Personal Identity

(Or:  What Does it Take To Survive?)



You survive for as long you exist.  And you exist whenever there is a person who is you.  But what are the necessary and sufficient conditions for there to be a person who is you?  This is the problem of personal identity.


A person can change in many ways over a lifetime.  The size and shape of our bodies change, as do our beliefs and personalities.  In those respects, you are more similar to most other adults than you are to your childhood self.  And yet, you are a different person from other adults, but the same person that your mother gave birth to.  What makes this true?


At this point, we should acknowledge that we do sometimes talk as though we are not the same people that we were as children, as in


Of course, I wasn’t yet the person that I’ve now become”. 


For that matter, we sometimes hear people say things like this,


He’s just not the same man I fell in love with.”


When we talk in this way, we are not being literal.  We are expressing our belief that a person has undergone very significant changes.  If I say to my old college roommate, “You aren’t the Tony I used to know”, I certainly do not mean that Tony has been replaced by an imposter, as in the movie Invasion of the Body Snatchers.  Nonetheless, I may be saying something true.  The weak sense of personal identity is the sense in which my old roommate is a “different person” because he is now so different.  But the strong (literal) sense of personal identity is what we are concerned about in a philosophy class.  If the same person doesn’t exist any more in the strong sense, then that person is dead.



Derek Parfit’s Teletransporter Thought Experiment

from “Divided Minds and the Nature of Persons”


Suppose that you enter a cubicle in which, when you press a button, a scanner records the states of all the cells in your brain and body, destroying both while doing so.  This information is then transmitted at the speed of light to some other planet, where a replicator produces a perfect organic copy of you.  Since the brain of your Replica is exactly like yours, it will seem to remember living your life up to the moment when you pressed the button, its character will be just like yours, and it will be in every other way psychologically continuous with you.”




Are you willing to press the button?


Let’s add that “you” will be paid a million dollars for participating in the experiment when “you” arrive at the other planet.  (By “you”, I mean the duplicate, of course).


If you believe that you will survive pressing the button (and hence, the destruction of your body), then presumably this is because you believe that the duplicate will be you.


Are you sure it will be you?  Consider the following variation:


Slight Delay.  When you press the button, your body will not be destroyed right away.  In fact, you are able to talk to the duplicate via satellites for a few minutes while they warm up the disintegrator.  (The technology itself doesn’t require that your body be destroyed, you see, but it has to be destroyed for legal reasons).  If the duplicate is you, then that would mean that you are talking to yourself.  But you are talking to someone on another planet…



Bodily Continuity



If you are not willing to press the button, presumably this is because you believe you will not survive the destruction of your body.


This suggests that what matters for survival is survival of the body.


The Same-Body Theory:  X, who exists at an earlier time, and Y, who exists at a later time, are the same person if and only if they have the same body.



Can you think of even a single case in the real world where this theory has failed?  It would have to be a case where either X and Y are the same person but have different bodies, or a case where X and Y are different people but have the same body.



Do you have the same body that you did fifteen years ago?  Unless you have used a teletransporter, then the answer is yes.  But what makes it the same body?  A body is a physical object.  So it is natural to suppose



The Same-Matter Requirement:  X is the same body as Y only if X and Y are made of the same physical stuff (the same matter).








There’s just one little problem.  Most of the cells in your body have been replaced with new cells, many times over.  The body that you have today contains very little of the matter from your body of fifteen years ago.  According to the Same-Matter Requirement, you have a different body!  Rather than accept this conclusion, philosophers reject the Same-Matter Requirement.


The Same-Matter Requirement:  X is the same body as Y only if X and Y are made of the same physical stuff (the same matter).


Instead, we say that X and Y are the same body only if they are physically continuous.  As Rachels put it, “Years ago, when I last saw my roommate, someone could have (in theory) started observing him, and they could have traced the long, unbroken path through space and time that led him into my presence today.”  As long as the changes in matter are gradual, and the body in question never ceases to exist at any point, we allow that it is the same body (of course, this also supposes that the body has not undergone any truly radical changes, like changing into something that isn’t a human body at all).




