olson 2003b).This is sometimes called the problem of "synchronic identity", asopposed to the "diachronic identity" of the Persistence Question (andthe "counterfactual identity" of the How could I have been? Questionbelow). We must handle these terms with care, however, for they areapt to give the impression that identity comes in two kinds,synchronic and diachronic: a serious blunder. The truth is simply thatare two kinds of situations where we can ask how many people (or otherthings) there are: synchronic situations involving just one moment anddiachronic ones involving a stretch of time.What am I? What sort of things, metaphysicallyspeaking, are you and I and other human people? What is our basicmetaphysical nature? For instance, what are we made of? Are we madeup entirely of matter, or partly or wholly of something else? If weare made of matter, which matter makes us up at any moment?What parts do we have, if any? Are we substances —metaphysically independent beings — or is each of us a state oran aspect of something else, or perhaps some sort of process orevent?One possible answer to this question is that we are biologicalorganisms. Surprisingly perhaps, most philosophers reject this. (Wewill return to it later.) Another is that we are partless immaterialsubstances (or compound things made up of an immaterial soul and amaterial body: see Swinburne 1984). Hume suggested that each of us is"a bundle of perceptions" (1978: 252; see also Quinton 1962 and Rovane1998: 212). A popular view nowadays is that we are material things"constituted by" human animals: you are made of the same matter as acertain animal, but you and the animal are different things becausewhat it takes for you to persist is different (Shoemaker 1984:112–114 and 1999, Baker 2000). Another is the idea that we aretemporal parts of animals (Lewis 1976). There is even the paradoxicalview that there is nothing that we are: we don't really exist at all(Russell 1985: 50, Wittgenstein 1922: 5.631, Unger 1979).How could I have been? How different could I havebeen from the way I actually am? Which of my properties do I haveessentially, and which only accidentally or contingently? Could I, forinstance, have had different parents? Frank Sinatra and Doris Daymight have had children together. Could I have been one of them? Orcould they only have had children other than me? Could I have died inthe womb before I acquired any mental features? Are there possibleworlds just like the actual one except for who is who — wherepeople have "changed places" so that what is in fact your career is mycareer and vice versa? Whether these are best described as questionsabout personal identity is debatable. (They are not about whetherbeings in other worlds are identical with people in the actual world:see van Inwagen 1985.) But they are often discussed in connection withthe others.What matters in identity? What is the practicalimportance of facts about our identity and persistence? Why should wecare about it? Why does it matter? Imagine that surgeons aregoing to put your brain into my head, and that neither of us has anychoice about this. Will the resulting person — who will presumablythink he is you — be responsible for my actions, or for yours? (Orboth? Or neither?) Suppose he will be in terrible pain after theoperation unless one of us pays a large sum in advance. If we wereboth entirely selfish, which of us would have a reason to pay?The answer may seem to turn entirely on whether the resulting personwould be you or I. Only you can be responsible foryour actions. The only one whose future welfare you cannot rationallyignore is yourself. You have a special, selfish interest in your ownfuture, and no one else's. What matters in identity, we might say, isidentity. But some deny this. They say that someone else could beresponsible for your actions. You could have an entirely selfishreason to care about someone else's well-being for his own sake. Icare, or ought rationally to care, about what happens to the manpeople will call Olson tomorrow not because he is me, butbecause he is then psychologically continuous with me as I am now (seeSection 4), or because he relates to me in some other way that doesn'timply that he and I are one. If someone other than me werepsychologically continuous tomorrow with me as I am now, he would havewhat matters to me, and I ought to transfer my selfish concern tohim. (See Shoemaker 1970: 284; Parfit 1971, 1984: 215, 1995; Martin1998.)That completes our survey of problems. Though these eight questionsare related, it is hard to find any interesting common feature thatmakes them all questions about personal identity. In any case they aredifferent, and it is important not to confuse them.2. Understanding the Persistence QuestionLet us turn now to the Persistence Question. Few concepts have beenthe source of more misunderstanding than identity over time. ThePersistence Question is often confused with other questions or statedin a tendentious way. It is important to get it right.The question is what is necessary and sufficient for a past or futurebeing to be you. Suppose we point to you now, and then point to ordescribe someone or something existing at another time — a certain agedman or woman, say. Then we can ask whether we are pointing to onething twice, or pointing once to each of two things. This is aquestion about the identity or persistence of a person overtime. (There are precisely analogous questions about the identity overtime of other objects, such as dogs.) The Persistence Question askswhat determines the answer to such questions, or makes statementsabout our identity over time true or false.The Persistence Question is about numerical identity. To saythat this and that are numerically identical is to say that