Abstract

Suppose we transplant the left hemisphere of one person, Alexandra, into the skull of another person, Alex, from whom both cerebral hemispheres have been removed; and transplant Alexandra’s right hemisphere into the skull of another person, Sandra, both of whose cerebral hemispheres have been removed. Both of the resulting persons will then have some of Alexandra’s brain and probably almost all of her memories and character. But since at most only one of them can be Alexandra, being Alexandra must, by the “principle of the identity of composites,” involve having another essential non-physical part—her soul. It is our soul and only our soul which makes us who we are.