Immanuel Kant's Critique of Pure Reason — trans. F. Max Müller

In the metaphysical deduction of the categories their a priori origin was proved by their complete accordance with the general logical functions of thought, while in their tra72scendental deduction we established their possibility as knowledge a priori of objects of an intuition in general (§ 20, 21). Now we have to explain the possibility of our knowing a priori, by means of the categories, whatever objects may come before our senses, and this not according to the form of their intuition, but according to the laws of their connection, and of our thus, as it were, prescribing laws to nature, nay, making nature possible. Unless they were adequate to that purpose, we could not understand how everything that may come before our senses must be subject to laws which have their origin a priori in the understanding alone.

Supplement XIV 763

First of all, I observe that by the synthesis of apprehension I understand the connection of the manifold in an empirical intuition, by which perception, that is, empirical consciousness of it (as phenomenal), becomes possible.

We have forms of the external as well as the internal intuition a priori, in our representations of space and time : and to these the synthesis of the apprehension of the manifold in phenomena must always conform, because it can take place according to that form only.

Time and space, however, are represented a priori, not only as forms of sensuous intuition, but as intuitions themselves (containing a manifold), and therefore with the determination of the unity of that manifold in them (see transcendental .Esthetic ^).

Therefore unity of the synthesis of the manifold without or within us, and consequently a connection to which everything that is to be represented as determined in space and time must conform, is given a priori as the condition of the synthesis of all appi-ehension simultaneously with the intuitions, not in them, and that synthetical unity can be no other but that of the connection of the manifold of any intuition whatsoever in an original consciousness, according to the categories, only applied to our sensuous intuition.

Consequently, all synthesis, without which even perception would be impossible, is subject to the categories ; and as experience consists of knowledge by means of connected perceptions, the categories are conditions of the possibihty of experience, and valid therefore a priori also for all objects of experience.

1 Space, represented as an object (as required in geometry), contains more than the mere form of intuition, namely, the comprehension of the n anifold, which is given according to the form of sensibility, into a perceptible (intuitable) representation, so that the form of intuition gives the manifold only, while the yt?r/;/rt'/ z«/wzV?c;; gives unity of representation. In the Esthetic I had simply ascribed this unity to sensibility, in order to show that it precedes all concepts, though it presupposes a synthesis not belonging to the senses, and by which all concepts of space and time become first possible. For as by that synthesis (the understanding determining the sensibility) space and time are first given as intuitions, the unity of that intuition a priori belongs to space and time, and not to the concept of the understanding. (See § 24.)

764 Stipplefnent XIV

If, for instance, I raise the empirical intuition of a house, through the apprehension of the manifold contained therein, into a perception, the necessary unity of space and of external sensuous intuition in general is presupposed, and I draw, as it were, the shape of the house according to that synthetical unity of the manifold in space. But this very synthetical unity, if I make abstraction of the form of space, has its seat in the understanding, and is in fact the category of the synthesis of the homogeneous in intuition in general : that is, the category of quantity, to which that synthesis of apprehension, that is, the perception, must always conform.^