Abstract

Parfit’s Branch Line argument is intended to show that the relation of survival is possibly a one-many relation and thus different from numerical identity. I offer a detailed reconstruction of Parfit’s notions of survival and personal identity, and show the argument cannot be coherently formulated within Parfit’s own setting. More specifically, I argue that Parfit’s own specifications imply that the “R-relation”, i.e., the relation claimed to capture of “what matters in survival,” turns out to hold not only along but also across the branches representing the development of a reduplicated person. This curious fact of ‘interbranch survival,’ as I call it, has gone unnoticed so far. The fact that the R-relation also holds across branches creates a trilemma for Parfit’s approach. Either the envisaged notion of personal identity is circular, or the R-relation fails as a reconstruction of the common sense notion of survival, or talk about persons ‘branching’ (being reduplicated etc.) remains semantically empty. In the paper’s last section I suggest that my criticism does not detract from the larger systematic significance of Parfit’s argument. The argument is simply terminologically miscalibrated. Even though Parfit’s branch line argument fails to establish the conceptual separability of survival and identity, it can be used to show the separability of sameness and numerical identity, which should have similar implications for meta-ethics as the original argument.