they areone and the same: one thing rather than two. This isdifferent from qualitative identity. Things are qualitativelyidentical when they are exactly similar. Identical twins may bequalitatively identical — there may be no telling them apart— but they are not numerically identical, for there are two ofthem: that's what makes them twins. A past or future person needn't,at that past or future time, be exactly like you are now in order tobe you — that is, in order to be numerically identical withyou. You don't remain qualitatively the same throughout your life. Youchange: in size, appearance, and in many other ways. So thequestion is not what it takes for a past or future being to bequalitatively just like you, but what it takes for a past or futurebeing to be you, rather than someone or something other thanyou. (Someone might say, as Hume apparently did, that a past or futurebeing couldn't be you unless he or she were thenqualitatively just like you are now. That would be a highlycontentious metaphysical claim. It amounts to denying that anythingcan survive any change whatever: even blinking your eyes would befatal, resulting in your ceasing to exist and being replaced withsomeone else. It would mean that you did not exist even a momentago. There would be no point in asking the persistence question ifthis were the case. Virtually all discussions of personal identityover time assume that it is possible for a person to change.) The confusion of qualitative with numerical identity is one source ofmisunderstanding about the Persistence Question. Here isanother. People sometimes ask what it takes for someone to remainthe same person from one time to another. The idea is that if Iwere to alter in certain ways — if I lost most of my memory, or mypersonality changed dramatically, or I underwent a profound religiousconversion, say — then I should no longer be the person I wasbefore. The question of what it takes for someone to remain the sameperson is not the Persistence Question. It is not in fact a questionabout numerical identity. If it were, it would answer itself, for Inecessarily remain numerically the same for as long as I exist.Nothing could make me a numerically different person from theone I am now. Nothing can start out as one thing and end up asanother, numerically different thing. This has nothing to do withpersonal identity in particular, but is simply a fact about the logicof identity.Those who say that after a certain sort of adventure youwould be a different person, or that you would no longer bethe person you once were, presumably mean that you would still exist,but would have changed in some profound and important way. If theperson resulting from the adventure were not numerically identicalwith you, it would not be the case that you were "a differentperson". Rather, you would have ceased to exist and been replaced bysomeone else. Those who say these things are usually thinking ofone's individual identity in the Who am I? sense: they are talkingabout the possibility of your losing some or all of the propertiesthat make up your individual identity and acquiring new ones . Thishas nothing to do with the Persistence Question.It is unfortunate that the words ‘identity’ and‘same’ are used to mean so many different things:numerical identity, qualitative identity, individual psychologicalidentity, and more. To make matters worse, some philosophers speak of"surviving" in a way that doesn't imply numerical identity, so that Icould "survive" a certain adventure even though I won't existafterwards. Confusion is inevitable.Here is a more insidious misunderstanding. Many people try to statethe Persistence Question like this:Under what possible circumstances is a person existing at one timeidentical with a person existing at another time? In other words, what does it take for past or future person to be you?We have a person existing at one time, and a person existing atanother time, and the question is what is necessary and sufficient forthem to be one person rather than two.This question is too narrow. We may want to know whether you were everan embryo or a foetus, or whether you could survive in a persistentvegetative state. These are clearly questions about what it takes forus to persist, and an account of our identity over time ought toanswer them. (Their answers may have important ethical implications:it is relevant to the morality of abortion, for instance, whethersomething that is an embryo or foetus at one time can be an adultperson at another time, or whether, by contrast, the adult person isalways numerically different from the foetus.) But many philosophersdefine ‘person’ as something that has certain mentalfeatures. Locke, for instance, famously said that a person is "athinking intelligent being, that has reason and reflection, and canconsider itself as itself, the same thinking thing, in different timesand places" (1975: 335). And neurologists say that early-term foetusesand human beings in a persistent vegetative state have no mentalfeatures at all. If anything like Locke's definition is right, suchbeings are not people. In that case we cannot ask whether you wereonce an embryo or could come to be a vegetable by asking what it takesfor a past or future person to be you.We can see the problem more clearly by considering a particularanswer to question 1:Necessarily, a person who exists at one time is identicalwith a person who exists at another time if and only if the formerperson can, at the former time, remember an experience the latterperson had at the latter time, or vice versa. That is, a past or future person is you just in the case that you cannow remember an experience she had then, or she can then remember anexperience you are having now. (This view is also sometimes