Let’s imagine two women: Millie and Bonnie.
Millie has a body-wasting disease. Her body is slowly breaking down, organ by organ. The only organ which is untouched is her brain. However, the body that supports that brain is deteriorating, and she is near to dying.
Bonnie is a healthy person who suffers an unfortunate accident. She takes a sharp blow to her head which damages her brain, killing her instantly, but leaving her body intact and healthy.
Due to the magic of non-existent futuristic medical technologies, doctors transplant Millie’s brain into Bonnie’s body.
Who is the resulting person? Who is walking around? Is it Millie because it’s her mind, or is it Bonnie because it’s her body? Or is it someone else?
For legal purposes, we identify a person by their physical attributes: fingerprints, retinal patterns, dental history, face. According to that methodology, this person is Bonnie, because she has Bonnie’s physical attributes. If she used Bonnie’s passport, she would be able to travel because her face and fingerprints match the photo and fingerprints in the passport. She would be accepted as Bonnie.
However, this person does not have any of Bonnie’s knowledge or memories. She remembers Millie’s life and friends and education. If we asked her to sit an academic exam about a topic that Bonnie learned, she would fail, but she would pass an exam about a topic that Millie learned; she is therefore entitled to use Millie’s academic qualifications as her own. Her encephalograph would show Millie’s brain patterns. She would recognise Millie’s family and friends.
If Millie and Bonnie each committed a crime before the transplant, should the post-transplant person be held responsible for either crime, or both, or neither? If she was in court, a witness would identify her as Bonnie, who was at the scene of one of the crimes. However, she would not remember committing Bonnie’s crime, but would remember Millie’s crime. Can she be held responsible for a crime she doesn’t remember committing (that brain is now dead)? Can she be held responsible for a crime noone saw her commit (that body is now dead)?
So… who is she? Is she Millie or Bonnie, or is is she some new composite person: Minnie?
Partly inspired by a couple of science fiction works:
'I Will Fear No Evil', by Robert A Heinlein
The Star Trek DS9 episode 'Dax', by Peter Allan Fields and D.C. Fontana