Individual identity is sometimes discussed under the term self. And ‘self’ does sometimes mean ‘person’. But it also means something different: some sort of immaterial subject of consciousness.
Let us first look up the terrain of personal identity.
It raises a wide range of questions that are at best loosely connected.
Who am I?
Outside of philosophy, ‘personal identity’ usually refers to certain properties to which a person feels a special sense of attachment or ownership.
It may be, for instance, that being a philosopher and loving music belong to my identity. One’s personal identity in this sense is contingent and changeable: different properties could have belonged to to the way one defines oneself as a person, and what properties these are can change over time
Persistence. What does it take for a person to persist from one time to another—to continue existing rather than cease to exist?
Suppose you point to a child in an old class photograph and say, “That’s me.” What makes you that one, rather than one of the others? This is sometimes called the question of personal identity over time. An answer to it is an account of our persistence conditions.
How do we find out who is who? What evidence bears on the question of whether the person here now is the one who was here yesterday?
One source of evidence is first-person memory: if you remember doing some particular action, or at least seem to remember, and someone really did do it, this supports the claim that that person is you. Another source is physical continuity: if the person who did it looks just like you, or even better if she is in some sense physically or spatio-temporally continuous with you, that too is reason to think she is you. Which of these sources is more fundamental?
(good examples include Shoemaker 1963, 1970 and Penelhum 1967, 1970).
Population. If the persistence question asks which of the characters introduced at the beginning of a story have survived to become those at the end of it, we may also ask how many are on the stage at any one time.
You may think the number of people at any given time is simply the number of human organisms there are then . But this is disputed. Some say that cutting the main connections between the cerebral hemispheres results in radical disunity of consciousness, and that because of this, two people share a single organism (see e.g. Nagel 1971;
What am I? What sort of things, metaphysically speaking, are you and I and other human people? What are our fundamental properties, in addition to those that make us people? What, for instance, are we made of? Are we composed entirely of matter, as stones are, or are we partly or wholly immaterial?
Here are some of the main proposed answers (Olson 2007):
We are biological organisms.
We are material things “constituted by” organisms.
We are temporal parts of animals: each of us stands to an organism as the first set stands to a tennis match (Lewis 1976).
We are spatial parts of animals: brains, perhaps, or parts of brain.
We are part less immaterial substances—souls—or compound things made up of an immaterial soul and a material body (Swinburne 1984: 21).
We are collections of mental states or events: “bundles of perceptions”, as Hume said .
There is nothing that we are: we don’t really exist at all .
There is no consensus or even a dominant view on this question.
What matters in identity?
What is the practical importance of facts about our persistence?
Why does it matter? What reason have you to care whether you yourself continue to exist, rather than someone else just like you existing in your place?
Imagine that surgeons are going to put your brain into my head and that neither of us has any choice about this. Suppose the resulting person will be in terrible pain after the operation unless one of us pays a large sum in advance. If we were both entirely selfish, which of us would have a reason to pay? Will the resulting person—who will presumably think he is you—be responsible for your actions or for mine? (Or both, or neither?)
The answer may seem to turn entirely on whether the resulting person would be you or I. Only I can be responsible for my actions.
Understanding the Persistence Question
The question is roughly what is necessary and sufficient for a past or future being to be someone existing now.
The persistence question asks, in this case, whether you would still exist. And the answer to that question is Yes: if you are a different person, then you still exist, just as you do if you remain the same person. When we speak of remaining the same person or of becoming a different person, we mean remaining or ceasing to be the sort of person one is.
This has to do with one’s individual identity in the sense of the who am I? question. It has nothing to do with persistence through time.
The persistence question is often taken to ask what it takes for the same person to exist at two different times.
Locke, for instance, said that a person is “a thinking intelligent being, that has reason and reflection, and can consider itself as itself, the same thinking thing, in different times and places” (1975: 335). Presumably this implies that something is a person at a given time only if it has those mental properties then. And neurologists say that early-term foetuses and human beings in a persistent vegetative state have no mental properties at all
That is, a past or future person is you just in the case that you (who are now a person) can now remember an experience she had then, or she can then remember an experience you are having now. Call this the memory criterion
The memory criterion purports to tell us which past or future person you are, but not which past or future thing. It says what it takes for someone to persist as a person, but not what it takes for someone to persist without qualification. So it implies nothing at all about whether you could come to be a vegetable or a corpse. For the same reason it tells us nothing about whether you were ever an embryo. (Olson 1997: 22–26, Mackie 1999: 224–228).
Accounts of Our Identity Through Time
There are three main sorts of answers to the persistence question in the literature. The most popular are psychological-continuity views, according to which the holding of some psychological relation is necessary or sufficient (or both) for one to persist.
