According to Kant, justifying the application of mathematics to objects in natural science requires metaphysically constructing the concept of matter. Kant develops these constructions in the Metaphysical Foundations of Natural Science (MAdN). Kant’s specific aim is to develop a dynamic theory of matter to replace corpuscular theory. In his Preface Kant claims completely to exhaust the metaphysical doctrine of body, but in the General Remark to MAdN ch. 2, “Dynamics,” Kant admits that once matter is reconceived as basic forces, it (...) is no longer possible to construct the concept of matter. I argue that Kant’s admission is only the tip of the problem, and that none of Kant’s commentators has fully grasped the problems infecting the MAdN that underlie Kant’s admission. I show that Kant’s proof that matter consists of forces is fallacious. I then re-analyze the circularity in Kant’s definition of density, criticizing both Adickes’ formulations and his later dissolution of it. I also show that a third circularity infects the relations between Kant’s treatment of “Dynamics” and “Mechanics” (MAdN ch. 3). These three fundamental problems demonstrate the untenability of Kant’s metaphysical method, and they require the radical revision of the relation between mathematics and metaphysics Kant undertakes in his opus postumum. I show that some of Kant’s most surprising and critical later claims about the Critical philosophy are correct, and that they require the sorts of remedies Kant contemplates in the opus postumum. (I defend the essentially correct analyses offered by Burkhard Tuschling and Eckart Forster against criticisms by Michael Friedman.). (shrink)