  Block and Fodor’s apparent move from the “conceptual coherence” of the inversion scenario to its possibility can be questioned. Even though one cannot know a priori that not-p (that is, p is conceptually coherent), p may yet be impossible. For example, one cannot know a priori that gold does not have the atomic number 79, but it is a necessary truth that gold has this atomic number. (This sort of separation between conceptual coherence and possibility is not uncontroversial, but is commonly  accepted.[32])