- It was the split-brain cases which drew me into philosophy. Our knowledge of these cases depends on the results of various psychological tests, as described by1 Donald MacKay. These tests made use of two facts. We control each of our arms, and see what is in each half of our visual fields, with only one of our hemispheres. When someone's hemispheres have been disconnected, psychologists can thus present to this person two different written questions in the two halves of his visual field, and can receive two different answers written by this person's two hands.
- Here is a simplified imaginary version of the kind of evidence that such tests provide. One of these people looks fixedly at the centre of a wide screen, whose left half is red and right half is blue. On each half in a darker shade arc the words, 'How many colours can you see?' With both hands the person writes, 'Only one'. The words are now changed to read, 'Which is the only colour that you can see?' With one of his hands the person writes 'Red', with the other he writes 'Blue'.
- If this is how such a person responds, I would conclude that he is having two visual sensations - that he does, as he claims, see both red and blue. But in seeing each colour he is not aware of seeing the other. He has two streams of consciousness, in each of which he can see only one colour. In one stream he sees red, and at the same time, in his other stream, he sees blue. More generally, he could be having at the same time two series of thoughts and sensations, in having each of which he is unaware of having the other.
- This conclusion has been questioned. It has been claimed by some that there are not two streams of consciousness, on the ground that the subdominant hemisphere is a part of the brain whose functioning involves no consciousness. If this were true, these cases would lose most of their interest. I believe that it is not true, chiefly because, if a person's dominant hemisphere is destroyed, this person is able to react in the way in which, in the split-brain cases, the sub-dominant hemisphere reacts, and we do not believe that such a person is just an automaton, without consciousness. The sub-dominant hemisphere is, of course, much less developed in certain ways, typically having the linguistic abilities of a three-year-old. But three-year-olds are conscious. This supports the view that, in split-brain cases, there are two streams of consciousness.
- Another view is that, in these cases, there are two persons involved, sharing the same body. Like Professor MacKay, I believe that we should reject this view. My reason for believing this is, however, different. Professor MacKay denies that there are two persons involved because he believes that there is only one person involved. I believe that, in a sense, the number of persons involved is none.
Footnote 1: In "MacKay (Donald) - Divided Brains - Divided Minds".
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