 The two main options among modal realists (philosophers who accept possible worlds in their ontology) are David Lewis’s extreme or genuine modal realism and ersatzism (or actualism or abstractionism: these terms all have slightly different connotations, which we’ll ignore here). It is a common thought among impossible worlds theorists that impossible worlds should just inherit the ontological status of their possible mates: whatever your favorite metaphysics of possible worlds is, impossible worlds are of the same kind. This has been called the Parity Thesis (see Rescher &amp; Brandom 1980). As Graham Priest puts it: