  Unfortunately, these points of clarification do not resolve all of the issues that are immediately raised by Kant's pronouncements about what is required for natural science proper. One further issue that is relevant here concerns the concept of matter that is at the heart of the Metaphysical Foundations. Kant introduces it in the Critique of Pure Reason (A847–848/B875–876) as the concept of something that is impenetrable, extended, and inert. Yet, in the beginning of the Preface of the Metaphysical Foundations, he describes it as whatever is an object of outer sense, and later he argues that the “basic determination of something that is to be an object of the outer senses had to be motion, because only thereby can these senses be affected” (4:476). Whatever weight one accords Kant's justification of the connection between matter, outer sense, and motion, one faces a dilemma. If the concept of matter, most fundamentally, is simply the concept of any object of outer sense, then how is it still empirical in any genuine sense (and what has become of the structural difference Kant draws between the Critique of Pure Reason and the Metaphysical Foundations)? If, by contrast, impenetrability, extension, and movability are deemed the basic traits of the concept of matter, then how can one know a priori that any object we might encounter in outer sense must behave in accordance with the laws that would govern matter so defined?