Date of Degree


Document Type


Degree Name





Graham Priest

Committee Members

Michael Devitt

Barry Loewer

David Papineau

Subject Categories

Metaphysics | Philosophy of Science


many worlds, identity, counterparts, quantum physics, mwi


The Many Worlds Interpretation of quantum mechanics - arguably our most powerfully predictive scientific theory to date - describes a branching Universe composed of an infinite number of quasi-classical macroscopic physical worlds. Though elegant in its straightforward rendering of the mechanics, the Many Worlds Interpretation presents a challenge for understanding identity over time. If we wish to preserve the notion of strict numerical identity, we are faced with the choice between: denying the transitivity of identity; very short-lived lives with near constant death; or accepting that the world is filled with many more individuals than we previously dreamed. In adopting a perdurantist account of identity over time, I argue for this last option. But questions remain about the relationship that branching individuals have to those from whom they’ve split. In this dissertation, I develop a novel account that I call Many Worlds Counterparts. This theory takes its inspiration from Lewis’s Counterpart Theory in offering a modal analysis of de re possibility, but avoids the major challenges that Lewis’s theory faces.