Section 4.1: The Self as Substance
According to Soul Theory,
X is the same person as Y if and only if X has the same soul as Y.
Soul Theory, which obviously moves away from physicalistic theories of identity such as animalism, takes us in the direction of the non-physical (at least at first glance). But this is, perhaps, what makes the suggestion at least on the face of it problematic. Clearly, to know when X is the same person as Y, we would need to know when X has the "same soul" as Y? Of course, knowing when one soul is the same as another soul requires a previous bit of knowledge -- knowing what souls are in the first place. How can we know what a soul is, much less know when one soul is the same as another soul?
Leaving this problem behind us, let's move to what the theory seems to be saying. To say that one has a soul, acording to this theory, is to say that the basis for one's memories and feelings and desires -- the whole basis for one's personality -- is made possible by an entity known as the soul. The idea here is that even if one loses this memory or that, or even if one's personality changes, that which underlies these things -- the soul -- remains unchanged and as such provides the basis for saying that it is the same person over time.
Let's think of it this way (as S and V do) -- think of the soul's relationship to thoughts like the relationship between a pincushion and the pins that are stuck in it. Obviously, taking some of the pins out and putting new ones in it does not affect our ability to say that, overall, we have "the same thing" over the changes. This is possible, provided that the identity condition for the "thing" is "same pincushion."
There are intuitions behind Soul Theory, not all of them being religious. We tend to think that a person who is a complete victim of amnesia is still the same person as before. They may act differently, we say, but it is still the same person. Since none of the memories, desires, or personality of the old person remains, it seems as if the pincushion or soul is what is providing the criterion of identity.
Other than the general problem of identifying what souls are in the first place, two arguments have been suggested against soul theory. One criticizes it as a necessary condition for personal identity, the other attacks it as not a sufficient condition for personal identity.
Leibniz's King of China TE: here Leibniz asks if we would be willing to have our souls switched into the body of Bill Gates (to update the example) if it meant that all of the "pins" were switched (so Bill Gates' pins are put into your pincushion and your pins are put into his pincushion). By Soul Theory, you would become very rich, even though you would have all of the memories and desires of Bill Gates. But Leibniz thinks that no one would agree to this, proving that no one finds it intuitive that "same soul" is a sufficient condition for personal identity.
Locke's Nestor TE: here we get the reverse problem. In this case, Locke asks what we would think if our pins were removed from our souls and placed into another person's soul and body. If so, then your memories and desires would be present in another person. According to Locke, our intuitions here are that we have moved our position to another body, and so it seems that our intuitions suggest that souls are not even necessary conditions for personal identity.