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Personal Identity Theory SG
Personal identity seems at first to be a non-issue, until we consider it in light of the problem of change. How can a thing change? If it does, it no longer has all the same properties, so it is no longer identical to its former self. (This, of course, applies to you as well.) If it is not identical to its former self, in what sense can it be the same thing? This is a question identity theory tries to answer. With respect to personal identity, there are three possible answers, discussed below.
Stanford Encyclopedia Article on Personal Identity
A more general article on identity issues (the problem of change.) Read section 5 on personal identity and Locke. Read the rest if you are interested in the more general problem of change.
1. With respect to personal identity:
      How do we make sense of someone’s observation that person P has changed?
      Person P is different from the way she was before
      But person P is not different from her earlier self the way she is different from her sister
      How can you be the same and not the same?
2. The more general problem of personhood: an answer to the questions of what makes something a person might help answer the question of how one can be the same person over time. Example: Plato and Descartes thought that a person was an immaterial soul or mind. The physical person is a kind of “accessory” that could be shed, so that the person could live on after the death of the body (and brain.) Thus the continuity of a soul guarantees the continuity of identity.
I. Physical Continuity Theory
The crude version: I am the same person I was yesterday because I inhabit the same body. But this is inadequate: what do we mean by "sameness?" (My body is very different now from what it was when I was a child.)
Let's refine the theory first:
Am I the same person I was when I was a child? Perhaps I am the same person because I have the same DNA. But then are identical twins the same persons? So this, too, seems inadequate.
Is it a matter of social recognition? (Others recognize me, so I am the same?) But what if you went somewhere where no one recognized you? Would you be a different person?
Twins do not travel the same path through space-time. So the best physical-continuity theory seems to be that identity is a matter of tracing out a continuous path through space and time. I am the same person I was because I still occupy the same space-time path I did as a child (it's just longer now!)
Therefore, let us take physical continuity theory to say that I am the same person because I trace out a single continuous path through space and time. (Notice that this is a more thoughtful thesis)
Problems for this theory:
1. Locke points out that if I lose a hand, it is no longer me (the “fission” problem: my hand now traces out a new space-time path backwardly continuous with mine) Thus on the space-time continuum theory, there now seem to be two of me.
2. Locke also imagines something like a 'body-switch' scenario. Say, some movie in which a guy is switched into his girlfriend's body. Where is the 'guy' now? Most of us would probably say he is in his girlfriend's body. This seems to suggest that personal identity is not (only) a matter of physical continuity. (Yeah, I know: you are saying "but this cannot happen!" But it doesn't really matter for philosophical purposes. The point is about what our intuitions are, and our intuitions suggest that most of us don't think that identity is purely a matter of physical continuity. If you don’t like this, imagine that someone’s brain is transplanted, and that his consciousness goes with his brain. Granted, the brain is part of the physical body, so this scenario complicates the question. See the article on personal identity in the Stanford Encyclopedia.)
Note here: the method of thought experiments. One is tempted to say: “But this cannot happen, and so the argument is meaningless.” But the point of science-fiction-like thought experiments is to see what our intuitions about philosophical issues are, when ordinary thinking about the issues are superficial. So, since in normal cases personalities and memories change only gradually, we invent bizarre cases of radical shifts or breaks and ask after our intuitions in those cases. Memento can be seen as a thought experiment of this kind.
3. Am I identical with myself as a fetus? Maybe so, but this seems to beg the question about the moral status (the “personhood”) of the fetus. Moreover, is a corpse the same person as the living person was? The space-time-path theory seems to suggest it.
4. What about Leonard? On the space-time-path theory, Leonard is the same from scene to scene, and is also continuous with the pre-injury Leonard. Are you satisfied with this result? Do you think he is the same person throughout the film?
II. Psychological Continuity Theory (Descartes, Locke)

Locke defines a person as "a [being] that has reason and reflection [i.e. is rational and self-conscious in a way that animals are not] the same thinking thing at different times and in different places." (italics mine) This seems to prejudice the case against physical continuity theory, but remember that Locke has objected (see above) to that theory.

Descartes says: “Is it not the same ‘I’ who is now doubting almost everything, who nonetheless understands some things, who affirms that this one thing is true, denies everything else, desires to know more, is unwilling to be deceived, imagines many things even involuntarily, and is aware of many things that apparently come from the senses?” Here, the “I” is the mind: a mental substance (see 254-55)
Again, let's refine this:
How is Descartes sure it is the same “I” having all his thoughts? What does he mean? There are at least two possibilities:
1. Stream of consciousness. Descartes says (being careful): “I am, I exist – that is certain. But for how long? For as long as I am thinking. For it could be that were I totally to cease from thinking, I should totally cease to exist.” One obvious problem here: what about when I am asleep (and not dreaming?) Do I receive a new identity upon waking?
2. John Locke’s answer: a person’s genuine memories tie the past and present selves together. I am the same person I was as a child because I have the memories of that child.
Problems for Locke's theory:
What about those times I do not remember? Am I continuous with the person that existed at those times? It would be very weird if I were continuous with some of my past “selves” but not others! So we appeal to the transitive property of identity, as follows: obviously, I have memories of yesterday, and yesterday, I had memories of the day before, etc., on into my past. The transitive property of identity is supposed to link me to my child self. (Transitivity is a property of the ‘=’ sign [identity]: if x=y and y=z, then x=z. So my memories of yesterday make it the case that I am the same person as the me of yesterday, and for the same reasons that person is identical with the person of the day before, so the current “I” am identical to the person of two days ago, and so on into my past.) But this is a problem in someone like Leonard’s case! Are the current versions of Leonard (who lack the relevant memory links) different people?
Hume objects to both 1 and 2: (see 340) “When I enter most intimately into what I call myself, I always stumble on some particular perception or another…. I never can catch myself at any time without a perception, and never can observe anything but the perception…. [Mankind] are nothing but a bundle or collection of different perceptions, which succeed each other with an inconceivable rapidity, and are in a perpetual flux and movement.”
In other words, how do you know it’s the same you that has all these memories? You have no perception of your own consciousness, since your consciousness is doing all the perceiving, and you have no perception of that consciousness’ continuity, since there is no continuous perception running through the shifting bundles of perceptions you do experience. In fact, how do you know you have more than a merely momentary existence? (i.e. this current moment.) What if there were an evil genius fooling you about your memories? Remember, this is the standard Descartes himself thought genuine knowledge must meet- that even if there were an evil genius doing this, I still could not doubt it. But memories can be doubted (obviously.) So there seems to be no metaphysical certainty that I am the same person, even from moment to moment.
Another problem: What counts as a "genuine memory?" A genuine memory seems to need to meet three requirements:
1.     A previous perception occurred
2.     That perception caused the current memory
3.     The memory must accurately represent the previous perception
Hume’s point about Locke's theory then becomes this: we (and Leonard!) have no way of checking! We have memories but no way of identifying the proper causal links to the past. (Because all the possible markers involve memory!) As Leonard says, “Memory’s not perfect. It’s not even that good. Ask the police. Eyewitness testimony is unreliable.”
3. Dispositional Theory 
There is a third version of psychological continuity theory. “I” am to be identified with a stable set of fundamental dispositions to act in certain ways. (Like a fundamental character or personality.) For instance, for Kierkegaard, “I” achieve an identity over time by having commitments which unify my will (my dispositions) and give me integrity. We will see this when we do existentialism.
What about Leonard? He claims to unify himself through his purpose. But he seems to have to re-discover that purpose constantly. Also, dispositions do not tie him in any obvious way to his pre-injury self. (Since his dispositions seem to have changed.)
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