 Another motivation for believing that conceptual issues are crucial for reduction stems from the idea that the identity of properties or kinds expressed by two predicates P1 and P2 manifests in the fact that there is an a priori route from P1 to P2. This interpretation is implicit in the received view attacked early on by Smart (1959). It also plays a role in Levine 1993, 1998 and Chalmers 1996; the idea is that for reductions to be explanatory, they must not leave out any aspect of the phenomenon to be explained. If qualia-concepts or concepts of conscious states pick out mental states in terms of features that cannot be fully explained in terms of the vocabulary of the reducing science, the explanation and hence the reduction is not successful. Even if our mental terms and neural terms refer to the same states, if the former pick them out using mental concepts that cannot be neurally explained, we will not have reduced the mental to the neural. There will be a residual explanatory gap (Levine 1993, 1998).