 A more pressing question is whether some of the forms of necessity discussed in sections  1  and  2  can be defined in terms of the others by relativization or restriction. Consider epistemic and metaphysical necessity first, and suppose for the sake of the argument that dualism is true and the two properties are indeed different forms of necessity. Can one of them be defined in terms of the other by one of the aforementioned methods? Not if there are both necessary a posteriori and contingent a priori propositions, since relativization and restriction only allow us to define one property in terms of another if the extension of one is a subclass of that of the other. However, the existence of contingent a priori truths is more controversial than that of necessary a posteriori propositions, and someone trying to define epistemic necessity in terms of metaphysical necessity or vice versa may repudiate the contingent a priori and hold that the extension of epistemic necessity is included in that of metaphysical necessity. Then such a philosopher could try (a) to define metaphysical necessity from epistemic necessity by relativization to some suitable class, or (b) to define epistemic necessity from metaphysical necessity by restriction.