As we have seen, the two fundamental problems of the theory of knowledge singled out by Popper in 1933 are the problem of induction-that is, “The question whether inductive inferences are justifi ed, or under what conditions”1-and the problem of demarcation-that is, “the problem of fi nding a criterion which would enable us to distinguish between the empirical sciences on the one hand, and mathematics and logic as well as ‘metaphysical’ systems on the other.”2 Following Kant, Popper called the former “Hume’s problem” and he thought it appropriate to refer to the latter as “Kant’s problem.”3