To: Walter F.
From: Geoffrey Klempner
Subject: Problem cases for personal identity
Date: 23 November 2005 11:40

Dear Walter,

Thank you for your email of 13 November, with your third essay for the Philosophy of Mind program, in 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 'chosen to approach this question via the subject/ object dichotomy'. What exactly does such an approach imply?

You could be saying that we have to reckon with the difference - or distance - between the subjective and objective views in giving criteria for personal identity over time. Thus, you go on to say, 'We must admit we have high standards when it comes to ourselves. There doesn't seem to be any physical or non-physical nugget that could remain intact throughout our lifetime, not to mention eternity.'

To me, this looks like a very low 'standard' in terms of setting criteria. It seems to me, from my subjective standpoint, that I could still be me without this body. I could still be me without any given subset of my memories (which, over an extended length of time means that there need be no memories in common between me at time 1 and me at time 2). Could I, through gradual degrees, become a spoon? or acquire the memories of Colonel Custer? If you are tempted to say, No, just describe a sufficiently slow and steady series of physical and/or mental changes between me with my body and me embodied in a spoon, or me with my memories and me with the memories of someone who knows himself as 'Colonel Custer'.

We need to take a step back. The issue is about identity, but there are two kinds of 'problem cases' with regard to identity. The first kind, illustrated by Chrisopher Buckingham, 'the morning star is the evening star', and 'my mother is my father', is essentially a-temporal. The morning star is *seen* at a different time from the evening star, but their identity is not a case of 'identity over time'. 'The morning star' and 'the evening star' both refer to one and the same object existing *now*. Problem cases with identity at a time rather than over time always revolve around the question of knowledge. As Frege observed in his famous paper, 'On Sense and Reference' we don't know everything about an object, we only know it via certain 'modes of presentation'. So it is possible to be ignorant of the fact that two such modes of presentation identify one and the same object now.

The second kind of problem case is the one that we are concerned with. It is by no means confined to persons. The question could have asked, Can one give adequate criteria for the identity of a motor car over time? Illustrate your answer with thought experiments describing "problem cases" of the identity of motor cars.' One could keep replacing old or worn out components in your motor car, including body panels, until no original parts are left. Is it still 'one and the same' car? In one important sense, Yes. Spatio-temporal continuity plays a central role in our conception of identity of physical things over time. An entity can change over time yet remain one and the same entity. The human body is another example - I read somewhere that most of the physical material in the average human body is replaced over a period of seven years.

Some philosophers have gone to the extreme of arguing that identity over time is never true 'identity'. But this seems to be a rather pointless move, because it renders a large part of our language obsolete. More relevant, for our purposes, is the claim argued by Derek Parfit in 'Reasons and Persons' that the concept of personal identity over time is dispensable. Parfit's answer to the question about criteria for personal identity would simply be, 'No'.

You highlight the importance of language in determining what we take 'ourselves' to be. The concept of a 'person' embodied in our language involves both physical and mental components. Because in our experience these normally do not 'pull apart' - no-one has yet succeeded in constructing a body-duplication machine - the concept is never tested to destruction. But what if it was? What if body-duplicating machines became part of everyday life? Or what if people lived for thousands of years?

Why should we even be bothering ourselves with such seemingly pointless questions?

I think there is a point. The point is in the way these thought experiments expose the conflict between the subjective and objective views. Subjectively, it seems impossible to doubt that if I went into a duplicating machine, one of the resulting persons would be 'I' and the other one 'not-I'. But this intuition is indefensible, once we recognize the necessity of an objective criterion - or, what amounts to the same thing - the dependence of the concepts of identity and personal identity on language. The only way to defend the intuition is by resurrecting a metaphysical Cartesian nugget of individuality whose identity is totally transparent to itself, never open to doubt.

If anyone is tempted by that idea, then an easy way to get rid of it is to imagine that God created you ten seconds ago, along with all 'your' apparent memories.

I believe very strongly that there is something about the subjective view that can and should be defended. But the attempt to define a 'subjective criterion of personal identity' is hopeless. The subjective 'I' only exists in the 'now'. All that it knows is 'I-now'.

All the best,

Geoffrey