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What Makes You the Same Person Over Time?

Updated on June 17, 2014

How many you's are there?

What is nonsensical about the following claim? "There are several tokens of me. If one commits the crime, another is put forward for the punishment. This affords me a strategy for getting away with anything nefarious I want to do."
What is nonsensical about the following claim? "There are several tokens of me. If one commits the crime, another is put forward for the punishment. This affords me a strategy for getting away with anything nefarious I want to do."

Personal Identity

What philosophy studies under the term "personal identity" has to do with perdurance continuity of the same entity (person) over time. The verb "endure" can be used for this claim. A person X endures over a time continuum t if and only if the same person X is present at every distinct unit of the time t, {t1, ..., tk}. An archaic verb that originally meant the same is "perdure," but, in contemporary philosophic debates, this verb now is used in a distinct sense. While "endures" means that the person is entirely present at any given moment of the period over which this person exists, "perdures" means that only a temporal part of the person exists at any given moment tj with the entire person existing only if the entire t-period is factored in. This unusual view, inspired by what seem to be certain metaphysical corollaries of Relativistic Physics, is known as Perdurantism or Mereology.

It is surprising that much ado would be made over continuity of the same person over time. We intuitively consider a person as the same person over time when we hold this person accountable for what he or she can be shown to have done in the past. We also expect the same person to receive rewards or penalties in the future after he or she has been judged worthy of such. Since we take the principle "no one should be rewarded or punished for something someone else has done", then it is clear that we are not bothered by the problem of same-person continuity on an everyday level. Yet, closer scrutiny shows readily that this is a recalcitrant problem. One after another, we can bring up criteria of personal identity (what makes someone the same person over time) only to discover that no criterion is immune to criticism. Matters become even more interesting when it turns out that we can produce cases that are puzzling also because they seem to challenge some logical principles associated with personal identity.

The stakes are high. Suppose that someone believes in immortality, or in resurrection of the dead, or in reincarnation. It has to be assumed that it is the same person that endures beyond demise of the body, or the same person that is brought back to life, or reincarnated. For instance, could divine justice be meted out against someone, who, resurrected, is not the same as the person who committed certain immoral deeds? Does it make sense to say that a person is reincarnated in a new body if it is actually ... not the same person that inhabits the new body?

What happens if some popular theories, in religion and spirituality, can actually be shown to run into logical absurdity on account precisely of messing up personal identity considerations? When we think how sloppy popular views tend to be, it should be no surprise if many promising and popular views harbored absurdities on this account. On the other hand, it is also true that the subject itself - personal identity - is quite formidable and affords opportunities of adjustment even for theories that sound far -fetched. For instance, resurrection theory, you would think, would easily break down when judged under the lens of personal identity. Isn't personal identity - continuity of the same person - lost when there is interruption in space and time (the person is said to die and then, later and somewhere else, reappear as the same person!) And yet, it is not as easy as it seems at first to show that spatiotemporal continuity is itself a necessary criterion that personal identity has to satisfy.

As you embark on your study of this subject, realize that this is about continuity and endurance over time of the same subject. It is not about some specific attribute of the person - like personality. When we say "she is not the same person" we don't mean that personal identity has been affected; in fact, we assume that she IS the same person, so we can say that SHE, the same person, has changed so dramatically. Watch out for this confusion - it is the most common among beginning students of the subject.

Continuity of the Same Person Over Time

What Makes you the Same Person Over Time?

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The first expression says, "for anything, there is a unique entity that co-refers with it." In Free Logic, this means that there is one and only one thing to be uniquely denoted by any name. The second expression says, "everything is self-identical".
The first expression says, "for anything, there is a unique entity that co-refers with it." In Free Logic, this means that there is one and only one thing to be uniquely denoted by any name. The second expression says, "everything is self-identical".

Numerical Identity

There are certain principles associated with the study of personal identity which, remember, has to do with continuation of the same person over time. A principle can be logical - ultimately based on the meanings of key logical connectives in a language. For instance, "if 'p and q' is true then 'p' must be true" is a logical principle - if treated as a principle. The principle "you cannot in two different locations at the same time" is not logical; call it metaphysical. In every logically conceivable world which is like ours in the way nature works overall, this principle must be true - if it is a good metaphysical principle. Clearly, this principle is relevant to the study of personal identity but it is weaker than the next principle - assuming that this next principle is logical, which means that it cannot be falsified regardless of what kind of world we are talking about. This next principle is known as the "principle of numerical identity."

