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.