As we saw in chapter 10, Kant believes the transcendental ideas of reason perform two positive functions: first, the idea of the unconditioned generates regulative principles for scientific explanations; second, the ideal of the ens realissimum provides a basis of moral faith for practical reason. The last part of the Critique sketches an account of both functions. Despite the brevity of his account here, Kant claims that reason is essential to the operations of the understanding. In spelling out this relation, Kant completes his revolutionary theory of the intellect. As we saw earlier in the Analytic, in analyzing concepts as predicates of possible judgments, Kant overturned the traditional view that judging presupposes conceiving. Here he completes the reversal by showing how judgment presupposes the higher-order functions of reason.

The final section of the Critique is the Transcendental Doctrine of Method. Although this contains four chapters, only the first two offer substantive discussions. In chapter I, the Discipline of Pure Reason, Kant contrasts the methods of philosophy and mathematics. The significant aspects here concern his theory of mathematical construction, and his views on definitions, axioms, and demonstrations. In chapter II, the Canon of Pure Reason, Kant outlines the moral theology required by practical reason, sketching his conceptions of the good and the morally ideal world. Here he argues that the moral law requires us to postulate the existence of God and the immortality of the soul.