I give a brief overview of Albert Casullo’s Essays on A Priori Knowledge and Justification, followed by a summary of his diagnostic framework for evaluating accounts of a priori knowledge and a priori justification. I then discuss Casullo’s strategy for countering deficiency arguments against empiricism. A deficiency argument against empiricism can be countered by mounting a parallel argument against moderate rationalism that shows moderate rationalism to be defective in a similar way. I argue that a particular deficiency argument put forth (...) by George Bealer in “The Incoherence of Empiricism” can withstand a parallel challenge mounted by Casullo.I then consider Casullo’s preferred analysis of the concept of a priori justification, which identifies a belief’s being justified by some nonexperiential source as the feature by virtue of which it is justified a priori. On the analysis, an apriori-justfied belief that is justified to a degree that is sufficient for knowledge is not taken to be empirically indefeasible. I argue that Casullo could avail himself of an empirical indefeasibility requirement that is consistent with his minimal and fallibilist conception of a priori knowledge. Doing so would capture a feature of the concept of a priori knowledge that is of particular interest and significance. (shrink)