- Life is dangerous but, as the lore of science fiction has it, so is survival. A moderately malfunctioning teletransporter may create a physical and psychological duplicate of you on Mars while failing to destroy the original on Earth. In this way you have apparently succeeded in surviving twice at once, and thus should be identical with either survivor. But since you cannot be identical with two survivors, it seems that we should say that neither of them qualifies as your survivor.
- Such fictional scenarios of fission or reduplication by teletransportation, or cloning, or brain transplantation1 are well-known gambits in the current debate about personal identity. With technological fancy and surgical graphicness varying to taste, these illustrations commonly serve to unhinge accounts of personal identity based on physical or psychological continuity2. If two persons, Y and Z, can be shown to be physically or psychologically continuous with person X, such continuity cannot be what personal identity consists in.
- This common use, however, is not the only one that has been made of fission or reduplication scenarios. In his discussion of physical and psychological "branch line cases" Derek Parfit urges a rather different and, I believe, systematically much more momentous conclusion. If it is conceivable that the graph representing the continuation of a person's physical or mental existence is not linear, then, Parfit’s argues, the relation of personal identity is not necessarily coextensive with the relation of survival. “Branch line cases“ thus entail according to Parfit a conceptual distinction between the notion of identity and the notion of survival.
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