will, regarded as the faculty of desire, is (in this view) one of the many natural causes in the world, viz. that cause which acts in accordance with concepts. All that is represented as possible (or necessary) by means of a will is called practically possible (or necessary); as distinguished from the physical possibility or necessity of an effect, whose cause is not determined to causality by concepts (but in lifeless matter by mechanism and in animals by instinct). Here, in respect of the practical, it is left undetermined whether the concept which gives the rule to the causality of the will, is a natural concept or a concept of freedom.
But the last distinction is essential. For if the concept which determines the causality is a natural concept, then the principles are technically practical; whereas, if it is a concept of freedom they are morally practical. And as the division of a rational science depends on the distinction between objects whose cognition needs distinct principles, the former will belong to theoretical Philosophy (doctrine of Nature), but the latter alone will constitute the second part, viz. practical Philosophy (doctrine of Morals).
All technically practical rules (i.e. the rules of art and skill generally, or of prudence regarded as skill in exercising an influence over men and their wills), so far as their principles rest on concepts, must be reckoned only as corollaries to theoretical Philosophy. For they concern only the possibility of things according to natural concepts, to which belong not only the means which are to be met with in nature, but also the will itself (as a faculty of desire and consequently a natural faculty), so far as it can be determined conformably to these rules by natural motives. However, practical rules of this kind are not called laws (like physical laws), but only precepts; because the will does not stand merely under the natural concept, but also under the concept of freedom, in relation to which its principles are called laws. These with their consequences alone constitute the second or practical part of Philosophy.
The solution of the problems of pure geometry does not belong to a particular part of the science; mensuration does not deserve the name of practical, in contrast to pure, geometry, as a second part of geometry in general; and just as little ought the mechanical or chemical art of experiment or observation to be reckoned as a practical part of the doctrine of Nature. Just as little, in fine, ought housekeeping, farming, statesmanship, the art of conversation, the prescribing of diet, the universal doctrine of happiness itself, or the curbing of the inclinations and checking of the affections for the sake of happiness, to be reckoned as practical Philosophy, or taken to constitute the second part of Philosophy in general. For all these contain only rules of skill (and are consequently only technically practical) for bringing about an effect that is possible according to the natural concepts of causes and effects, which, since they belong to theoretical Philosophy, are subject to those precepts as mere corollaries from it (viz. natural science), and can therefore claim no place in a special Philosophy called practical. On the other hand, the morally practical precepts, which are altogether based on the concept of freedom to the complete exclusion of the natural determining grounds of the will, constitute a quite special class. These, like the rules which nature obeys, are called simply laws, but they do not, like them, rest on sensuous conditions but on a supersensible principle; and accordingly they require for themselves a quite different part of Philosophy, called practical, corresponding to its theoretical part.
We hence see that a complex of practical precepts given by Philosophy does not constitute a distinct part of Philosophy, as opposed to the theoretical part, because these precepts are practical; for they might be that, even if their principles were derived altogether from the theoretical cognition of nature (as technically practical rules). [A distinct branch of Philosophy is constituted only] if their principle, as it is not borrowed from the natural concept, which is always sensuously conditioned, rests on the supersensible, which alone makes the concept of freedom cognisable by formal laws. These precepts are then morally practical, i.e. not merely precepts or rules in this or that aspect, but, without any preceding reference to purposes and designs, are laws.
II. OF THE REALM OF PHILOSOPHY IN GENERAL
So far as our concepts have a priori application, so far extends the use of our cognitive faculty according to principles, and with it Philosophy.
But the complex of all objects, to which those concepts are referred, in order to bring about a knowledge of them where it is possible, may be subdivided according to the adequacy or inadequacy of our [cognitive] faculty to this design.
Concepts, so far as they are referred to objects, independently of the possibility or impossibility of the cognition of these objects, have their field which is determined merely according to the relation that their Object has to our cognitive faculty in general. The part of this field in which knowledge is possible for us is a ground or territory (territorium) for these concepts and the requisite cognitive faculty. The part of this territory, where they are legislative, is the realm (ditio) of these concepts and of the corresponding cognitive faculties. Empirical concepts have, therefore, their territory in nature, as the complex of all objects of sense, but no realm, only a dwelling-place (domicilium); for though they are produced in conformity to law they are not legislative, but the rules based on them are empirical and consequently contingent.
Our whole cognitive faculty has two realms, that of natural concepts and that of the concept of freedom; for through both it is legislative a priori. In accordance with this, Philosophy is divided into theoretical and practical. But the territory to which its realm extends and in which its legislation is exercised, is always only the complex of objects of all possible experience, so long as they are taken for nothing more than mere phenomena; for otherwise no legislation of the Understanding in respect of them is conceivable.
Legislation through natural concepts is carried on by means of the Understanding and is theoretical. Legislation through the concept of freedom is carried on by the Reason and is merely practical. It is only in the practical [sphere] that the Reason can be legislative; in respect of theoretical cognition (of nature) it can merely (as acquainted with law by the Understanding) deduce from given laws consequences which always remain within [the limits of] nature. But on the other hand, Reason is not always therefore legislative, where there are practical rules, for they may be only technically practical.
Understanding and Reason exercise, therefore, two distinct legislations in regard to one and the same territory of experience, without prejudice to each other. The concept of freedom as little disturbs the legislation of nature, as the natural concept influences the legislation through the former.—The possibility of at least thinking without contradiction the co-existence of both legislations, and of the corresponding faculties in the same subject, has been shown in the Critique of pure Reason; for it annulled the objections on the other side by exposing the dialectical illusion which they contain.
These two different realms then do not limit each other in their legislation, though they perpetually do so in the world of sense. That they do not constitute one realm, arises from this, that the natural concept represents its objects in intuition, not as things in themselves, but as mere phenomena; the concept of freedom, on the other hand, represents in its Object a thing in itself, but not in intuition. Hence, neither of them can furnish a theoretical knowledge of its Object (or even of the thinking subject) as a thing in itself; this would be the supersensible, the Idea of which we must indeed make the basis of the possibility of all these objects of experience, but which we can never extend or elevate into a cognition.
There is, then, an unbounded but also inaccessible field for our whole cognitive faculty—the field of the supersensible—wherein we find no territory, and, therefore, can have in it, for theoretical cognition, no realm either for concepts of Understanding or Reason. This field we must indeed occupy with Ideas on behalf of the theoretical as
