Thursday, 19 May 2011
Swinburne's Brain Hemisphere Thought Experiment
It's an attempt to show that mind and body MUST be separate things
It goes like this:
In theory, I could take the two hemispheres of your brain and put each one in a new body.
In theory, both of those bodies could be 'alive' - with half of your brain in each one.
In this situation, it'd be hard to say which one was really 'you' wouldn't it?... which one has your mind?
In theory, both could be conscious*
So where is your mind?
We're not sure are we?
So, I know exactly where all the bits of your body are
But, I'm not sure where your mind is
So, mind and body must be separate things mustn't they?
Therefore, this is support for substance dualism
(*in fact, there's some debate amongst scientist over whether the right hemisphere could ever really be conscious - but it doesn't control language, so how could we ask it anyway?)
Here's the argument in Swinburne's own words:
"Knowledge of what happens to bodies and their parts will not show you for certain what happens to persons. Let me illustrate this with the example of brain transplants.
The brain, as you will know, consists of two hemispheres, and a brain-stem. There is good evidence that humans can survive and behave as conscious being if much of one hemisphere is destroyed. Now suppose my brain (hemisphere plus brain-stem) was divided into two, and each half brain taken out of my skull and transplanted into the empty skull of a body from which a brain has just been removed; and there to be added to each half-brain from some other brain (e.g. the brain of my identical twin) whatever other parts (e.g. more brain stem) are necessary in order for the transplant to take and for there to be two living persons with lives of conscious experiences. Which of these two resulting persons would be me? Probably both would to some extent behave like me and make my memory claims; for behaviour and speech depend, at any rate in very large part, on brain-states, and there is very considerable overlap between the _information_ carried by the two hemispheres which gives rise to behaviour and speech. But both persons would not be me. For if they were both identical with me they would be the same person as each other (if a is the same as b, and b is the same as c, then a is the same as c) and they are not. They now have different experiences and lead different lives. There remain three other possibilities - that the person with my right half-brain is me, or that the person with my left half-brain is me, or that neither is me. But we cannot be certain which holds. It follows that that mere knowledge of what happens to bodies does not tell you what happens to persons.
It is tempting to say that it is a matter of arbitrary definition which of the three possibilities is correct. But this temptation must be resisted. There is a crucial factual issue here - which can be shown if we alter our thought experiment a little. Suppose that I am captured by a mad surgeon. He explains that he is going to perform this operation on my brain, in consequence of which there will be two living persons, one made partly out of my right brain hemisphere and the other made partly out of my left brain hemisphere. He announces that he will give one of these later persons ten million dollars and that he will subject the other one to torture. He allows me to choose which of the later persons will get ten million dollars and which will be tortured; that is, to choose whether the person who has my left half-brain will become a rich man while the one who has my right half-brain will suffer, or whether if will be the other way around.
How ought I to choose in order to become rich? It is evident that whether I shall survive the operation and whether my life will be happy or sad are factual questions. (Only someone under the grip of some very strong philosophical dogma would deny that), and yet, as I await the transplant and know exactly what will happen to my brain, each of the two choices would be very risky - if I choose that the person with my left half-brain will be rewarded, I do not know whether it will be me; and also, if I choose that the person with my right half-brain will be rewarded, I do not know if that person will be me.
And even after the operation no one will know for certain whether I have survived, or which of the later persons is me. Even if one subsequent person resembles the earlier me more in character and memory claims than does the other, that one may not be me. Maybe I've survived the operation but am changed in character and have lost much of my memory as a result of it, in consequence of which the other subsequent person resembles the earlier me more in his public behaviour than I do. And even if a fourth possibility, that they are both to some extent me, were (despite its apparent incoherence) correct, neither science nor philosophy could show that to us for certain, for all the evidence which could ever be obtained would be compatible with the other possibilities as well.
Reflection on this thought experiment shows that however much we come to know for certain about what has happened to my brain (and other parts of my body), and however much we come to know for certain about which mental properties are instantiated in which subsequent persons, we would not know for certain what has happened to me. What we would not know is which substance each of the later persons is. But since we do know - we may suppose - what has happened to each atom of my body, I must be different from my body. I must have a further essential immaterial part whose continuing in existence makes the brain (and so body) to which it is linked my brain (and my body), and to this something I give the traditional name of 'soul'. I am my soul plus whatever brain (and body) it is connected to. Normally my soul goes when my brain goes, but in unusual circumstances (such as when my brain is split) it is uncertain where it goes