attributedto Locke, though it is doubtful whether he actually held it.) Call itthe Memory Criterion. The Memory Criterion may seem to imply that if you were to lapse intoa persistent vegetative state, the resulting vegetable would not beyou, as it would be unable to remember anything. You would have ceasedto exist, or perhaps passed on to the next world. But in fact itimplies no such thing. That is because this is not a case involving aperson existing at one time and a person existing at another time(assuming that a human vegetable is not a person). The MemoryCriterion tells us which past or future person you are, butnot which past or future thing. It tells us what it takes forone to persist as a person, but not what it takes for one topersist without qualification. So it implies nothing at all aboutwhether you could persist as a vegetable. For the same reason it saysnothing about whether you were ever an embryo. (Olson 1997: 22–26,Mackie 1999: 224–228).So we hadn't better ask question 1. Instead we ought to ask what ittakes for any past or future being, person or not, to be youor I:Under what possible circumstances is a person who exists at onetime identical with something that exists at another time(whether or not it is a person then)?Philosophers typically ask 1 rather than 2 because they assume thatevery person is a person essentially: nothing that is in facta person could possibly exist without being a person. (By contrast,something that is in fact a student could exist without being astudent: no student is essentially a student.) If that is true, thenwhatever is a person at one time must be a person at every other timewhen she exists. This assumption makes questions 1 and 2 equivalent.Whether it is true, however, is a serious question (an instance of theHow could I have been? Question). If you are a person essentially, itfollows that you could not possibly have been an embryo, or come to bea vegetable (supposing, again, that such things aren't people). Theembryo that gave rise to you is not numerically identical withyou. You came into existence only when that being developed certainmental capacities. The assumption that you are a person essentiallyalso rules out our being animals, for no animal is a personessentially: every human animal started out as an unthinking embryo,and may end up as an unthinking vegetable.Whether we are animals and whether we were once embryos aresubstantive questions that an account of personal identity ought toanswer. They are not matters to be settled in advance by the way weset up the debate. So we cannot assume at the outset that we arepeople (in something like Locke's sense) essentially. Asking question1 prejudges the issue by favoring some accounts of what we are andwhat it takes for us to persist over others. In particular, asking 1effectively rules out the Somatic Approach described in the nextsection. It is like asking which man committed the crime before rulingout the possibility that it might have been a woman.3. Accounts of Our Identity Through TimeAlmost all proposed answers to the Persistence Question fall into oneof three categories. The first is the PsychologicalApproach, according to which some psychological relation isnecessary or sufficient (or both) for one to persist. You are thatfuture being that in some sense inherits its mental features —beliefs, memories, preferences, the capacity for rational thought,that sort of thing — from you; and you are that past being whosemental features you have inherited in this way. There is dispute overjust what sort of inheritance this has to be — whether it mustbe underpinned by some sort of physical continuity, for instance, orwhether a "non-branching" requirement is needed. There is alsodisagreement about what mental features need to be inherited. (I willreturn to some of these issues.) But most philosophers writing onpersonal identity since the early 20th century have endorsed someversion of the psychological approach. The memory criterion mentionedearlier is an example. Advocates of the Psychological Approach includeJohnston (1987), Garrett (1998), Hudson (2001), Lewis (1976), Nagel(1986, 40), Noonan (2003), Nozick (1981), Parfit (1971; 1984, 207),Perry (1972), Shoemaker (1970; 1984, 90; 1997; 1999), and Unger (1990,ch. 5; 2000). A second idea is that our identity through time consists in some brutephysical relation. You are that past or future being that has yourbody, or that is the same biological organism as you are, or thelike. Whether you survive or perish has nothing to do withpsychological facts. I will call this the SomaticApproach. It is comparatively unpopular. (It should not beconfused with the view that physical evidence has some sort ofpriority over psychological evidence in finding out who is who. Thathas to do with the Evidence Question and not the PersistenceQuestion.) Its advocates include Ayers (1990: 278–292), Carter(1989), Mackie (1999), Olson (1997), van Inwagen (1990), and Williams(1956–7, 1970).You may think the truth lies somewhere between the two: we need bothmental and physical continuity to survive; or perhaps either wouldsuffice without the other. Views of this sort are usually versions ofthe Psychological Approach as I have defined it. Here is a testcase. Imagine that your cerebrum — the upper brain that is primarilyresponsible for your mental features — is transplanted into myhead. Two beings result: the person who ends up with your cerebrum andmost of your mental features, and the empty-headed being left behind,which may still be biologically alive but will have no mental featuresat all. Those who say that you would be the one who gets your cerebrumusually say so because they believe that some relation involvingpsychology suffices for you to persist: they accept