Pure mathematics and pure natural science would not require fortheirownsecurityand certainty a deduction such as we have just concluded with respect to them both; for the former rests upon its own evidence, while the latter, although arising from the pure sources of the understanding, is dependent upon the complete substantiation of experience, a witness it is unable altogether to repudiate and do without, seeing that with all its certainty, as philosophy, it can never compete with mathematics. Both these sciences required the foregoing investigation, not for their own sake, but for the sake of another science, namely, metaphysics.
Metaphysics is concerned not merely with natural conceptions, having invariably an application in experience, but, in addition to these, with pure conceptions of Reason, which can never be given in any possible experience; that is, with conceptions whose objective reality (as distinguished from simple cobwebs of the brain), and with assumptions whose truth or falsity can be confirmed or discovered by no experience. This part of metaphysics is precisely that which constitutes its essential purpose, all else being merely a means thereto, and hence this science requires such a deduction for its own sake. The third problem, now before us, concerns, as it were, the essence and speciality of metaphysics, namely, the occupation of Reason with itself alone, inasmuch as it broods over its own conceptions and the knowledge of objects supposed to arise immediately from them, without having need of the mediation of experience, or indeed without the possibility of being able to attain thereto by its means. Without a satisfactory solution of this problem, Reason can never be just to itself. The empirical use to which Reason limits the understanding, does not exhaust its own function. Each special experience is but a portion of the whole sphere of its domain. But the absolute totality of all possible experience, though in itself no experience, constitutes nevertheless for Reason a necessary problem, to the mere presentation of which it demands quite different conceptions from the pure conceptions of the understanding, the use of which is only immanent, i.e., referable to experience, so far as it can be given; whereas the conceptions of Reason extend to the completeness, i.e., the collective unity of all possible experience, thereby passing beyond any given expe- rience and becoming transcendent.
As, then, the understanding required the Categories for experience, so Reason contains in itself the ground of Ideas, by which I understand necessary conceptions the subject of which cannot be given in any experience. The latter are as inherent in the nature of Reason as the former in the nature of the Understanding, and if they carry with them an illu- sion that may easily mislead, this illusion is unavoidable, although we may very well guard ourselves from being misled by it.
As all illusion consists in the subjective ground of judgment being taken for objective, the self-knowledge of pure Reason, in its transcendent (exaggerated) use, is the only preservative against the aberrations into which Reason falls when it misapplies its function, and refers its transcendent character, concerning only its own subject and its direction in all immanent uses, to the object itself.