 It is one thing to hold that a priori knowledge requires enabling empirical experience to acquire the concepts that are the basis of that sort of knowledge, and quite another thing to hold that those concepts must be grounded in such experience. The latter rules out some seemingly obvious kinds of a priori knowledge (viz., some mathematical and normative knowledge, and knowledge of certain conditional propositions which seem merely to be about the relationship of concepts), and so seems too strong. It also seems to allow in a priori knowledge of the existence of, say, foxes and bachelors, and so seems too weak.