the PsychologicalApproach. Those who say that you would be the empty-headed vegetablesay so because they take your identity to consist in somethingnon-psychological, as the Somatic Approach has it.Both the Psychological and Somatic Approaches agree that there issomething that it takes for us to persist — that our identity throughtime consists in or necessarily follows from something other thanitself. A third view denies this. Mental and physical continuity areevidence for identity, it says, but do not always guarantee it, andare not required. No sort of continuity is absolutely necessary orabsolutely sufficient for you to survive — or at least none isboth necessary and sufficient. The only correct and completeanswer to the Persistence Question is that a person existing at onetime is identical with a being existing at another if and only theyare identical. There are no informative, non-trivial persistenceconditions for people. This is sometimes called the SimpleView (Chisholm 1976: 108ff., Swinburne 1984, Lowe 1996:41ff., Merricks 1998). It is often combined with the view that we areimmaterial or have no parts, though it needn't be. The Simple View ispoorly understood, and deserves more attention than it hasreceived. (For an interesting discussion see Zimmerman 1998.)It seems that the Persistence Question must have an answer. One ofthese three views, or another that I haven't mentioned, must betrue. If there is such a thing as you — if there is anythingsitting there and reading this now — then some conditions mustbe necessary and sufficient for it to persist. Those conditions willinvolve psychology, or some sort of brute physical continuity, orsomething else. Or there are no such conditions, as the Simple Viewhas it. Moreover, at most one such view can be true. We will revisitthis claim in Section 8, however.4. The Psychological ApproachMost people (most Western philosophy teachers and students, at anyrate) feel immediately drawn to the Psychological Approach. It seemsobvious that you would go along with your brain if it weretransplanted into a different head, and that this is so because thatorgan would carry with it your memories and other mentalfeatures. This would lead that person to believe that he or she wasyou. Why should this belief be mistaken? That suggests that ouridentity over time has something to do with psychology. It isnotoriously difficult, however, to get from this conviction to aplausible answer to the Persistence Question.What psychological relation might our identity through time consistin? We have already mentioned memory: a past or future being is you ifand only if you can now remember an experience she had then, or viceversa. This faces two well-known problems, discovered in the 18thcentury by Seargeant and Berkeley (see Behan 1979), but more famouslydiscussed by Reid and Butler (see the excerpts 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. Laterstill, in her dotage, she remembers her law career, but has entirelyforgotten paying the fine, and everything else she did in heryouth. According to the Memory Criterion, the young student is themiddle-aged lawyer, the lawyer is the old woman, but the old woman isnot the young student: an impossible result. If x andy are one and y and z are one, xand z cannot be two. Identity is transitive; memorycontinuity is not.Second, it seems to belong to the very idea of remembering that youcan remember only your own experiences. To remember paying a fine (orthe experience of paying) is to remember yourselfpaying. That makes it trivial and uninformative to say that you arethe person whose experiences you can remember — that is, to say thatmemory continuity is sufficient for personal identity. It isuninformative because you can't know whether someone genuinelyremembers a past experience without already knowing whether he is theone 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 thepast. The Memory Criterion tells us that Blott is Clott if Blott cannow remember an experience of Clott's that occurred at that pasttime. But Blott's seeming to remember one of Clott's experiences fromthat time counts as genuine memory only if Blott actually is Clott. Weshould have to know who was who before applying the theory that issupposed to tell us who is who. Saying that you are the person whoseexperiences you can remember is like saying that you are the personwho is entitled to your passport: true, but trivial. (Note, however,that this problem does not affect the claim that memory connectionsare necessary for identity. There is nothing trivial aboutthat.)One response to the first problem is to switch from direct to indirectmemory connections: the old woman is the young student because she canrecall experiences the lawyer had at a time when the lawyer rememberedthe student's life. The second problem is traditionally met byinventing a new concept, "retrocognition" or "quasi-memory", which isjust like memory but without the identity requirement: even if it isself-contradictory to say that I remember doing something I didn't do,I could still "quasi-remember" it (Penelhum 1970: 85ff., Shoemaker1970; for criticism see McDowell 1997). Neither solution gets us far,however, for the Memory Criterion faces a more obvious problem: thereare many times in my past that I can't remember or quasi-remember atall, and to which I am not even linked indirectly by an overlappingchain of memories. There is no time when I could recall anything thathappened to me while I was asleep last night. So the Memory Criterionhas the absurd implication that I did not exist then, and the man whoslept in my bed last night was someone else.A better way forward appeals to the notion of causal dependence(Shoemaker 1984, 89ff.). Let us say that a being at a later time