Psychological Continuity



The following thought experiments provide counter-examples to Same-Body Theory:



The Prince and the Cobbler

John Locke (1632-1704)


"For should the soul of a prince, carrying with it the consciousness of the prince's past life, enter and inform the body of a cobbler, as soon as deserted by his own soul, from the body of a cobbler, every one sees he would be the same person with the prince, accountable only for the prince's actions.”



The Emperor of China

Leibniz (1646-1716)


Let us suppose that some individual suddenly became the Emperor of China, but only on condition that he forgot what he had been, as if he had just been reborn:  does that not come to the same in practice, or in the effects that could be registered, as if he had to be annihilated and an Emperor of China created at the same instant at the same place?”


Three Feet to the Left



My body suddenly ceases to exist; it just disappears into thin air.  One second later, a body that is made of entirely different matter but is otherwise identical appears three feet to my left.  From the point of view of the new body, it is as though I had suddenly jumped three feet to my left.  Shall we have a funeral and mourn my death?  Shall you welcome the “new” person into the world?  If this seems ridiculous to you, imagine how ridiculous it would seem to person who now stands three feet to the left (your right).  Intuitively, what we have is the same person as before.  (If you agree but had a different opinion about the teletransporter case, then maybe you ought to consider that case again…)



The Emperor of China case shows that bodily continuity isn’t sufficient for same-personhood.  The Prince and the Cobbler case purports to show that bodily continuity is neither sufficient nor necessary (because the Prince and the Cobbler still exist, but in different bodies).  And Three Feet to the Left is an attempt to show that bodily continuity isn’t necessary.  Apparently, some kind of psychological continuity is required for same personhood.  A simple psychological continuity theory is



The Memory Theory:  X, who lived at an earlier time, is the same person as Y, who lives at a later time, if and only if Y can remember doing what X did, feeling what X felt, thinking what X thought, and so on.



The Memory Theory implies that in the case of the Prince and the Cobbler, the two have switched bodies.  It also implies that in the Emperor of China case the individual has not survived, but has been replaced by a new person that has the old body.  These implications agree with our intuitions about the two cases (don’t they?).



Lockean Argument from Responsibility (from PP, p. 61)


1)  It is fair to hold someone responsible for having done something if and only if he can remember having done it.


2)  It is fair to hold Y responsible for what X did if and only if X and Y are the same person.


3)  Therefore, X and Y are the same person if and only if Y can remember doing what X did.



Objection:  In general, responsibility seems to depend on sameness in the weak sense.” (PP, p. 61)  Suppose you have committed a crime.  If you become a “different person” in the weak sense (i.e. you have grown and changed; now you are such that you would never commit a crime, etc.) then, arguably, it would no longer be appropriate to hold you behind bars for the crime.  In the strong sense, though, you are still the same person who committed the crime.  This is true even if you don’t remember doing it.  So, premise 2) is false if ‘same person’ is intended in the strong (literal) sense.


Arguably, premise 1 is also false.  Imagine that Rob is thinking about robbing a store.  He knows that if he remembers committing the crime, then he will probably give himself away (for example, he would not pass a lie detector test).  So, before robbing the store, Rob takes a drug that will prevent him from remembering what he is about to do.  After the robbery, Rob puts the money in his bank account where he will happily find it later without knowing where it came from.  As planned, Rob robs, then forgets the robbery, and forgets putting the money in the bank.  Now, is it really plausible that Rob isn’t responsible for the crime?


The Lockean Argument is not sound.  But that does not show that the Memory Theory is false (bad arguments are sometimes advanced for true theories).



Objections to Memory Theory



Thomas Reid’s soldier


"Suppose a brave officer to have been flogged when a boy at school for robbing an orchard, to have taken a standard from the enemy during his first campaign, and to have been made a general in advanced life; suppose, also that, when he took the standard, he was conscious of his having been flogged at school, and that, when made a general, he was conscious of his haven taken the standard, but had absolutely lost consciousness of his flogging."


Is the general the same person as the boy?


According to Memory Theory, the boy is the same person as the officer and the officer is the same person as the general.  It would seem to follow that the boy is the same person as the general.  But the general does not have the memories that the boy did.  Therefore, Memory Theory has the surprising result that the boy and the general are not the same person.  (If this sounds like an unwarranted conclusion to draw on the basis of the loss of one memory, we can add that all of the boy’s memories are lost by the time he becomes the general).