You are that future 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 whose mental features you have inherited in this way. But most philosophers writing on personal identity since the early 20th century have endorsed some version of this view. The memory criterion mentioned earlier is an example. Advocates of psychological-continuity views include Johnston (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 sort of answer is that our persistence consists in some brute physical relation. You are that past or future being that has your body, or that is the same biological organism as you are, or the like. It has nothing to do with psychological facts. Call these brute-physical views. (Don’t confuse them with the view that physical evidence has some sort of priority over psychological evidence in finding out who is who. That has to do with the evidence question.) Their advocates include Ayers (1990: 278–292), Carter (1989), Mackie (1999), Olson (1997), van Inwagen (1990), and Williams (1956–7, 1970).
One may think the truth lies somewhere between the two: we need both mental and physical continuity to survive, or perhaps either would suffice without the other. This usually counts as a psychological-continuity view as we have defined it.
Here is a test case. Imagine that your brain is transplanted into my head. Two beings result: the person who ends up with your cerebrum and (presumably) most of your mental features, and the empty-headed being left behind, which may be biologically alive but has no mental features. Those who say that you would be the one who gets your brain usually say so because they believe that some relation involving psychology suffices for you to persist. Those who say that you would be the empty-headed vegetable say so because they take your persistence to consist in something entirely non-psychological, as brute-physical views have it.
Both psychological-continuity and brute-physical views agree that there is something that it takes for us to persist—that there are informative, nontrivial necessary and sufficient conditions for a person existing at one time to be a thing existing at another time.
Most people—most Western philosophy teachers and students, anyway—feel immediately drawn to psychological-continuity views (Nichols and Bruno 2010 give experimental evidence for this). If your brain were transplanted, and that organ would carry with it your memories and other mental features, the resulting person would be convinced that he or she was you. Why should this conviction be mistaken? This can make it easy to suppose that the person would be you, and that this would be so because he or she is psychologically continuous with you. It is notoriously difficult, however, to get from this thought to an attractive answer to the persistence question.
First, suppose a young student is fined for overdue library books. Later, as a middle-aged lawyer, she remembers paying the fine. Later still, in her dotage, she remembers her law career, but has entirely forgotten not only paying the fine but everything else she did in her youth.
According to the memory criterion the young student is the middle-aged lawyer, the lawyer is the elderly woman, but the elderly woman is not the young student. This is an impossible result: if x and y are one and y and z are one, x and z cannot be two. Identity is transitive; memory continuity is not.
Second, it seems to belong to the very idea of remembering that you can remember only your own experiences. To remember paying a fine (or the experience of paying) is to remember yourself paying. That makes it trivial and uninformative to say that you are the person whose experiences you can remember—that is, that memory continuity is sufficient for personal identity.
One response to the first problem is to modify the memory criterion by switching from direct to indirect memory connections: the old woman is the young student because she can recall experiences the lawyer had at a time when the lawyer remembered the student’s life. The second problem is traditionally met by replacing memory with a new concept, “retrocognition” or “quasi-memory”, which is just like memory but without the identity requirement: even if it is self-contradictory to say that you remember doing something you didn’t do but someone else did, you could still “quasi-remember” it (Penelhum 1970: 85ff., Shoemaker 1970; for criticism see McDowell 1997).
Neither move gets us far, however, as both the original and the modified memory criteria face a more obvious problem: there are many times in one’s past that one cannot remember or quasi-remember at all, and to which one is not linked even indirectly by an overlapping chain of memories. For instance, there is no time when you could recall anything that happened to you while you dreamlessly slept last night. The memory criterion has the absurd implication that you have never existed at any time when you were unconscious. The person sleeping in your bed last night must have been someone else.
A better solution replaces memory with the more general notion of causal dependence (Shoemaker 1984, 89ff.).
It still leaves important questions unanswered, however. Suppose we could somehow copy all the mental contents of your brain to mine, much as we can copy the contents of one computer drive to another, and that this erased the previous contents of both brains. Whether this would be a case of psychological continuity depends on what sort of causal dependence counts. The resulting being (with my brain and your mental contents) would be mentally as you were before, and not as I was. He would have inherited your mental properties in a way—but a funny way. Is it the right way? Could you literally move from one organism to another via “brain-state transfer”? Psychological-continuity theorists disagree (Shoemaker 1984: 108–111 and 1997, Unger 1990: 67–71; see also van Inwagen 1997).
A more serious worry for psychological-continuity views is that you could be psychologically continuous with two past or future people at once. If your cerebrum—the upper part of the brain largely responsible for mental features—were transplanted, the recipient would be psychologically continuous with you by anyone’s lights (even though there would also be important psychological differences).
