  What is the intended interpretation of ‘imagine’ in P1 and P2? (In the literature ‘conceive’ is used interchangeably.) The usual answer (Levine 2001, Ch. 2) is this: \(p\) is imaginable iff not-\(p\) is not a priori. On this interpretation, P1 amounts to this: it is a priori that if the explanans holds, so does the explanandum; that is, that the explanans a priori entails the explanandum. This can be motivated by appeal to the “deductive-nomological” model of explanation (see Hempel 1965, and the entry on  scientific explanation);  this model is controversial, and in fact Levine rejects it, but he argues that there is a broader motivation: