Depending on how you interpret the question, there is potentially a lot of mileage in this. I want to address the concept of personal identity because I really fail to understand the problem that a lot of philosophers seem to have with it. If there is a problem, it is about accepting the way things are.
What makes a person a person? What makes a person the same person they were a minute ago? Or yesterday? What am I? I think a lot of the problem grows from a tension between general conceptions of identity in space and time conflicting with the idea that people are special in their identity.
Identity outside the realms of maths and logic is a messy affair. Ships get replaced bit by bit. People change their minds and behaviour. Organs are transplanted. Plant grow from seeds and people grow from infancy. Despite these changes, we still identify a person as that same person, simplistic matters of mistaken identity and error aside. Indeed, errors are possible, but in most cases the identity is sound and useful. Otherwise I’d not know if you were my mother, a stranger, work colleague or whatever. Cunning philosophers offer many thought experiments to eat away at this kind of identification, involving switching brains between people, removing parts of the body until the person ‘disappears’ and suchlike.
My answer is simple: Personal identity is a convenient and pragmatic label for a space/time phenomenon (that we call a person). By and large people do not change beyond recognition, or become instantaneously cloned, or have brain switches. As long as the person is pretty similar, we would say they were the same, or just out of sorts perhaps. Certainly we prioritise the wealth of behavioural and related information in most cases, so a personality swap would likely result in us saying the person was the personality. So much information is contained within a person’s brain, like a treasure chest we can only access through limited means, that it is usually deemed the significant majority – as an iceberg below the water. However, the body is also a perfectly valid part of the person, so it is just pragmatic preference for the mind that leads us to identify the person with the personality! If we systematically delete parts of a mind, hypothetically speaking, at what point does it stop being the same person? Well, if the body was still intact, that would probably suffice for the personal identifier even when they were a vegetable, although it would also be valid to say they were a different person, or no longer there at all. Pragmatic preference again, and both approaches would be intelligible in conversation. If the body had been switched, and the mind was systematically erased, the person becomes less and less identifiable just as a pile of sand becomes less of a pile as grains are removed. The person status here becomes arbitrary – maybe when the last part of their mind you remember disappears? At that point, pragmatics kick in – what are you going to do with the body? ‘Is it Julie any more?’ is less important than ‘What do we do with her?’. Basing such decisions on essentially vague personal identity seems harsh, but this is exactly what we have to do.
What these curious thought experiments challenging our notions of personal identity do offer, is an opportunity to consider how we would deal with such cases. It’s not a clear cut matter by any means, but when things happen beyond our language’s scope, we need to adapt. They also helpfully erode immaculate conceptions of ourselves as perfectly discrete and identifiable entities.
If you are still set on yourself as having an essential part, like a soul or your mind, then consider what would happen if you removed parts of it, and if it has no parts to remove, then ask how it can be identified with you at all? Is it hiding in your big toe? As far as I can tell, the only ‘soul’ that I could have that would make sense is the complete summation of every part of me, every thought and experience, everything I have ever done or affected, every atom that was ever part of my body. In short, much of the world would be included too – a holistic and complete ‘oversoul’ rather than a reductionist particle ‘soul’ with no defining qualities hiding somewhere. It’s possible that the entire universe might actually be included given the possibility for some weird physics claims about action/interaction at a distance, making for some wonderful hippy claims that ‘we’re all one with the universe’. Otherwise, my ‘soul’ would be incomplete – just as indeterminate and essentially arbitrary as another label.
How would I tell what thoughts were mine, say? Well indeed -a holistic ‘oversoul’ concept runs into similar indeterminacy as other labels in practice, but the theoretical status is sound. Should it turn out that thoughts can indeed be shared by different people then I’m happy to accept that oversouls can be as well! As for what use the term would be – actually, I think the greatest practical offshoot is the reflective self evaluation that ensues when we try to pin it down in the first place! Accepting the inherent ambiguity and uncertainty of self is one of the fundamentals of the Egophage project.