Derek Parfit and the Paradox of Personal Identity

Derek Parfit: The Argument of Fission

The effective argument imagines a scenario where one person splits into two. First, people have survived operations in which an entire hemisphere of one brain is removed. While the person may be changed in very significant ways, we do not consider them to be numerically distinct from the original person who chose to undergo the procedure.

Also, if a brain could be transplanted into a different body, the person would go where the brain goes; in other words, our brains are essential to our identity.

Third, we can assume that if it were possible to, say, destroy one hemisphere and transplant the other into a new body, the resulting person would be numerically identical to the original person that existed prior to this procedure. In other words, the survival of half of your brain is enough to constitute your survival.

The Fission Scenario and Survival

Fourth, imagine that we take your brain and transplant each hemisphere into two new bodies. There are three possibilities to consider:

• (A) You do not survive.
• (B) You survive as one of the two people.
• (C) Both survive.

Fifth, survival and identity are distinct; identity cannot be identical with more than one thing. Finally, even when we know all the relevant information, we seem unable to come up with a determinate answer to the question of identity in such cases. Parfit thinks that such cases cannot be easily answered because they have no answer. It is natural to believe that there is something more about all possible cases, and this must be a fairly profound fact.

The Practical Importance of Personal Identity

First, in order to sort out who is innocent and who is guilty, we must know the identity of the criminal. Our natural concern and anticipation for the future both presuppose judgments about identity.

For example, if you think physical continuity of the body is required for a person to continue on, you will only accept the possibility of an afterlife that requires a quite literal physical resurrection of one's body. If you come to believe that only mental continuity is required, you can have a more flexible vision of heaven.