Part Three of Derek Parfit’s Reasons and Persons is titled “Personal Identity”. One of its central claims is what Parfit calls the Reductionist View: that persons are not “separately existing entities” over and above their brains and bodies. What is important about being the same person at different times consists primarily in psychological continuity and connectedness.
Another, related claim is that being the same person is not in itself very important. In particular, it is not a rational justification for self-concern. If I know that someone in the future will not be myself, that is not a good reason not to anticipate having that person’s experiences. What is important are the underlying, real relations of psychological continuity and connectedness. And even they do not have exactly the same importance that we tend to believe personal identity has.
Part Three of Reasons and Persons contains 150 pages of closely-reasoned arguments which are by and large original, compelling, and illuminating. I will not try to restate all of Parfit’s arguments, or to comment on them all; instead, I strongly recommend his book to anyone interested in this subject. In this post, I will review one of Parfit’s more important lines of argument in Chapter 12, “Why Our Identity is Not What Matters.”
Parfit begins this chapter by making a refreshing break from the philosophical practice of thought-experiments, building instead on actual cases documented in medical literature. These are the famous ‘split-brain’ cases, in which surgeons severed the corpus callosum, the main bundle of nerve fibres connecting the left and right hemispheres of the human brain, as a treatment for epilepsy. Cutting the connection reduces the severity of epileptic attacks by preventing seizures from spreading from one hemisphere to the other. But there are side-effects.
The effect, in the words of one surgeon, was the creation of ‘two separate spheres of consciousness’. (p 245) (more…)