Derek Parfit is a philosopher who, in Part 3 of his book Reasons and Persons, explores the question of personal identity. He asks what aspect of a person defines their identity. Parfit goes about with thought experiments to examine how he would define a person’s identity to be. Two theories he discusses are the theory of Physical Criterion and the theory of Wide Psychological Criterion. By using two theory desiderata (general traits that strong theories should possess), explanatory power and existing beliefs, it can be shown that the personal identity theory of Wide Psychological Criterion is the most plausible one.
At the beginning of his book, Parfit begins with two thought experiments that make the reader question how they would intuitively define personal identity. One of them is the simple teleporter case, where a person enters a teleporter, the teleporter scans the person’s exact molecular structure, destroying his body in the process, and sends this data to another machine elsewhere where the person’s exact molecular structure is recreated (Parfit, 1984). The other is called the branch line case, where the person’s body is scanned in the teleporter, but his body is not destroyed in the process. A duplicate of himself is recreated elsewhere using the data scanned from the teleporter, creating a situation where two individuals exist simultaneously that seemingly share the same identity (Parfit, 1984).
One of the theories that Parfit suggests to define identity in these thought experiments is the Physical Criterion, where a person is considered to be numerically identical if and only if enough of their living brain from a prior point in time continues to exist to be the brain of the person now (Parfit, 1984) (numerical id...
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...sed on their behavior tell us the contrary, and leans towards the Wide Psychological Criterion. Because of this, we would regard and identify them differently. Thus, our prior belief favors the Wide Psychological Criterion over the Physical Criterion.
The Wide Psychological Criterion is clearly the favored theory in describing and defining personal identity. The way that we would interact with individuals is based on their personality and psychological state. The physical state of the brain is neither as significant nor as tangible as one’s personality and psychological state. In addition, in certain scenarios the Physical Criterion fails to cohere with our prior beliefs on identifying persons based on our intuitions, whereas the Wide Psychological Criterion coheres well. This coherence is strong evidence supporting the validity of the Wide Psychological Criterion.