In order to convince ourselves of the correctness of this Deduction of the concept before us, and the necessity of assuming it as a transcendental principle of cognition, just consider the magnitude of the problem. The problem, which lies _a priori_ in our Understanding, is to make a connected experience out of given perceptions of a nature containing at all events an infinite variety of empirical laws. The Understanding is, no doubt, in possession _a priori_ of universal laws of nature, without which nature could not be an object of experience; but it needs in addition a certain order of nature in its particular rules, which can only be empirically known and which are, as regards the Understanding, contingent. These rules, without which we could not proceed from the universal analogy of a possible experience in general to the particular, must be thought by it as laws (_i.e._ as necessary), for otherwise they would not constitute an order of nature; although their necessity can never be cognised or comprehended by it. Although, therefore, the Understanding can determine nothing _a priori_ in respect of Objects, it must, in order to trace out these empirical so-called laws, place at the basis of all reflection upon Objects an _a priori_ principle, viz. that a cognisable order of nature is possible in accordance with these laws. The following propositions express some such principle. There is in nature a subordination of genera and species comprehensible by us. Each one approximates to some other according to a common principle, so that a transition from one to another and so on to a higher genus may be possible. Though it seems at the outset unavoidable for our Understanding to assume different kinds of causality for the specific differences of natural operations, yet these different kinds may stand under a small number of principles, with the investigation of which we have to busy ourselves. This harmony of nature with our cognitive faculty is presupposed _a priori_ by the Judgement, on behalf of its reflection upon nature in accordance with its empirical laws; whilst the Understanding at the same time cognises it objectively as contingent, and it is only the Judgement that ascribes it to nature as a trancendental purposiveness (in relation to the cognitive faculty of the subject). For without this presupposition we should have no order of nature in accordance with empirical laws, and consequently no guiding thread for an experience ordered by these in all their variety, or for an investigation of them.