 Such a definition may get the extension of the definiendum right. But a definition may be intended to do much more than that: it may be meant to tell us what it \(is\) for something to fall under the concept to be defined. Suppose that someone tried to define the property of being an equiangular triangle as that of being a triangle whose sides are of equal length. While this is extensionally correct, it does not give us the right account of what it is for something to be an equiangular triangle (what it is for something to have that property has something to do with the sizes of its angles, not with the lengths of its sides). It could be argued that definitions of type (a) and (b) face similar difficulties. For example, a definition of kind (a) entails that a proposition’s being metaphysically necessary consists in its being epistemically necessitated by a certain class of propositions. But that would make metaphysical necessity an epistemic property, and dualists typically want to resist that idea. Similarly for definitions of type (b). Whether something is epistemically necessary (in the sense of being a priori) seems to be a purely epistemic matter. A priori propositions may also be metaphysically necessary, but their metaphysical necessity isn’t part of what makes them a priori, and therefore shouldn’t be mentioned in a definition of a prioricity.