From Wikimedia Commons
Source: From Wikimedia Commons

Dear Professor Davies,

Today my friends and I were discussing a question about consciousness, and I would love to hear your opinion: under perfect circumstances, if we were able to cut a person symmetrically down the middle of a brain and regrow both halves using stem cells to become two people, what would happen to the original person's consciousness? I would love to hear your opinion.

Thanks, B.H.

Dear B.H.,

To me, this question is a little like this one: Suppose you have a bowl of water, and that water has wetness. You pour half the water into another bowl, and then put in more water so that both bowls are full again. What happened to the wetness of the original bowl of water?

I think that consciousness is only possible in humans with large areas of brain at work. Personally, I think that a hemisphere (but probably not a cerebellum, for instance) is enough to have consciousness. But when someone is a split brain patient (that is, their corpus collosum is cut) they effectively have two consciousnesses running at the same time. Arguably, they already have two consciousnesses going before the split--counting the number of consciousnesses is problematic.

So rather than thinking that the person's "original" consciousness has moved or changed, I think it's more accurate to think about what the two brain halves would be conscious of. For example, the right and left hemispheres often differentiate in what they specialize in. It could be that the new brain made from the original right hemisphere has severe language problems, because language is mostly a left-brain thing. So the new brain (with its new left hemisphere grown from stem cells and hasn't learned a language yet) might not be conscious of language.

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