ispsychologically connected with someone who exists at anearlier time if and only if the later being has the psychologicalfeatures she has at the later time in large part because theearlier being has the psychological features she has at the earliertime. Having a current memory (or quasi-memory) of a past experienceis one sort of psychological connection — the experience causes thememory of it — but there are others. And let us say that you are nowpsychologically continuous with a past or future being if andonly if your current mental features relate to those she has then by achain of psychological connections. Then we can say that a person whoexists at one time is identical with something existing at anothertime if and only if the first is, at the first time, psychologicallycontinuous with the second as she is at the second time. This still leaves important questions unanswered. Suppose we couldsomehow copy all the mental contents of your brain onto mine, therebyerasing the previous contents of both brains. The resulting beingwould be mentally like you were before, and not like I wasbefore. Whether this would be a case of psychological continuitydepends on what sort of causal dependence counts. The resulting personwould have inherited your mental properties in a way, but not in theusual way. Is it the right way? Could you literally move from onehuman animal to another via "brain-state transfer"? Advocates of thePsychological Approach disagree (Unger 1990: 67–71, Shoemaker1997). (Schechtman 1996 gives an interesting objection to thepsychological-continuity strategy, without abandoning thePsychological Approach.)5. FissionWhatever psychological continuity comes down to in the end, a moreserious worry for the Psychological Approach is that you could bepsychologically continuous with two past or future people. Ifyour cerebrum were transplanted, the resulting being would bepsychologically continuous with you by anyone's lights. ThePsychological Approach implies that she would be you. Now the cerebrumhas two hemispheres, and if one of them is destroyed the resultingbeing is also psychologically continuous with the original person.(Hemispherectomy — even the removal of the left hemisphere,which controls speech — is considered a drastic but acceptabletreatment for otherwise-inoperable brain tumors: see Rigterink 1980.)So the Psychological Approach implies that if we destroyed one of yourcerebral hemispheres and transplanted the other, you would be the onewho got the transplanted hemisphere.But now let the surgeons transplant both hemispheres, each into adifferent empty head. (We needn't pretend, as some authors do, thatthe hemispheres are exactly alike.) Call the resulting people Leftyand Righty. Both will be psychologically continuous with you. Ifany future being who is psychologically continuous with youmust be you, it follows that you are Lefty and you areRighty. That implies that Lefty is Righty, for two things cannot benumerically identical with one thing. But Lefty and Righty are clearlytwo. So you can't be both. We can make the same point in another wayby supposing that Lefty is hungry at a time when Righty isn't. If youare Lefty, you are hungry at that time. If you are Righty, youaren't. If you are both Lefty and Righty, you are both hungry and nothungry at once: a contradiction.The Psychological Approach appears to have the impossible consequencethat one thing could be identical with two things. Short of giving upthat approach altogether, there would seem to be just two waysout. One is to say that, despite appearances, "you" were really twopeople all along. There are two different but exactly similar peoplein the same place and made of the same matter at once, doing the samethings and thinking the same thoughts. The surgeons merely separatethem (Lewis 1976, Noonan 2003: 139–42; Perry 1972 offers a morecomplicated variant). The "multiple-occupancy view", as this iswhimsically called, is implausible for a number of reasons, not leastbecause it implies that we can't know how many people there are at agiven time until we know what happens later. (It is usually combinedwith "four-dimensionalism", the metaphysical thesis that allpersisting objects are extended in time and made up of temporal parts:see Section 8.)The other solution is to take back the claim that psychologicalcontinuity by itself is sufficient for one to persist. You areidentical with a past or future being who is psychologicallycontinuous with you as you are now only if no other being isthen psychologically continuous with you. (There is no unacceptablecircularity in this. We don't need to know the answer to thePersistence Question in order to know how many people there are at anyone time; that question comes under the Population Question.) Thismeans that neither Lefty nor Righty is you. If both your cerebralhemispheres are transplanted, that is the end of you — though you wouldsurvive if only one were transplanted and the other destroyed. This isthe "non-branching view" (Shoemaker 1984: 85, Unger 1990: 265, Garrett1998: ch. 4; for criticism see Noonan 2003: 12–15 and ch. 7). It toois hard to believe. If you could survive with half your brain, howcould preserving the other half mean that you don't survive?For that matter, the non-branching view implies you would perish ifone of your hemispheres were transplanted and the other left inplace. And if "brain-state transfer" gives us psychologicalcontinuity, you would cease to exist if your total brain state werecopied onto another brain without erasing yours. ("Best-candidate"theories such as Nozick 1981 attempt to avoid this.)The non-branching view makes the What Matters? question particularlyacute. Faced with the prospect of having one of your hemispherestransplanted, there