To: Francis W.
From: Geoffrey Klempner
Subject: Personal identity and replacement by a superior copy
Date: 1 February 2007 09:53

Dear Francis,

Thank you for your email of January 22 with the revised version of your second essay for The Possible World Machine, in response to the following question:

'Imagine you are Michael Harding. As you lie injured on the road, you are told that a brain scanner is going to be used to map your memories and personality, and the information used to program the brain of a new body cloned from one of your own cells. The moment the new 'you' gains consciousness, the old 'you' will be painlessly destroyed. How do you feel about that prospect? – Justify your answer by reference to one of the competing philosophical accounts of the relation between mind and body.'

I enjoyed reading this essay. You have gone for the 'brave' option of dualism, while most of my students thrash about trying to answer this question on the basis of the assumption of physicalism.

Your dualism is not 'epiphenomenal' but 'interactionist'. Epiphenomenal dualism accepts that the brain is the source of all thought and experience, but hold that it somehow 'emits' non-physical consciousness which gives us the illusion that we are in control of our bodies, while in reality a zombie whose brain did not produce or emit consciousness would do and say exactly what we do and say.

The classic dualist view, however, was formulated by Descartes in his 'Meditations on First Philosophy'. What is notable about Descartes' attempt to put dualism on a strong philosophical foundation is his rejection of any notion of the soul as 'ethereal ectoplasm', the stuff that Spiritualists think ghosts are made of. In Descartes' words, the soul is not a 'wind or a vapour'. If there is a soul, Descartes argued, it cannot have any of the attributes of body. In other words, a soul has no length or breadth. It is not located in space. Its connection with a particular body is determined purely through causal interaction. Soul A is the soul of body B if and only if A causally interacts with B. The soul is not 'in' a body, but rather hooks up to it through cause and effect.

The claim that the soul cannot have any spatial attributes follows from Descartes' ingenious argument for mind-body dualism. It is logically conceivable, Descartes argued, that I could be having all the experiences I am now having, even if I was being 'deceived by an evil demon' into thinking that there exists a physical world. It follows that anything physical cannot be part of my 'essence', what it is that makes me, me. As a matter of contingent fact, there is a physical world and I have a physical body, but I would still be me without a body or in a reality which was purely mental, made up by the evil demon.

Now, you ask, 'Can the scanner scan Michael's soul?'. Given the above, the answer must surely be no. Not just because the soul is ethereal or insubstantial but because it has nothing in common with the physical.

However, thought experiments can be changed. Instead of a brain scanner, imagine an evil demon who is able to duplicate souls, or swap souls into different bodies. Imagine that one day your best friend starts acting as if he was someone else entirely. Then, the next day, he is back to normal, then the next day someone else entirely and so on. On the assumption that it is the soul that determines 'who we are', a possible explanation would be that an evil demon was having fun and swapping two souls in one body.

If you accept that, then consider the next thought experiment. There is an evil demon who is swapping your best friend's soul on a daily basis. But you do not notice because the two souls are identical in their mental properties.

Come to think of it, how do you in fact know that the 'soul' that you refer to as 'Francis' or 'myself' is in fact the same soul that hooked up to your body one minute ago? All you have are your present memories. As Bertrand Russell once remarked, there is no way to tell, on the basis of memory, that the Earth was not created five minutes ago and we and our apparent memories along with it.

You offer the traditional 'interactionist' soul theory as an explanation of why human beings are the way they are, and all my remarks are based on the assumption that this is true. But you can see from the way these thought experiments are going that there are serious difficulties with the theory as stated.

We have a perfectly good idea of what it means to say that there are two physically identical bodies, e.g. the two versions of Mike Harding. But that is because different physical objects are (as a matter of logic) located in different places. Space is what enables us to identify physical things uniquely. Attributes alone can never do this because you can always conceive of the possibility of a perfect duplicate.

But what about souls? We said that souls do not have any spatial properties. So it doesn't make sense to suppose that there are two identical souls, one 'here' and one 'there'.

We have a perfectly good idea of what it means to say that one and the same object persists through time, while undergoing changes which do not affect its identity. I have a laptop which was once shiny and now and now has a few scratches, upgraded memory, lots of additional software. But it is the same laptop. It has traced a continuous path through space-time.

But a soul does not trace a path. It's identity is determined purely by what it can 'remember'. Yet we have seen that there is no detectable difference between true memories and false ones.

These difficulties have led philosophers to the conclusion that the concept of a soul, as described, is incoherent because it is impossible to establish conditions for its identity, either at one time, or over time.

It is interesting that a lot of what you say about the 'soul' is in fact believed by Scientologists, who have a fanciful theory that human beings are like 'horses' being 'ridden' by tiny (and therefore undetectable!) physical beings from another planet. This theory, although preposterous, at least has the virtue of not being logically incoherent, because the 'soul' in this case (I've forgotten the name they use for it) is a physical entity.

All the best,

Geoffrey