Critique of Judgment

We proceed quite correctly if, as usual, we divide Philosophy, as containing the principles of the rational cognition of things by means of concepts (not merely, as logic does, principles of the form of thought in general without distinction of Objects), into theoretical and practical. But then the concepts, which furnish their Object to the principles of this rational cognition, must be specifically distinct; otherwise they would not justify a division, which always presupposes a contrast between the principles of the rational cognition belonging to the different parts of a science.

Now there are only two kinds of concepts, and these admit as many distinct principles of the possibility of their objects, viz. natural concepts and the concept of freedom. The former render possible theoretical cognition according to principles a priori; the latter in respect of this theoretical cognition only supplies in itself a negative principle (that of mere contrast), but on the other hand it furnishes fundamental propositions which extend the sphere of the determination of the will and are therefore called practical. Thus Philosophy is correctly divided into two parts, quite distinct in their principles; the theoretical part or Natural Philosophy, and the practical part or Moral Philosophy (for that is the name given to the practical legislation of Reason in accordance with the concept of freedom). But up to the present a gross misuse of these expressions has prevailed, both in the division of the different principles and consequently also of Philosophy itself. For what is practical according to natural concepts has been identified with the practical according to the concept of freedom; and so with the like titles, ‘theoretical’ and ‘practical’ Philosophy, a division has been made, by which in fact nothing has been divided (for both parts might in such case have principles of the same kind).