According to the principle of numerical identity, one and only one entity can claim to be person X for any concievalbe person X. Or, no more than one persons can be the same person. There are conceivable situations that seem to pose a challenge to this principle. Alternatively, one could claim that those situations must be logically absurd since they violate this principle - assuming that the principle of numerical identity is logical.

What does the principle of numerical identity mean really? Does it have the same meaning as the principle of self-identity -- "any object, called for instance "a", is identical with itself and, so, "a = a" is also a logical principle since "a" must be co-referring with any name that names the same thing." Were more than one persons to claim to be the same person, we would have: let's say, person X1 is the same as person X2. Then, it cannot be the case that X1 and X2 are, nevertheless, different persons. We take it that X1 and X2 are separate The word "person" is used with the same meaning throughout (otherwise, we would have a logical error known as equivocation.)

Here is another proof. Suppose that X1 and X2 lay claim to being the same person even though they are also presumed, to begin with, as being distinct. If X1 and X2 make this claim, then they should have the same attributes exactly. Person X1 must have the property "being-identical-with-X1." Self-identity - which we suspected to be related to numerical identity - is a logical principle. So, we can also say that X1 has the property "cannot-possibly-be-distinct-from-X1." Since X1 and X2 have all properties in common, then X2 must also have this property of X1 we just constructed. Now, let's see what this means: X2 must have the property "cannot-possibly-be-distinct-from-X1." Yet, we accepted putatively that X1 and X2 are distinct! So, we have reached a contradiction and we must reject the assumption of distinction. (This proof method is called, in Latin, reductio ad absurdum; it is also called, in some contexts, Indirect Proof and Proof by Contradiction.)

Here is another proof - but this one requires accepting a non-logical principle which we cannot prove. The added principle is due to Leibniz and is called "the principle of sufficient reason." In an old-fashioned version, the principle demands that everything (object or event) has a cause. In a more modern version, the principle demands that a satisfactorily complete explanation is available for anything (regardless as to whether we have in access to the explanation in fact or not.) Now, if two persons X1 and X2 lay claim to being the same person, as before, they must share all their attributes. Yet, we could not possibly - not even in principle - produce a complete explanation as to why there are two rather than one person. Suppose we took the set of all the common attributes to be satisfactorily complete as an explanation for person X1. If person X2 is distinct from X1, the complete account or explanation of X2 should include the property "distinct-from-X1." Yet, this property is not available since, then, X1 should also have it - and if X1 had the property "distinct-from-X1", this would throw X1's explanation into absurdity! We prove again that X1 and X2 cannot be distinct - they are the same person.

As a matter of practical application, the study of theories or criteria of personal identity should observe this principle of numerical identity. Any theory or criterion that runs afoul of this criterion, should be rejected.

The Most Popular Criterion?

What is fascinating about personal identity is that criteria as to what makes you the same person over time, which come across as sensible and unproblematic at first, run into trouble. Can we come across a satisfactory view as to what makes one the same person over time? To be sure, there is also a view that we are not the same person over time - let's say, because this would require that non-concrete entities, (like the abstract entity that arches over all your concrete moment-to-moment selves), are real. You can find more on the classical version of this view under "bundle theory" and under "David Hume."

Perhaps the most popular criterion is memory (better, rightly caused memory stream) accompanied by accurate self-awareness. John Locke proposed this criterion but he ran into embarrassment (or did he?) when he retracted it for the following hypothetical: suppose person X, who is discovered to have been a serial killer, is now on trial but can be documented properly to be suffering from severe deterioration of mental faculties to the extent that he has no sustained memories of what he ever did and no self-awareness. Under the memory criterion, we would have to declare him as no longer being the same person. If we also accept as a good moral principle that we should not punish person X2 for what a distinct person, X1, has done, then, it follows that we should not punish the presumed serial killer on trial. The person the jury faces is not the same person with the person who committed the heinous crimes. (It doesn't matter whether the crime is much less serious form the point of view of personal identity - or does it?) Locke pulls back in this case and declares that society may have good reasons (they have to be defensible, justifiable good reasons) to declare the person on trial as being the same person with the serial killer regardless of the metaphysical indications to the contrary. By doing this, Locke switches to a different theory of personal identity - one that has not been popular and comes across as counterintuitive, which we can call "the forensic theory of personal identity."

