 Why have some thought that a proposition that is a priori justified cannot be defeated by empirical evidence? Kant said that a priori knowledge is “knowledge that is absolutely independent of all experience” (Kant 1787 [1965: 43(B3)]). But it might be that the requirement that a priori knowledge be absolutely independent of all experience is too stringent. Enabling experiences may be required. It avoids this difficulty to hold instead that a priori knowledge and justification are independent of all experience beyond what is needed to grasp the relevant concepts involved in the relevant proposition (see, below,  sec. 4.1).  The fact that a priori knowledge is not independent of all experience does not show that it is empirically defeasible, but it does defeat the argument that it is not empirically defeasible because it is independent of all experience.