would seem to be no reason to prefer that theother be destroyed. On the contrary: wouldn't you rather have bothpreserved, even if they go into different heads? Yet on thenon-branching view that is to prefer death over continuedexistence. This is what leads Parfit and others to say that you don'treally want to continue existing. At any rate you don't wantit for its own sake. Insofar as you are rational, at least, you onlywant there to be someone psychologically continuous with you in thefuture, whether or not that person is strictly you. More generally,facts about who is identical with whom have no practicalimportance. What matters practically is who is psychologicallycontinuous with whom. (Lewis 1976 and Parfit 1976 debate whether themultiple-occupancy view can preserve the conviction that identity iswhat matters practically.)This threatens to undermine the entire Psychological Approach.Suppose you would care about the welfare of your two fission offshootsin just the way that you ordinarily care about your own welfare, eventhough neither of them would be you. Then you would care about whathappened to the person who got your whole brain in the originaltransplant case, even if she would not be you. Even if youwould regard that person as yourself for all practical purposes— if you would anticipate her experiences just as you anticipateyours, for instance — that would in no way support the claimthat she was you. Our reactions to the brain-transplant casewould not support the claim that our identity over time consists inpsychological continuity, but only the claim that psychologicalcontinuity is what matters practically. In that case we may wonderwhether we have any reason to accept the Psychological Approach.It is sometimes said that fission is not a special problem for thePsychological Approach, but afflicts all answers to the PersistenceQuestion equally, apart (perhaps) from the Simple View. Whether thisis so is an interesting question. Even if it is, though, the fissionproblem looks especially worrying for the Psychological Approach, forit threatens the support for that view without affecting the argumentsfor rival views. (It does not undermine arguments for the SomaticApproach, for instance.)6. The Thinking-Animal ProblemThe Psychological Approach faces another problem (Carter 1989, Ayers1990: 278–292, Snowdon 1990, Olson 1997: 80f., 100–109,2003a). It arises because that view implies that we are not humananimals, for no sort of psychological continuity is either necessaryor sufficient for a human animal to persist.Every human animal starts out as an embryo, and may end up in apersistent vegetative state. Neither an embryo nor a human vegetablehas any mental features at all, and so neither is psychologicallycontinuous with anything. This shows that a human animal can persistwithout any sort of psychological continuity: psychological continuityis not necessary for animal identity. If you needpsychological continuity to persist, you cannot be an animal.The brain-transplant story shows that no sort of psychologicalcontinuity is sufficent for an animal to persist. If your cerebrum istransplanted into another head, the one who gets that organ, and noone else, will be psychological continuous, at that time, with you asyou were before the operation. Do the surgeons thereby move a humanorganism from one head to another? It seems not. They simply move anorgan from one animal to another, just as they might do with a kidneyor a liver. We can have continuously physically realized,non-branching psychological continuity between one human animal andanother. Thus, no sort of psychological continuity suffices for animalidentity. If it suffices for your identity — if you would goalong with your transplanted cerebrum and leave your animalbehind — then you are not an animal. Not only are you notessentially an animal. You are not an animal at all, evencontingently: nothing that is even contingently an animal would goalong with its transplanted cerebrum.The problem with this is that there is a human animal located whereyou are. That animal would seem to have the same thoughts and othermental features as you have. (It shares your brain.) But thePsychological Approach implies that you are not that animal. Itfollows that there is a conscious, intelligent being otherthan you now sitting in your chair and thinking your thoughts. Thereare twice as many thinking beings as the census-takers report. Thisought to lead you to wonder which thinker you are. You maybelieve that you are the non-animal — the one with psychologicalpersistence conditions. But the animal has the same reasons forbelieving that it has psychological persistence conditions asyou have for supposing that you do. Yet it is mistaken. How do youknow that you're not the one making the mistake? Even if you are thenon-animal thinker with the psychological persistence conditions, itseems that you could never know it.Three solutions to this problem have been proposed (apart from givingup the Psychological Approach altogether, that is). Some say thathuman animals have psychological persistence conditions (Wiggins 1980:160, 180; McDowell 1997: 237; for criticism see Olson 1997:114–119). Despite appearances, the Psychological Approach iscompatible with our being animals, and so the problem doesn'tarise. The surgeons do not move your cerebrum from one animal toanother in the transplant story. Rather, one animal has its parts cutaway until it is the size of a cerebrum. It thereby ceases to be ananimal (a detached cerebrum is not an animal), and is moved across theroom and given a new complement of parts, at which point it comes tobe an animal once more. This view has not proved popular, however.A second solution is to deny that human animals can think in the