Peacock (2011) also defends the modal entailment account from the problem of irrelevant necessary existents. In particular, he argues that the ontological costs of theories positing necessary existents can be compared by considering, not the theories' actual commitments (which cannot be known prior to determining the truth of the theories), but by considering “what ontology we would regard a theory as implicitly committed to, were it true” (Peacock 2011: 93). This suggests that there is some counterfactual relation between propositions according to which—to return to Jackson's example—non-actual possible worlds would have to exist if modal realism were true, but it is not the case that non-actual possible worlds would have to exist if materialism were true. This counterfactual relation, of course, could not be given a standard possible worlds analysis lest it apply vacuously to impossible antecedents. But even supposing a semantics for such a counterfactual relation could be given, the natural question to put to Peacock is: why not use that relation, in place of modal entailment, to characterize ontological commitment? For it is this counterfactual relation, not modal entailment, that will have to be brought to bear when comparing the ontological costs of theories positing necessary existents.[26] In sum: neither Jackson nor Peacock give any positive reason to hold on to a modal entailment account, which is puzzling in light of the fact that each is committed to there being an entailment relation—be it epistemological or counterfactual—that better fits the role that the notion of ontological commitment was introduced to play.