Briefly, a person at t2 is identical with a person at t1, iff
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There is psychological continuity in the form of overlapping chains of strong psychological connectedness.
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There is no branching (for example if you get duplicated, this clause is added cause identity is (apparantly) necessarily a one-one relation (though I don’t believe Personal Identity is as I shall explain in an essay which will shortly make an appearance in the essays/theses section)).
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The continuity has the right kind of cause. (In other words someone who spontaneously appears by random occurance and claims to be Aristotle is not Aristotle, just happens to be qualitively similar, there has to be a causal relation between Aristotle at t1 and Aristotle at t2).
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Personal identity is nothing more than holding facts such as 1-3.
A brief explanation of strong psychological connection: His theory can be seen as an extension of Locke’s original one of identity being overlapping memory connections, but instead of relying just on memory connections Parfit looks at connection between all our psychological states, such as memories, beliefs, desires, dispositions, the connection between intentions and subsequent acts and so on. A connection is termed ‘strong’ if there is a significant amount of resemblance of one to the other. Thus while I have strong connection to myself a week ago, I certainly do not have the same connection to myself 10 years ago and there will only be a small amount of connection.
Parfit then goes on to argue Identity is not what matters to us:
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Identity cannot be many-one, one-many, it is logically one-one.
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In a case of fission (where we split into two people), it must either be that
(a) We survive as both, (b) we survive as one or the other, (c) we survive as neither.
(b) is stupid, for (c), how can a double success be a failure, (a) is logically incoherent, identity is a one-one relation, therefore, identity is not what matters to us, all that matters iin survival is bearing Relation R to someone(psycholgical continutiy and/or connectedness to someone).
That’s a little of a crash course. If you can get get your hands on a copy of Parfit’s Reason and Persons do so, it’s really good, though I’ve only had time to read Part III so far, which is on identity, so I don’t know if the rest is good. Skeptic, what do you think of the rest of it?
In response to your question (a good un), our personality changes as we experience events and that change is permanent even if we can’t actually remember them.
One might argue that our Freudian Id might remember it though (is it the id or the ego, argh! I might be wrong in saying id there). Depends if you buy a Freudian view though.