It is infinitely important for Reason not to let slip the mechanism of nature in its products, and in their explanation not to pass it by, because without it no insight into the nature of things can be attained. Suppose it admitted that a supreme Architect immediately created the forms of nature as they have been from the beginning, or that He predetermined those which in the course of nature continually form themselves on the same model. Our knowledge of nature is not thus in the least furthered, because we cannot know the mode of action of that Being and the Ideas which are to contain the principles of the possibility of natural beings, and we cannot by them explain nature as from above downwards (_a priori_). And if, starting from the forms of the objects of experience, from below upwards (_a posteriori_), we wish to explain the purposiveness, which we believe is met with in experience, by appealing to a cause working in accordance with purposes, then is our explanation quite tautological and we are only mocking Reason with words. Indeed when we lose ourselves with this way of explanation in the transcendent, whither natural knowledge cannot follow, Reason is seduced into poetical extravagance, which it is its peculiar destination to avoid. On the other hand, it is just as necessary a maxim of Reason not to pass by the principle of purposes in the products of nature. For, although it does not make their mode of origination any more comprehensible, yet it is a heuristic principle for investigating the particular laws of nature; supposing even that we wish to make no use of it for explaining nature itself,--in which we still always speak only of natural purposes, although it apparently exhibits a designed unity of purpose,--_i.e._ without seeking beyond nature the ground of the possibility of these particular laws. But since we must come in the end to this latter question, it is just as necessary to think for nature a particular kind of causality which does not present itself in it, as the mechanism of natural causes which does. To the receptivity of several forms, different from those of which matter is susceptible by mechanism, must be added a spontaneity of a cause (which therefore cannot be matter), without which no ground can be assigned for those forms. No doubt Reason, before it takes this step, must proceed with caution, and not try to explain teleologically every Technic of nature, _i.e._ every productive faculty of nature which displays in itself (as in regular bodies) purposiveness of figure to our mere apprehension; but must always regard such as so far mechanically possible. But on that account to wish entirely to exclude the teleological principle, and to follow simple mechanism only--in cases where, in the rational investigation of the possibility of natural forms through their causes, purposiveness shows itself quite undeniably as the reference to a different kind of causality--to do this must make Reason fantastic, and