waythat we do. Thinking animals are not a problem for the PsychologicalApproach because there are none. Why can't human animals think? Youwould expect the explanation to be that they are material things, andno material thing can think. If any material thing couldthink, wouldn't it be an animal? But some argue that materialnon-animals can think even though animals cannot. Shoemaker says thatorganisms can't think because they have the wrong persistenceconditions: the nature of mental properties entails that psychologicalcontinuity must suffice for the bearers of those properties topersist, and it does not suffice for organisms persist. Materialthings with the right persistence conditions, however, can think. (SeeShoemaker 1984: 92–97, 1999, 2004, Olson 2002b. For a related view seeBaker 2000: 101–105.)Noonan (1998) proposes a third solution. He concedes that humananimals think as we do, and invokes an unorthodox view of personhoodand of first-person reference to explain how we can know that we arenot those animals. First, not just any rational, self-conscious beingis a person, but only one with psychological persistenceconditions. So human animals don't count as people. Second, personalpronouns such as ‘I’ refer only to people. So when theanimal associated with you says ‘I’, it doesn't refer toitself. Rather, it refers to you, the person who shares in thatutterance. When it says, "I am a person," it doesn't express the falsebelief that it is a person, but the true belief that youare. The animal is not mistaken about which thing it is, and neitherare you. You can infer that you are a person from the linguistic factsthat you are whatever you refer to when you say ‘I’, andthat ‘I’ never refers to anything but a person. (Fordiscussion see Olson 2002a.)7. The Somatic ApproachThere appears to be a thinking animal located where you are. It alsoappears that you are the thinking thing — the onlyone — located there. If things are as they appear, then you are thatanimal. The view that we are animals is sometimes calledAnimalism.Animalism does not imply that all animals, or even all human animals,are people. We may not want to call human embryos or animals in apersistent vegetative state ‘people’. Being a person maybe only a temporary property of you, like being a philosopher. Nordoes animalism imply that all people are animals. It is consistentwith the existence of wholly inorganic people: gods or angels orrational robots. It does not say that being an animal is part of whatit is to be a person (a view defended in Wiggins 1980, 171 andWollheim 1984, ch. 1 and criticized in Snowdon 1996). Animalismleaves the answer to the Personhood Question entirely open. If we are animals, we have the persistence conditions of animals.And as we saw, animals appear to persist by virtue of some sort ofbrute physical continuity. So Animalism seems to imply a version of theSomatic Approach.A few philosophers endorse the Somatic Approach without saying thatwe are animals. They say that we are our bodies (Thomson 1997), or thatour identity through time consists in the identity of our bodies (Ayer1936: 194). This has been called the Bodily Criterion of personalidentity. Its relation to Animalism is uncertain. If a person's body isby definition a sort of animal, then perhaps being identical to one'sbody is the same as being an animal. Whether this is so depends in parton what it is for something to be someone's body — a surprisinglydifficult question (see van Inwagen 1980, Olson 1997: 144–149).The Somatic Approach is unpopular. We have already met the mainobjection to it: when we reflect on the cerebrum-transplant story, itseems obvious that you would go along with your transplanted cerebrum,even though the animal would stay behind. The claim that youwould stay behind, and that someone who thinks she is you and has fullmemories of your life and even has your brain might not beyou, might seem incredible (see Unger 2000). This is often taken to becompletely decisive.The Somatic Approach has the virtue of being compatible with ourbeliefs about who is who in actual cases. In every actual case, thenumber of people we think there are is equal to the number of humananimals. Every actual case in which we take someone to survive orperish is a case where a human animal survives or perishes. ThePsychological Approach, or at any rate for the view that psychologicalcontinuity is necessary for us to persist, does not share thisvirtue. When someone lapses into a persistent vegetative state, hisfriends and relatives rarely conclude that their loved one no longerexists, even when they believe that there is no mental continuity ofany sort between the human vegetable and the person. (They mayconclude that his life no longer has any value; but that is anothermatter.) And most of us believe that we were once foetuses. When wesee an ultrasound picture of a 12-week-old foetus, we ordinarily thinkwe are seeing something that will, if all goes well, be born, learn tospeak, and eventually become an adult human person. Yet none of us isin any way psychologically continuous with a 12-week-old foetus. Andone might set more store by our opinions about cases we have actuallyconfronted than by our opinions about science-fiction stories.Friends of the Somatic Approach can also try to account for theattraction of the transplant argument in a way that is consistent withtheir view. Whenever someone has memories, personality, and othermental traits just like yours, that is strong evidence for his beingyou. All the more so if he inherited those traits from you. Brutephysical continuity is also far less important to the narrativestructure of most stories than mental continuity. When we hear astory, we don't much care about which person at the end of it is