Personal Identity and What Matters

Identity is not what prudentially matters in survival
(this is originally and most famously argued in Parfit 1984,
245-280). Consider the case of division. a is one of three
identical triplets. In World 1 his equipollent cerebral
hemispheres are removed from his head and each is
inserted into the (suitably emptied) skull of one of his
brothers, resulting in the existence of two persons, b and c.
In World 2, only one of his hemispheres is transplanted
(while the other is destroyed), resulting in the existence of
b*. Note that, assuming the necessity and transitivity of
identity, b* is identical to neither b nor c.