Thus the main question, How are synthetic judgments a priori possible? divides into the subordinate questions, How is pure mathematics possible?  How is pure natural science possible, and, How is metaphysics (in two senses:  metaphysics in general, and metaphysics as science) possible?  The Transcendental Aesthetic (the critique of sensibility or the faculty of intuition) answers the first of these questions; the Transcendental Analytic (the critique of the understanding), the second; and the Transcendental Dialectic (the critique of “reason” in the narrower sense) and the Transcendental Doctrine of Method (Methodenlehre), the third.  The Analytic and the Dialectic are the two parts of the Transcendental “Logic” (critique of the faculty of thought), which, together with the Aesthetic, forms the Transcendental “Doctrine of Elements” (Elementarlehre), in contrast to the Doctrine of Method.  The Critique of Pure Reason follows this scheme of subordinate division, while the Prolegomena co-ordinates all four parts in the manner first mentioned.

Let us anticipate the answers.  Pure mathematics is possible, because there are pure or a priori intuitions (space and time), and pure natural science or the metaphysics of phenomena, because there are a priori concepts (categories) and principles of the pure understanding.  Metaphysics as a presumptive science of the suprasensible has been possible in the form of unsuccessful attempts, because there are Ideas or concepts of reason which point beyond experience and look as though knowable objects were given through them; but as real science it is not possible, because the application of the categories is restricted to the limits of experience, while the objects thought through the Ideas cannot be sensuously given, and all assumed knowledge of them becomes involved in irresolvable contradictions (antinomies).  On the other hand, a science is possible and necessary to teach the correct use of the categories, which may be applied to phenomena alone, and of the Ideas, which may be applied only to our knowledge of things (and our volition), and to determine the origin and the limits of our knowledge—­that is to say, a transcendental philosophy.  In regard to metaphysics (knowledge from pure reason), then, this is the conclusion reached:  Rejection of transcendent metaphysics (that which goes beyond experience), recognition and development of immanent metaphysics (that which remains within the limits of possible experience).  It is not possible as a metaphysic of things in themselves; it is possible as a metaphysic of nature (of the totality of phenomena), and as a metaphysic of knowledge (critique of reason).

The interests of the reason are not exhausted, however, by the question, What can we know? but include two further questions, What ought we to do? and, What may we hope?  Thus to the metaphysics of nature there is added a metaphysics of morals, and to the critique of theoretical reason, a critique of practical reason or of the will, together with a critique of religious belief.  For even if a “knowledge” of the suprasensible is denied to us, yet “practical” grounds are not wanting for a sufficiently certain “conviction” concerning God, freedom, and immortality.