We are forced to conclude that either


(a) Memory Theory is wrong, or

(b) personal identity is not transitive.


A relation, R, is “transitive” means


              Whenever xRy and yRz, then xRz


For example, the relation richer than is transitive.  If Bill is richer than Oprah and Oprah is richer than Phil, then Bill is richer than Phil.


Normally, we assume that personal identity is transitive.  But the idea that it isn’t deserves serious consideration.  In a human lifetime, people undergo many changes, but not enough to bring their personal identity into question.  But while this is ordinarily true, we can imagine possible situations in which it isn’t…


The Case of the Incredible Changing Woman


Vera, a young victim of brain cancer, volunteers to undergo some new and experimental treatments.  The treatments cure her cancer but they have profound side-effects.  One effect is that she gradually loses her memories.  She loses them in the order she acquired them, so that in thirty years, she will have a completely new set of memories.  Another effect is that her personality gradually changes, so that in thirty years she will have an entirely new personality.


It has been 40 years since the cancer treatments.  The body that is continuous with Vera’s is now 55 years old and has a completely new set of memories and a completely new personality, not to mention a new name.  If asked, “Melissa” says she thinks of Vera like a grandmother, and knows about her only what people have told her.  Does Vera still exist?  It might be tempting to reply that there is a different person only in the weak sense.  But this case involves changes much more radical than would normally occur.  The changes are gradual, and there are enough similarities over any ten year period that the same person can be said to exist during that time.  But by the time that 30 (let alone 40) years have passed there seems to be a different person, and not just in the weak sense.


If Vera gradually lost her memories but the rest of her personality remained intact, then we would be much more inclined to say that she remains the same person (in the strong sense) 40 years later.  Perhaps what this shows is that the Memory Theory is inadequate not because it denies that same personhood is transitive (it is correct to deny that), but because it leaves personality out of the analysis.  But before injecting personality into the analysis, there are two other objections to Memory Theory we should consider…


Real Memories versus False Memories


Often, people think they remember something but are mistaken (p. 63)


Whenever you seem to remember something, you have a certain kind of experience that we will call an apparent memory.  Some apparent memories represent things as they actually happened.  We will call these real memories.  Others represent things falsely:  we will call these false memories.


At any given time, we have lots of memories that we aren’t experiencing.  We can apply the same real/false distinction to these as well.


In Problems from Philosophy, p.63, James Rachels writes:


When Y remembers doing what X did, is the memory real or apparent?  Obviously, [Memory] theory cannot be referring to apparent memories.  If it were, then the theory would imply that any lunatic who “remembered” being Napoleon would really be Napoleon.  So the theory must be referring to real memories—X is the same person as Y if Y accurately remembers being X.  This seems correct, but only because it is an empty tautology.  Of course, if Y accurately remembers being X, then he was X—that is what “accurate” means.


Is Memory Theory guilty of a conceptual error of this magnitude?  Is Memory Theory an empty truism which tells us nothing substantive about personal identity?  Rachels is correct in noting that it would be if the Memory Theory referred only to real memories.


But suppose we understand Memory Theory as referring to apparent memories, both real and false (never mind whether this is true to Locke’s original idea).  What Memory Theory says, then, is that being a certain person is a matter of having a certain apparent memories.  Some of these represent the world accurately, some do not, but together they form a collection that distinguishes one person from another.  Does it follow that “any lunatic who “remembered” being Napoleon would really be Napoleon”?  No. 

Even if the lunatic has some of the apparent memories that Napoleon had (not that we would be able to verify this!), he will certainly not have all, or even most of them.  That is, unless we are considering a case where all the memories of Napoleon have somehow been magically or scientifically transferred to the “lunatic”.  But in that case, we don’t have an obvious counter-example to Memory Theory--we have a Prince and the Cobbler type situation, or perhaps a case where Napoleon Lives Again.


