Wittgenstein said that “there are no true a priori propositions” (Lectures, 1930–32, p. 13; Tractatus, 2.225), Carnap tirelessly denied the synthetic a priori, and Schlick went so far as to define empiricism as the rejection of synthetic a priori knowledge. In spite of all this there can be no doubt that the major contribution of Wittgenstein's and Carnap's epistemologies in the early 1930s was their interpretations of all a priori knowledge, both analytic and synthetic. Their theories of philosophical grammar and of logical syntax may well be regarded as the first genuine alternatives to Kant's conception of the a priori.

By the beginning of the twentieth century, no philosopher of consequence was satisfied with Kant's solution to the problem of the a priori. Many had come to understand far better than Kant what was involved in particular instances of a priori knowledge; but efforts to build general accounts of what that form of knowledge was and the way it was grounded were far less successful. In Part I we examined some of the alternatives to Kant's theory that had been put forth by Kantians or anti-Kantians.