The non-branching approach On the most popular revision of the psychological-continuity view in light of this problem, what is necessary and sufficient for personal identity over time is
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Righty are identical to each other But they aren’t: they are two, not one.6 Conse-quently, psychological continuity cannot be sufficient – and hence not necessary and sufficient – for personal identity over time
5 The non-branching approach
On the most popular revision of the psychological-continuity view in light of this problem, what is necessary and sufficient for personal identity over time is
psychological continuity that has not taken a branching form (Shoemaker 1984, 85; cf Parfit 1984, 263) Thus a person existing at t is identical to an individual existing at a future time t* only if the latter individual is the only one who is at t* psychologically continuous with the former as he is at t.7 This clause is not satis-fied in the fission case: upon fission, no one is uniquely psychologically continu-ous with Henry as he is before fission So, on this non-branching version of the psychological-continuity view, Henry is identical with neither Lefty nor Righty (nor with anyone else existing after fission)
One objection to the non-branching approach is that, precisely because it implies that fission is the end of Henry, it makes fission as bad as ordinary death This, many think, is not a sensible result: ordinary death seems far worse than fis-sion Whereas it seems prudentially reasonable for Henry to make great sacrifices
in order to prevent his own death, many believe that it would be prudentially irrational for him to do so in order to prevent fission – for instance, by seeing to it that one of his hemispheres is destroyed
Another objection is that the non-branching approach violates an attractive
“intrinsicality” requirement My identity with an individual seems to be wholly
a matter between him and me; after all, numerical identity is arguably the most intimate relation of all Thus while, for instance, my being the only one who is equally tall as a certain individual does not solely depend on the intrinsic features
of the relation between us, my being identical with a future individual seems to do
so (Noonan 1985) On the non-branching approach, however, it depends in part on
extrinsic features, namely, on whether there are future individuals other than him
who are psychologically continuous with me
In Parfit’s view, however, such objections draw the wrong lessons from the fission case (1984, 261–270) They derive their force from the assumption that identity is what prudentially matters in survival, but what the fission case reveals
is precisely that this assumption is mistaken After all, Parfit suggests, we should all agree that if Henry’s left hemisphere had been placed in a new body and the right one had been destroyed, then he would survive as the person ending up with his left hemisphere – call him “Only” – and his relation to Only would contain what matters.8 Because Henry’s relation to Lefty is intrinsically just like his rela-tion to Only, it too must contain what matters The fact that, in the fission case, Henry also bears this kind of relation to yet another future person – Righty – does
not mean that anything that matters is missing from his relation to Lefty Indeed,
since the parallel claims can be made about Henry’s relation to Righty, fission is