Returning now to the problem of a priori knowledge, which we left unsolved when we began the consideration of universals, we find ourselves in a position to deal with it in a much more satisfactory manner than was possible before. Let us revert to the proposition “two and two are four.” It is fairly obvious, in view of what has been said, that this proposition states a relation between the universal “two” and the universal “four.” This suggests a proposition which we shall now endeavour to establish: namely, “All a priori knowledge deals exclusively with the relations of universals.” This proposition is of great importance, and goes a long way towards solving our previous difficulties concerning a priori knowledge.