We begin by interpreting the ‘must’ as metaphysical necessity, and interpreting metaphysical necessity as quantification over all metaphysically possible worlds. According to the simplest modal account, a theory is (perhaps implicitly) committed to Ks whenever the existence of Ks is necessarily implied by the theory. This account isn't even Quinean in spirit—it makes no mention of the values of the variables—but it serves as a useful foil to Quinean accounts, and will come in for later discussion (§2.2). It is assumed that each possible world has an associated domain of entities: roughly, all the entities that exist in the world. (These entities, however, need not all be in the range of the first-order quantifiers.) Moreover, it is assumed that when sentences of first-order predicate logic are evaluated relative to a possible world, the sentences' quantifiers range only over entities in the domain of the world in question. This leads to what will be called a modal entailment criterion (other entailment criteria make use of entailment relations other than metaphysical necessitation): 