If the notion of ontological commitment is to play a role in theory choice or clarifying ontological disputes, then the ontological commitments of a theory must be rationally accessible; failure to acknowledge the ontological commitments of one's theory must be a rational failing (see Michael 2008). That puts the following epistemic constraint on the entailment relation that is invoked by an entailment account of ontological commitment: the entailments of a theory must be knowable by any ideally rational agent, that is to say, must be knowable a priori. Does modal entailment meet this constraint? With respect to the examples considered thus far—theories positing necessary existents—it may depend on how “ideally rational” and “a priori” are understood; some philosophers would argue that entities that necessarily exist can be known a priori to exist. But if (following Kripke 1980) we accept that there are necessary propositions that cannot be known a priori, then modal entailment will run afoul of the epistemic constraint. Suppose, for example, that ‘Water is H2O’ is (metaphysically) necessary but not knowable a priori. Then the theory containing the single sentence, ‘Water exists’, modally entails that hydrogen exists. But, according to the epistemic constraint, the existence of hydrogen should not be an entailment on an entailment account of ontological commitment. It appears that some sort of a priori entailment relation must take the place of modal entailment in the analysis of ontological commitment: