 Casullo recommends a different approach to defending rationalism. He thinks rationalists should start from common ground and that they should “enlist empirical support for the existence of a priori knowledge” (2012a: 248–249). This may be because he thinks that nonexperiential mental states are the basis of a priori justification and “nonexperiential mental state” is a natural kind term. This seems plausible because Casullo thinks that “experience” is a natural kind term (see, below,  sec. 6.4).  He may think that the reference of all natural kind terms must be discovered empirically and so think that what he takes as the basis of a priori justification must be discovered empirically. Suppose the nonexperiential mental states that Casullo thinks are the bases of a priori justification are what other philosophers call “intuitions”. If a priori knowledge rests partly on a priori justification, and that rests on “intuitions”, that could explain why Casullo recommends empirical inquiry as a means to discover what intuitions are like as a first step in explaining how they can provide a priori justification.