“Suppose first that I am one of a pair of identical twins, and that both my body and my twin’s brain have been fatally injured. Because of advances in neuro-surgery, it is not inevitable that these injuries will cause us both to die. We have between us one healthy brain and one healthy body. Surgeons can put these together.

If all of my brain continues both to exist and to be the brain of one living person, who is psychologically continuous with me, I continue to exist. This is true whatever happens to the rest of my body. ...

It is in fact true that one hemisphere is enough. There are many people who have survived, when a stroke or injury puts out of action one of their hemispheres. With his remaining hemisphere, such a person may need to re-learn certain things, such as adult speech, or how to control both hands. But this is possible. ... [So] I would survive if my brain was successfully transplanted into my twin’s body. And I could survive with only half my brain, the other half having been destroyed. Given these two facts, it seems clear that I would survive if half my brain was successfully transplanted into my twin’s body, and the other half was destroyed.”