thesame animal as a certain person at the beginning. We care about who ispsychologically continuous with that person. These facts might lead usto think that the one who got your transplanted cerebrum would be youeven if, because you are an animal, this is not the case.This last thought suggests that the Somatic Approach, like thenon-branching view, might imply that identity has no practicalimportance. The one who got your transplanted cerebrum may have allthat matters in identity. You might have a reason, before thetransplant, to care selfishly about what will happen to him or herafterwards. He might be responsible for your actions, entitled to yourbank account, and so on. Yet on the Somatic Approach he would not beyou.8. Wider IssuesWe have compared the virtues of the two main accounts of our identityover time. We saw that the Psychological Approach, despite its initialattraction, faces problems with fission. The usual "non-branching"solution is both implausible in itself and suggests that identity hasno practical importance, which in turn undermines the original supportfor the Psychological Approach. And it implies that we are notanimals, raising the awkward problem of how we relate to theapparently intelligent animals we call our bodies. The SomaticApproach — in particular when combined with the view that we areanimals — is also intuitively attractive, and solves thethinking-animal problem. But it has implausible consequences aboutour identity over time. Is there any way to move the discussionforward?I believe that the debate turns on more general issues in metaphysicsand the philosophy of mind. For instance, advocates of thePsychological Approach appear to be committed to the view that eachnormal human organism is associated with a non-organism that thinksand is conscious. They will need an account of the metaphysicalnature of this non-organism, and of how it relates to the animal. Ifthey hope to solve the thinking-animal problem by denying that humananimals can think, they will need an account of the nature of mentalproperties that is consistent with this.Some general metaphysical views suggest that there is no unique rightanswer to the question of what it takes for us to persist. Perhaps thebest-known example is the ontology of temporal parts or"four-dimensionalism" (see Heller 1990, ch. 1, Sider 2001). It saysthat a persisting thing exists at different times by having differenttemporal parts located at those times, much as a spatially extendedthing exists in different places by having different spatial partslocated in those places. All persisting things have earlier and laterparts in the way that tennis matches do. For every period of time whenyou exist, short or long, there is a temporal part of you that existsonly then.What things are we, on this view? There are many likelycandidates. Suppose you are a material thing, and that we know whatdetermines your spatial boundaries. That should tell us what counts asyour current temporal part or "stage" — the temporal part of youlocated now and at no other time. That stage is a part of a vastnumber of temporally extended objects (Hudson 2001: ch. 4). Forinstance, it is a part of a being whose temporal boundaries aredetermined by relations of psychological continuity, in the sensedefined in Section 4, among its stages. That is, one of the beingsthinking your current thoughts is an aggregate of person-stages, eachof which is psychologically continuous with each of the others and notwith anything else. The view that we persist by virtue ofpsychological continuity suggests that that is what you are.Your current stage is also a part of a being whose temporal boundariesare determined by relations of psychological connectedness(Section 4 again). 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 not to anythingelse. This may not be the same as the first being, for some stages maybe psychologically continuous with your current stage but notpsychologically connected with it. The view that psychologicalconnectedness is necessary and sufficient for us to persist suggeststhat we are beings of the second sort (Lewis 1976). Your current stageis also a part of an animal. And it is a part of many bizarre andgerrymandered objects, such as Hirsch's "contacti persons" (Hirsch1982, ch. 10). Some even say that you are your current stage itself(Sider 2001, 188–208).Four-dimensionalism implies that you share your current thoughts withcountless beings that diverge from one another in the past orfuture. This makes it hard to say which thing you are, or which thingsany of us are. And because each of these beings persist through timeby virtue of something different, it is equally hard to say what ouridentity over time consists in. How could we ever know? We could saythat we are the beings we refer to when we say ‘I’, ormore generally the beings that our personal pronouns and proper namesrefer to. That seems true enough; but it is unlikely, givenfour-dimensionalism, that our personal pronouns succeed in referringto just one sort of thing. It is more likely that each utterance of apersonal pronoun refers ambiguously to many different candidates: tovarious sorts of psychologically interrelated aggregates, to ananimal, and perhaps to others as well. That would make itindeterminate which things, even which kinds of things, we are. Andinsofar as the different candidates have different persistenceconditions, it would be indeterminate what our identity over timeconsists in. 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Olson,‘Personal Identity’, in The Blackwell Guide to thePhilosophy of Mind, edited by S. Stich and T. Warfield, Oxford:Blackwell, 2003. Copyright © 2007 byEric T. Olson<e.olson@shef.ac.uk> |
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