"Dogmatic" metaphysics pursues the kinds of questions outlined in the Third Part as prompted by ideas of reason. These questions ask about the nature of the soul, the possibility of freedom, the ultimate constituents of matter, the existence of God, and so on. Metaphysics relies entirely on the faculty of reason, and Kant tries to show us that reason cannot get us any closer to answering these questions. The faculty of reason cannot connect with anything outside the mind, and certainly not with things in themselves.

If we recall, a science is a body of synthetic a priori knowledge. That is, it is a field of study that makes interesting, non- analytic judgments, but does so without any reference to experience. In order to make synthetic judgments without reference to experience, our mental faculties must be able to make significant connections within their own pure concepts. Our faculty of sensibility can use its pure intuitions of space and time to make mathematics and geometry. Our faculty of understanding can use its pure concepts to make natural science possible. Our faculty of reason has ideas, so the pressing question is what kind of synthetic judgments can these ideas produce?

We have seen that the ideas of reason pose all sorts of metaphysical questions that reason cannot answer. We have also seen that in doing so, reason pushes itself to the bounds of human knowledge, giving a sense of completeness and unity as to what we can know. Reason, then, has a sense of what kind of knowledge is possible, and so is ideally suited to examining the different mental faculties and determining precisely how knowledge is structured. The Prolegomena itself has essentially employed this technique: throughout, Kant has been investigating the different kinds of knowledge we have and the grounds on which this knowledge is justified. His conclusions that there are three mental faculties (sensibility, understanding, and reason), that the faculty of sensibility contains pure intuitions of time and space, or that the faculty of understanding is structured according to the concepts listed in his table of categories, are all conclusions reached through a critical investigation of the structure of knowledge.

While "dogmatic" metaphysics asks what we can know, Kant's critical metaphysics asks how we can know. A "critique" is an investigation that looks inward rather than outward, that investigates knowledge itself rather than the objects of knowledge. The Prolegomena is a shortened version of Kant's great work, the Critique of Pure Reason, which is an attempt to investigate how and what our faculty of reason is capable of.

Kant is not doing psychology. He is not trying to figure out how the mind works or anything like that. Rather, he is trying to figure out how knowledge works, and any claims he makes about the workings of the mind are based on his conclusions as to how knowledge must be structured in the mind.

One of the most significant conclusions of Kant's critical philosophy is that many concepts we think of as objective—like space, time, or causation—are in fact part of the way we structure knowledge. These concepts, as Kant shows in the Third Part, are often the source of perplexing metaphysical conundrums. In showing that these concepts are not to be found in the world, but rather in our own faculties, Kant is essentially redirecting metaphysics. He is telling us that we should not apply metaphysical concepts to the world but to our own faculties. All metaphysics can do for us is tell us how we know what we know. It cannot tell us what we cannot know.

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