If there were a modern day “lunatic” who had Napoleon’s apparent memories, then would it really be Napoleon?  Memory Theory implies yes.  There are two problems with this answer; both of them are ones that we have come across before, and neither of them have to do with Real versus False memories.  The first problem is that we would want to know whether the “lunatic” had Napoleon’s personality.  Does he have the same beliefs as Napoleon?  Does he have the same know-how?  Does he react to situations in the same way that Napoleon did?  Does he have the same attitudes?  If the answer to all of these questions is no, then it isn’t Napoleon, even if the apparent memories are the same.  The second problem is that the “lunatic” has a body that is not even remotely physically continuous with Napoleon’s body.  This should remind us of Parfit’s teletransporter:  we might have a copy of Napoleon, but not Napoleon himself.



We have seen that having the same body is not sufficient for same personhood.  We have also seen that sameness of memory and personality does not seem to be sufficient either (because bodily continuity might be lacking).  It is tempting to combine these ideas into one package.  You might say it includes everything but the kitchen sink:


Mind and Body theory:  X, who lived at an earlier time, is the same person as Y, who lives at a later time if and only if X and Y have the same body and Y has most of X’s apparent memories and Y has most of X’s personality.


The qualifier most of is there to account for the fact that X may have gone through some significant changes in memory and personality while remaining the same person in the strong sense.


Unfortunately, the Kitchen Sink Theory is still not adequate as stated.  First of all, it would imply that you are not the person your mother gave birth to.  After all, you don’t remember being a newborn baby, nor do you have the same personality. The most of qualifier may help, but not enough.


Second, there is the following thought experiment:




An amazing new drug, Duo, has been invented that causes a human body to divide itself into two new bodies, each of which is just like the original.  This fission process involves the division of every cell in the body (including all of the neurons).  The process takes several hours and is disturbing to watch, as the body first stretches in the middle, then splits.  Intravenous fluid is administered to add the extra mass needed for two bodies.  After a couple of hours, there are what appear to be conjoined twins.  The twins soon separate, however, and each “twin” has all of the memories and personality of the original.



Suppose that Mr. Splits takes a dose of Duo and undergoes the fission process.  As a result, there are the two identical individuals.  Which one of them is the same person as Mr. Splits?  They both have the same memories and personality.  Psychologically speaking, they are both Mr. Splits.  Do either of them have the same body as Mr. Splits?  They can’t both have the same body, because one body can’t be two bodies.  On the other hand, there is absolutely no reason to suppose that one is the same body as the original but the other isn’t.  But if neither is the same body, then (according to Kitchen Sink Theory) we have to conclude that Mr. Splits has died, which seems absurd.  Apparently, we are at an impasse, and the Kitchen Sink Theory is of no help.


Moreover, it’s hard to see how any theory of personal identity would be able to tell us which of the resulting individuals is Mr. Splits.  Theories of personal identity work by specifying the relations that hold between X and Y that are necessary and sufficient for X and Y being the same person.  But in the Fission case, every relation that is relevant to personal identity and holds between Mr. Splits and one of the twins also holds between Mr. Splits and the other twin.  So, it looks like either both of them are Mr. Splits or neither of them are, and both of these answers are counter-intuitive.


Real life split brain cases (see pp. 66-68) are similarly vexing.  It seems absurd to say that the person whose corpus callosum has been severed has died, yet it also seems absurd that the two separate streams of consciousness that remain are that of one individual.


Cases like this have led some philosophers to wave a white flag and declare that personal identity isn’t all its cracked up to be.



Consider the following list:


· There are Perceptions, memories, thoughts, emotions, and other mental events.


The mental events are fleeting.  Some exist at one moment, only to be replaced by others the next.


· There are various relations of similarity and causality which hold between some of the mental events but not others.


No matter how different mental events are related, we can always raise doubts about whether they are mental events of the “same person” (teletransporter, Fission, etc.)


· There are various bodies (specifically, brains) in which the mental events occur.


Having the same body is neither necessary nor sufficient for being the same person.  (Prince and the Cobbler, Emperor of China, Three Feet to the Left, Fission)



There are no other elements of which persons could be composed.  Yet, no matter how we try to put these elements together, it seems we are unable to come up with a satisfactory analysis of ‘same person’.  This leads some to the conclusion that


There is no satisfactory analysis of ‘same person’ to be had.  In other words, same personhood cannot be defined.