The Forensic Theory might make better sense in the case of objects. See the following text on the famous ancient puzzle regarding the ship of Theseus. For now, consider some of the problems with the memory-awareness theory. One problem is that the theory has the consequence that someone suffering from advanced Alzheimer's must be presumed to be a different person from the person there was before a certain threshold in the condition, beyond which sense of self-awareness and matching memories were no longer available. It is not fatal to the theory that we might not be able to fix this threshold. The counterintuitive element is that we are supposed to declare a shift in personhood. Perhaps it is our sentimentality that stands in the way here. This consequence might not be as far-fetched as it may sound at first. I think that the following, less known critique, is more decisive:

Suppose we have a person, X, and stages in his life: Y1, Y2, Y3. At, or throughout, Y2, X remembered events that had indeed transpired throughout period Y1. X-at-Y2 must then be the same person with X-at-Y1. Now move to period Y3. We can assume without any absurdity that, now, at Y3, X remembers events at Y2 but not events at Y1. So, X-at-Y3 is the same person with X-at-Y2 but he is not same person with X-at-Y1. Let me use some natural, and informal, symbolization to sum up these results.

  1. X/Y1 = X/Y2 (matching memories)
  2. X/Y2 = X/Y3 (matching memories)
  3. not(X/Y1 = X/Y3) (memory discontinuity)
  4. but, given 1 and 2, we should have, X/Y1 = X/Y3 (we should take personal identity to have the transitivity property).

We actually ended up with a contradiction, as you can see. Isn't this a damning critique?

Can any other do any better?

The Ship of Theseus

This legendary philosophic challenge from classical antiquity addresses another problem, although it has something to do with personal identity too. We don't have the presumably endless and exciting debates this puzzle generated - only one short paragraph in Plutarch's Theseus giving us information about it.

The ship of legendary Athenian hero Theseus had been on display for a long time in Athens. Yet, one day, it was realized that not a single plank of the original ship remains. Plank after plank of the ship had been duly removed as they were rotting away, compromising the integrity of the ship, with the passage of time. The problem known as "sorites" - not to address here - is key to the controversy: at what point did the ship cease to be the same ship (assuming that it did), given that each and every one of the changes made to the ship over time is admittedly unable to make such a great difference as to make the ship cease to be the same ship? (The problem here is vagueness, and we can address it in another place.)

Suppose that an enterprising showman had been collecting the rotting planks as they were being thrown away and has now managed to put together a ship which he showcases as the "real ship of Theseus." There can't be two ships both of which are the same item (that would violate the principle of numerical identity we studied earlier.) Which one, if either, is the real ship of Theseus?

Perhaps we are bogged down in the wrong area: there may something deeply confused about metaphysical speculation - and notice that our default is set on making all those metaphysical speculations about what "really" this or that and about whether some thing or entity (what is that?) has been preserved or ceased to exist. Should we take the view, rather, that we can go pragmatic? Given good, defensible reasons we can consider item A as the "same" and enduring relevant item (the ship of Theseus, let's say) regardless of what metaphysical perturbations we consider to have befallen the item under consideration. It might be the case even that metaphysical puzzles - like the one we started considering - can be shown, time and again, to be originating in misuse of language or confusion about logical matters? Not everyone agree with this view, which is the famous Analytical Philosophy approach. An early philosopher who took a dim and mistrustful view of metaphysical conundrums was Thomas Hobbes who, in De Corpore, suggested what we called the pragmatic way out - and notice how this approach is like what we called earlier the Forensic Theory of identity.

Yet, the case of persons, unlike that of objects, makes it harder to accept forensic solutions. Locke's about-face - accepting good reasons for considering the amnesiac as the same person for juridical purposes - comes across as rather unreasonable. This demurral in the case of persons involves some of the most difficult issues regarding the status of consciousness and what criteria we accept for personhood. This adds even more complexity to the problem of personal identity.

Memory and Punishment

Is it Fair to Punish a Severe Amnesiac for Something He/She Did in the Past?

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The Soul and Personal Identity

The classical definition of the soul denotes an immaterial entity which, even though participating through the body in the spatiotemporal field, is not confined or confinable in space or time. If the definition is not logically absurd, there are claims that one can prove, by the definition alone, that a soul is indestructible. An additional property may be needed - that the soul is simple or has no constituent parts. When people think of after-death experiences and continuation of consciousness following cessation of key brain functions, this seems to be the view they are invoking. A characteristic popular confusion is to mix popular religious views that have a "resurrection" theory with the soul theory. If the soul is indestructible, then death cannnot happen; but without death, there can be no resurrection - because, by definition, resurrection presupposes that someone dies and then, at some later time, is raised back to life. The soul-view might have originated in ancient Egypt but it entered into philosophy on account of Plato's persistent efforts to back up the theory with proofs.

