To: Marcus S.
From: Geoffrey Klempner
Subject: Criteria for personal identity over time
Date: 7 October 2004 09:32

Dear Marcus,

Thank you for your email of 27 September with your third essay for the Philosophy of Mind program, entitled 'Personal Identity over Time'.

The essay is a response to the question, 'Can one give adequate criteria for the identity of a person over time? Illustrate your answer with thought experiments describing "problem cases" of personal identity.'

You have attempted something quite rare: a genuine attempt at original philosophical inquiry. I am impressed by this.

The solution which you propose sufficiently dissimilar from those I have come across in the literature to convince me that you have thought this out for yourself, although the method you have used is one that is tried and tested.

If we can discover the essence of things which belong to kind F - whatever F might be - then this promises to provide criteria, i.e. necessary and sufficient conditions, for the identity of individuals of kind F over time. That is the methodological principle which you have relied on.

How do we determine this essence when F is a human being? We look for what all human beings have in common. And so you arrive at your formula: 'True persons are identified by being in a state of human potential.'

Now, this might be criticized as an account of the essence of the kind 'human' because the concept human appears (implicitly) on both sides of the equation: The essence of a human being = being in a state of human potential. But this criticism can be easily met.

Suppose you asked me, 'What is the essence of the kind, "fish"?' and I replied, 'The essence of a fish is being in a state of fishy potential.'

This looks circular, but on closer inspection the formula provides a way of explaining the essence. 'Fishy potential' might be further explained as the ability to swim in water, to extract oxygen using gills, etc.' Anything which swims and has gills (etc.) is a fish. A fish out of water, not using its gills is still a fish, and so is a fish which is lying still at the bottom of the lake.

But now comes the second part of the process: how do we use the account of the essence of a human being (or a fish) to provide criteria for the identity of a human being (or a fish) over time?

The account hinges critically on the notion of 'same potential'. To illustrate how this works, you introduce your own problem case of personal identity, the story of Susan Jones.

Susan Jones' brain is transplanted into a new body. Then the brain turns to mush and all that is left is the living body of Susan Jones. Susan's mental capacities (potential) survived the brain transplant; her physical capacities (potential) survived the loss of her mental capacities.

If Susan's old body had not been discarded after the transplant operation but reconstructed and given the brain of another crash victim, we would certainly have problems with our account of identity. This is just a version of the 'splitting' problem which you do not address: the 'same' human potential can split into two human potentials each of which goes its own way. However, that is a general problem which faces a variety of different accounts of personal identity, and not a special difficulty with your account.

If we are dealing with fish, there really isn't a problem. No-one is that bothered whether what we have, after some lengthy experimental process, is the 'same fish' or not. - On second thoughts, if the fish undergoing vivisection was your pet fish, then you might have an interest in whether your pet had 'survived' the experiment, or 'died' and had its parts used to make a 'new' fish. (On the other hand, you might reassure yourself, 'At least Nemo's fins survived, even if Nemo is no longer here.')

With persons, the case is altogether different. And this is where your account requires an extra step. What you need is an explanation of our social, moral and legal *interest* in identifying persons. That is what the analysis of 'human potential' would plug into.

So, for example, according to the seventeenth century philosopher Locke's 'forensic' theory of personal identity in terms of continuity of memory, human potential embodies the characteristic human capacity of a sense of self (lacking in a fish), the consciousness of one's responsibility for the things one has done in the past, of the obligations one holds to other persons and so on.

This Lockean account gives the right answer in the brain transplant scenario, but the wrong answer in the brain wasting scenario. That is one advantage of your account over Locke. Susan Jones' loving husband, who agreed to be cryogenically frozen along with her in 1985, is united with her once more in 2042, then remains by her side during the long decline, never doubting that the body that he hugs, the lips that he kisses are *hers*. So long as Susan's new body survives with some physical capacity, even if it is only the capacity to breathe, she *is* still there.

A first rate piece of work - well done!

All the best,

Geoffrey