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).