 The unity argument says that grounding is a justified posit because of its unifying role (Audi 2012a; Rosen 2010; Schaffer 2009). On one way of casting the argument, it has three premises. First, there is substantial unity among the metaphysical relations (relations familiar from metaphysics such as functional realization and set formation that resist analysis in purely modal terms). Second, where there is substantial unity, plausibly there is a unifier, something that explains why the relations in question resemble one another in the manner they do. Third, if there is a unifier for the metaphysical relations, the best candidate is grounding.