 The problem is recognizably the familiar one of substitutivity in referentially opaque contexts, such as propositional attitudes and modal  contexts.[52]  Talk of good reasons being ones where the debater has a desire to know P but not an equivalent Q is indeed an opaque context. To analyse what goes wrong in the tempting inference, Dharmakīrti, in effect, made a recognizable move by distinguishing between types of identities: “being impermanent” and “being produced” are extensionally identical, but somehow not intensionally so—what he terms being the same concept, i.e., exclusion (apoha; vyāvṛtti). And in the opaque context substitution should only be made between conceptually identical terms.