What can we learn about personal identity from cases in which one person is split into two people?
1. Intro - we know that a person can survive (and retain the same personal identity) with only half their brain intact. The 'cases' the question refers to, then, are cases in which it is hypothesised that the brain is divided, as in corpus callosotomy procedures, and these two halves subsequently transplanted into two new, distinct bodies. Assuming the hypothesis is sound, what light does this shed on identity? The fission problem.
2. Fission - if P1's left hemisphere is taken and placed in new body B1, and P1's right hemisphere is implanted in B2, if both transplants are successful, who is P1?
The fact that neither B1 nor B2 seems to have exclusive claim to be P1 suggests the possibility that you could be psychologically continuous with more than one person, and if an account of personal identity leaves this option open, it is of no use, since any account that leaves open the possibility that one person might become two has failed to account for identity at all. This is a problem for both memory accounts and psychological continuity accounts of personal identity. What, then, can we suggest has happened in these split-brain operations?
1. multiple-occupancy - if there is to be a fission at some point in your future, then you are already two people, occupying the same space. Persons are made up of temporal parts just as they are made up of physical parts, and pre-fission the two persons' temporal parts coincided physically in one body, sharing the same thoughts and actions, but diverge at the point of fission, and thus their subsequent temporal parts are located separately. This argument relies upon a particular ontology of temporal parts, and reinforces the apparent truth that the question of personal identity is unlikely to be resolved until underlying metaphysical issues are themselves conclusively resolved.
2. non-branching psychological continuity - psychological continuity alone is not sufficient for identity: you are identical with a past or future being only if that being is psychologically continuous with you and no other being is. Thus if your brain is divided and both hemispheres transplanted, you expire at the moment of operation, and two new beings are created. This approach has the odd consequence that if one of your divided hemispheres is destroyed, you survive, but if both survive, you do not: one can survive hemispherectomy only if one hemisphere is destroyed! Equally, under this view, if brain-state transfer is a legitimate example of psychological continuity, then unless your original brain was erased of its content at the moment of transfer of your brain state to a new host, the 'real you' would also cease to exist, since two of 'you' would be in existence. So this view appears to suggest that in some cases humans would intuitively prefer to cease to exist, rather than continue (since intuitively, if asked, we would presumably ask that as much of our brain continues functioning and is preserved in whatever state or location). Parfit suggests this is not so unusual as it sounds - humans, insofar as they are rational, do not value continued existence over all else. What we value is the existence of someone in the future who is psychologically continuous with me, which in this case is possible even if it effectively means death for 'me'. Whilst the usual way to achieve this psychological continuity into the future is through continued existence, fission is simply an example of when that is not the case. This seems to cohere with Parfit's assertion that psychological continuity is more important practically than numerical identity. This answer to the problem of fission, whilst it might demonstrate that practically, psychological continuity is of greater importance than numerical identity, appears to concede that psychological continuity does not in fact entail persistence of identity at all. Parfit's argument appears to concede that the psychologically connected future selves about whom you care above the continuation of your own unique numerical identity are absolutely not you, so has this really answered the persistence question?
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