Prior to thinking about personhood as philosophers, we all would have agreed to the following statement:


“No matter what happens to me—whether it is a car accident or a teletransporter or a mind swap or whatever it is—at the end of the day I have either survived it or I haven’t.  Either I exist or I don’t; there is no middle ground.”


But if same personhood cannot be defined, then we have to accept the surprising conclusion that survival is not a black and white matter.  There are some cases in which we neither survive nor fail to survive.  How could this be? 


Perhaps same personhood, and hence survival, comes in degrees.



Actually, I think we can do a little better than leaving it at that…



Person Types and Person Tokens



              Time is an album by ELO, and I have it on a disk at home. Other people have it as well.  I could show you mine and truthfully say, “This is Time by ELO.”  And other people could say the same, referring to their own.  We don’t normally give such goings on a second thought, but we should.  There is something odd here.  On the one hand, there is only one album, Time, by ELO.  It’s not as if ELO made more than one album with the same title.  On the other hand, there are many Time albums out there, one of which I own.  How can this be?  How can there be one thing that is also many things?

              The same puzzle arises when we think about language.  There is only one letter ‘A’ but there are also many (there are 6 in this sentence alone).  The same goes for words and sentences.  And pictures, cars, games, and so on.


A TYPE is a general form that many different things can have.


A TOKEN of a type is a particular item that has that type (form).


              If I show you my Time album, I am really showing you my token of the Time album type.  The type itself is not identical to any one token, or any collection of tokens (the type would still exist even if all the tokens (copies) were destroyed).

In the case of a musical album, being a token of the album requires containing information that encodes a very precise sequence of sounds that can be played back.  In the case of a picture, being a token of the picture requires that it look a certain way.  There are many different types, and many different types of types, and in general, being a token of  type T will require something different, depending on what T is like.

It turns out that the word ‘album’ is ambiguous. It could refer to an abstract type, or to a particular token.  The same goes for anything to which the type / token distinction can be applied.



The Teletransporter Revisited


              The Type/Token distinction can be applied to persons.  Suppose that I press the button in the teletransporter case.  Do I survive?  Let’s think of it this way:  When my body is destroyed, one token of me is destroyed.  At the same time, another token of me is created on the other planet.  Me, considered as a person type, is a form or property that different things can have.  The duplicate on the other planet has a body and mind exactly like mine, with the same personality and (apparent) memories.  This is sufficient for it to be of my person type.


              A person token is an individual that has the property of being a person.  I believe that person tokens are bodies.  Not everyone agrees.  A dualist (such as Descartes) would say a person token is a nonphysical substance.  But even a dualist can distinguish between a person token and a person type.


              A person type is something abstract. It is what we might call a “person property”.  Something has a particular person property (e.g. being Dan) when it has all the features that are necessary for being that person.  These features are largely psychological, and we won’t attempt to say exactly what they are.  [Importantly, none of the necessary features are historical.  For example, being Dan doesn’t entail having been on planet earth.  If person types had historical entailments, then it wouldn’t be possible for there to be different tokens of the same person type.]


Two different kinds of survival:


Token survival:  I survive at future time t if and only if the same token that now has my person type exists at t (and still has my person type).


Type survival:  I survive at future time t if and only if there is some token that has my person type at t (whether or not it is a token that exists now).



Fission Revisited


We now have a simple way of describing fission.  Before the fission, there was one token of the type Mr. Splits.  The Duo drug caused the one token to split into two.  Now there are two tokens of the type Mr. Splits.  There is no need to agonize over which is the “real” Mr. Splits—they are both real tokens of the same person type.  Neither of the tokens has any more claim than the other to being the original token.



Judging from reactions to the teletransporter thought experiment and others like it, most people are inclined to identify survival as token survival.  But people are not consistent on this point.  In the Three Feet to the Left example, most of us are willing to count it as survival, which in this case would be type survival.  Is there any rational basis for preferring one kind of survival to the other?


Imagine that all music albums had only one copy in existence, and we had no way of making more copies.  Naturally, we would consider the destruction of the only copy of a good album to be tragic.  But if another copy could be made, then nothing of value would be lost.  Wouldn’t the same be true of people?