Kant's Critique of Reason was a new field purporting to inquire into a priori knowledge. His self-appointed successors invented another field, "epistemology", which they believed to be itself a priori. To convince themselves of this they followed a principle (first made explicit by Kuno Fischer) that a priori knowledge cannot be known a posteriori. This principle is a synthetic judgment which was widely accepted because it was disguised as an analytic judgment. It is interesting that by the same fallacy one can arrive at the opposite conclusion ("empiricist epistemology"). And in legal philosophy we can see that the dispute about the League of Nations has the very same structure.