Thus, there is an epistemological relation between propositions, “casts doubt on”, which holds between CC and C1 but not between CC (=BC) and B1. Or, turning this around once again, there is an epistemological relation—presumably some sort of a priori entailment relation—according to which modal realism entails that non-actual possible worlds exist, but materialism does not entail that non-actual possible worlds exist. Jackson says nothing to further characterize this relation, let alone provide a semantic analysis. But, however the relation is precisely characterized, the natural question to put to Jackson is: why not use that relation, in place of modal entailment, to characterize ontological commitment? For, clearly, this “epistemological” entailment relation better tracks our judgments as to the ontological commitments of theories; and it is the “epistemological” relation, not modal entailment, that will have to be brought to bear when comparing the ontological costs of theories positing necessary existents.