HKB
be considered an extension of our knowledge in a practical point of view, without contradicting the admission that for speculative purposes reason has not in the least gained by this.
Every employment of reason in respect of an object requires pure concepts of the understanding (categories), without which no object can be conceived. These can be applied to the theoretical employment of reason, i.e., to that kind of knowledge, only in case an intuition (which is always sensible) is taken as a basis, and therefore merely in order to conceive by means of- them an object of possible experience. Now here what have to be thought by means of the categories in order to be known are ideas of reason, which cannot be given in any experience. Only we are not here concerned with the theoretical knowledge of the objects of these ideas, but only with this, whether they have objects at all. This reality is supplied by pure practical reason, and theoretical reason has nothing further to do in this but to think those objects by means of categories. This, as we have elsewhere clearly shown, can be done well enough without needing any intuition (either sensible or supersensible) because the categories have their seat and origin in the pure understanding, simply as the faculty of thought, before and independently of any intuition, and they always only signify an object in general, no matter in what way it may be given to us. Now when the categories are to be applied to these ideas, it is not possible to give them any object in intuition; but that such an object actually exists, and consequently that the category as a mere form of thought is here not empty but has significance, this is sufficiently assured them by an object which practical reason presents beyond doubt in the concept of the summum bonum, the reality of the conceptions which are required for the possibility of the summum bonum; without, however, effecting by this accession the least extension of our knowledge on theoretical principles.