It could be said - though not without controversy - that the soul theory faces no personal identity challenge. This would account for the absence of the problem of personal identity from Plato's dialogues - something unusual given the voracious appetite for engaging philosophic challenges we come upon when we read Plato's work. As a single, uncompounded, non-material substance, the soul cannot change as material objects can (in fact, material MUST change on account of their spatiotemporally constructed constitution.) Of course, how the soul can also interact with material things, like the body, and how the soul can also be, in a sense, trapped or "entombed" in the body pose serious challenges to the soul theory, but the soul view might be immune to the specific problem of personal identity. If the person is a soul, then the guarantee that it is the same person that continues over time is like the guarantee that the entity represented by the numeral "2" (not to be confused with this numeral), the number two, is the same over time. Indeed, it sounds confused to even raise the question as to whether the number two is the same over time. For the persuasion known as Number Realism the number two is a real, mind-independent, although abstract, entity. The Platonic soul is in some respects like that - it is abstract, and so on. Of course, an immediate objection to all this may well be that Platonic philosophy confuses metaphysical things (selves, etc.) for logical things (the things we deal with in formal discourses.) Be that as it may, remember, the narrower issue we are posing right here is about personal identity: does the soul theory face the personal identity challenge? Maybe it does not.

On the other hand, consider the following hypothetical, regardless as to what one may think about a theology of punishment (the hypothetical has a specific target in metaphysical speculation). Take a person X, whose identity is continuous (if that's the right word, since the soul that is the person named X is not confined in time). Now, it's game over and the Judge is about to allocate rewards and punishments. X has no memories of the alleged mortal sins he had committed. Yet, there is no question that we are dealing with the same person since the soul is unaffected and cannot cease or discontinue in the way spatiotemporally confined things can. But is it fair to punish this person - or to reward him - given that he has no idea of what is attributed to him? Moreover, a better theory may be that punishment is strictly educational - a sadist would find other uses for punishment but the moral justification for punishment lies fundamentally, it could be argued, in the educational purpose of punishment. The amnesiac person X cannot be properly educated through punishment since he cannot match what he presumably did with the punishment inflicted on him now.

Cutting-Edge Challenges to Traditional Thinking About Personal Identity

Modern science has advanced by leaps and bounds and has given us an unprecedented map of the brain's array of electrochemical activities. By an evolutionary fluke, the brain is divided into two hemispheres. Even though the left hemisphere is "smarter", the redundant division of labor within the brain is such that both hemispheres could function with sufficient capacity for connecting a person to his or her past and self-awareness. Indeed, in the kind of case introduced by philosopher Derek Parfit, parallel transplantation of the two hemispheres into two distinct (and themselves brainless) bodies, would pose a serious challenge. Since we cannot exclude the possibility of success, we are left with the following options:

  1. two new persons emerge
  2. the left hemisphere person is the same with the donor while the right hemisphere person is a new person
  3. the right hemisphere person is the same person with the donor while the left hemisphere person is a new person
  4. both are the same person

Does option #4 violate numerical identity? Consider how this case resembles in relevant respects the case of someone who has the corpus callosum, which allows for interaction between the two hemispheres, severed. Wouldn't we rather take the view, as more intuitive appealing, that we do not have two new persons but one person? On the other hand, if numerical identity is a logical principle, then its violation, as we saw, results in absurdity and condemns the theory. Yet, it is not clear that numerical identity is violated in adopting #4 above. Notice that both left and right hemisphere allow for continuity of memories and self-awareness. Assume we have two distinct persons, X1 and X2, yet both are the same person with X given the criterion of memory-awareness. But, if X1 and X2 are the same with X, then they are the same and not two distinct persons!

What happens when new memories accrue? The body with the one hemisphere will have different ongoing experiences impacting it from the body with the other hemisphere. In this case, memory-awareness give us rather a bifurcation of persons and not the same person. On the other hand, however, we take it that one does not become a different person simply by having new memories accruing. Perhaps the criterion of memory-awareness is not sufficient, after all. This does not mean that we have to declare that we have two separate persons in the above thought experiment. It is interesting that challenging hypotheticals made conceivable by scientific advancement may be guiding our thinking about a problem like personal identity - instead of trying to proceed on an a priori basis about what criteria to select, as the case was in the past.

© 2014 Odysseus Makridis


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