Now this concept brings the Reason into a quite different order of things from that of a mere mechanism of nature, which is no longer satisfying here. An Idea is to be the ground of the possibility of the natural product. But because this is an absolute unity of representation, instead of the material being a plurality of things that can supply by itself no definite unity of composition,--if that unity of the Idea is to serve at all as the _a priori_ ground of determination of a natural law of the causality of such a form of composition,--the purpose of nature must be extended to _everything_ included in its product. For if we once refer action of this sort _on the whole_ to any supersensible ground of determination beyond the blind mechanism of nature, we must judge of it altogether according to this principle; and we have then no reason to regard the form of such a thing as partly dependent on mechanism--for by such mixing up of disparate principles no certain rule of judging would be left. For example, it may be that in an animal body many parts can be conceived as concretions according to mere mechanical laws (as the hide, the bones, the hair). And yet the cause which brings together the required matter, modifies it, forms it, and puts it in its appropriate place, must always be judged of teleologically; so that here everything must be considered as organised, and everything again in a certain relation to the thing itself is an organ. § 67. _Of the principle of the teleological judging of nature in general as a system of purposes_ We have already said above that the _external_ purposiveness of natural things affords no sufficient warrant for using them as purposes of nature in order to explain their presence, and for regarding their contingently purposive effects as the grounds of their presence according to the principle of final causes. Thus we cannot take for natural purposes, _rivers_ because they promote intercourse among inland peoples, _mountains_ because they contain the sources of the rivers and for their maintenance in rainless seasons have a store of snow, or the _slope_ of the land which carries away the water and leaves the country dry; because although this shape of the earth’s surface be very necessary for the origin and maintenance of the vegetable and animal kingdoms, it has nothing in itself for the possibility of which we are forced to assume a causality according to purposes. The same is true of plants which man uses for his needs or his pleasures; of beasts, the camel, the ox, the horse, dog, etc., which are indispensable to him as well for food as because they are used in his service in many different ways. In the case of things which we have no reason for regarding in themselves as purposes, such external relation can only be hypothetically judged as purposive.