But now suppose that both hemispheres are transplanted, each into a different empty head. (We needn’t pretend, as some authors do, that the hemispheres are exactly alike.) The two recipients—call them Lefty and Righty—will each be psychologically continuous with you. The psychological-continuity view as we have stated it implies that any future being who is psychologically continuous with you must be you. It follows that you are Lefty and also that you are Righty. But that cannot be: if you and Lefty are one and you and Righty are one, Lefty and Righty cannot be two. And yet they are. To put the point another way, suppose Lefty is hungry at a time when Righty isn’t. If you are Lefty, you are hungry at that time. If you are Righty, you aren’t. If you are Lefty and Righty, you are both hungry and not hungry at once: a contradiction.
But a healthy, adult human organism seems a paradigm case of a thinking being. This raises three apparent problems. First, if the organism we call your body can think, your not being an organism would imply that you are one of two intelligent beings sitting there and reading this entry.
More generally, there would be two thinking beings wherever we thought there was just one. Second, the organism would seem to be psychologically indistinguishable from you. That would make it a person, if being a person amounts to having certain mental or behavioral properties (as on Locke’s definition). In that case it cannot be true that all people (or even all human people) persist by virtue of psychological continuity. Some—the animal people—would have brute-physical persistence conditions.
Third, this makes it hard to see how you could know whether you were a non animal person with psychological persistence conditions or an animal person with brute-physical ones. If you thought you were the non animal, the organism would use the same reasoning to conclude that it was too. For all you could ever know, it seems, you might be the one making this mistake.
In the same way, psychological-continuity views raise the questions, “What am I? Am I non animal that would go with its transplanted brain, or an organism that would stay behind with an empty head?” And here too there seem to be no grounds on which to answer them.
These three objections have been called the “too-many-thinkers” or thinking-animal problem. The only way to avoid them altogether is to say that we are organisms (and that there are no beings who persist by virtue of psychological continuity).
Finally, psychological-continuity theorists can concede that human organisms are psychologically indistinguishable from us, but try to explain how we can still know that we are not those organisms. The best-known proposal of this sort focuses on personhood and first-person reference. It says that not just any being with mental properties of the sort that you and I have—rationality and self-consciousness, for instance—counts as a person. A person must also persist by virtue of psychological continuity. It follows that human animals are not people. Further, personal pronouns such as ‘I’, and the thoughts they express, refer only to people.
So when your animal body says or thinks ‘I’, it refers not to itself but to you, the person. The organism’s statement ‘I am a person’ does not express the false belief that it is a person, but the true belief that you are.
None of these objections arise on animalism, the view that we are organisms. This does not imply that all organisms, or even all human organisms, are people: as we saw earlier, human embryos and animals in a persistent vegetative state may not count as people. Being a person may be only a temporary property of you, like being a student
Assuming that organisms persist by virtue of some sort of brute-physical continuity, animalism implies a version of the brute-physical view. A few philosophers endorse a brute-physical view without saying that we are animals. They say that we are our bodies (Thomson 1997), or that our identity through time consists in the identity of our bodies (Ayer 1936: 194). This has been called the bodily criterion of personal identity. Its relation to animalism is uncertain.
Most versions of the brute-physical view imply that human people have the same persistence conditions as certain non people, such as dogs. And it implies that our persistence conditions differ from those of immaterial people, if they are possible. It follows that there are no persistence conditions for people as such.
The most common objection to brute-physical views is the repugnance of their implication that you would stay behind if your brain were transplanted (e.g. Unger 2000; for an important related objection see Johnston 2007). In other words, brute-physical views are unattractive in just the way that psychological-continuity views are attractive.
The debate between psychological-continuity and brute-physical views cannot be settled without considering more general matters outside of personal identity. For instance, psychological-continuity theorists need to explain why human organisms are unable to think as we do. This will require an account of the nature of mental properties. Or if human organisms can think, they must explain how we can know that we are not those organisms. This will turn on how the reference of personal pronouns and proper names works, or on the nature of knowledge.
Some general metaphysical views suggest that there is no unique right answer to the persistence question. The best-known example is the ontology of temporal parts mentioned in section 5. It says that for every period of time when you exist, short or long, there is a temporal part of you that exists only then. This gives us many likely candidates for being you—that is, many different beings now sitting there and thinking your thoughts. Suppose you are a material thing, and that we know what determines your spatial boundaries. That should tell us what counts as your current temporal part or “stage”—the temporal part of you located now and at no other time. That stage is a part of a vast number of temporally extended objects (Hudson 2001: ch. 4).
The temporal-parts ontology implies that each of us shares our current thoughts with countless beings that diverge from one another in the past or future. If this were true, which of these things should we be? Of course, we are the things we refer to when we say ‘I’, or more generally the referents of our personal pronouns and proper names. But these words would be unlikely to succeed in referring to just one sort of thing—to only one of the many candidates on each occasion of utterance. There would probably be some indeterminacy of reference, so that each such utterance referred ambiguously to many different candidates. That would make it indeterminate what things, and even what sort of things, we are. And insofar as the candidates have different histories and different persistence conditions, it would be indeterminate when we came into being and what it takes for us to persist (Sider 2001b).
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