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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.
 
Readings
 
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.
 
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.
But 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. On this theory, I cannot survive the death of my body. If physical continuity is the criterion for identity, then my identity vanishes when my physical continuity ceases. (It cannot be the same “I” that survives my death.) This may or may not be a problem, depending on one's views of these matters.
2. Imagine 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.)
3. Is Leonard (in Memento) the same person he was before his accident? Teddy doesn’t think so. On the physical continuity theory, he is, but there are problems in saying that he is. He is, after all, seemingly very different. The same problem seems to apply to the Leonard of five minutes ago!
 
II. Psychological Continuity Theory (Descartes, Locke)
Descartes: “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 memories of yesterday, and yesterday, I had memories of the day before, etc, on into my past. (The transitive property of memory that is supposed to link me to my child self.) 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 possibilities: (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? What if there were an evil genius fooling you about your memories? Remember, this is the standard Descartes himself thought genuine knowledge must meet. 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!)
 
Leonard: “Memory’s not perfect. It’s not even that good. Ask the police. Eyewitness testimony is unreliable.”
 
III. Dispositional Theory (Kierkegaard, Proust, Sartre(?))
 
“I” am my personality, or a set of fundamental dispositions to act in certain ways
 
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
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