 Counterpart theory was originally introduced in Lewis (1968) as a formal theory about possible worlds and their inhabitants, but the label quickly came to be used for a loosely defined approach towards trans-world identity and the interpretation of modal or temporal discourse. For the modal case, the central idea is that a de re modal claim like ‘Joe Biden might be immaterial’ is understood as saying that at some possible world, someone who sufficiently resembles Joe Biden in certain respects—a counterpart of Biden—is immaterial. Accordingly, ‘Biden is necessarily material’ is understood as saying that all of Biden’s counterparts at all worlds are material.