A transcendental principle is one by means of which is represented, _a priori_, the universal condition under which alone things can be in general Objects of our cognition. On the other hand, a principle is called metaphysical if it represents the _a priori_ condition under which alone Objects, whose concept must be empirically given, can be further determined _a priori_. Thus the principle of the cognition of bodies as substances, and as changeable substances, is transcendental, if thereby it is asserted that their changes must have a cause; it is metaphysical if it asserts that their changes must have an _external_ cause. For in the former case bodies need only be thought by means of ontological predicates (pure concepts of Understanding), _e.g._ substance, in order to cognise the proposition _a priori_; but in the latter case the empirical concept of a body (as a movable thing in space) must lie at the basis of the proposition, although once this basis has been laid down, it may be seen completely _a priori_ that this latter predicate (motion only by external causes) belongs to body.--Thus, as I shall presently show, the principle of the purposiveness of nature (in the manifoldness of its empirical laws) is a transcendental principle. For the concept of Objects, so far as they are thought as standing under this principle, is only the pure concept of objects of possible empirical cognition in general and contains nothing empirical. On the other hand, the principle of practical purposiveness, which must be thought in the Idea of the _determination_ of a free _will_, is a metaphysical principle; because the concept of a faculty of desire as a will must be given empirically (_i.e._ does not belong to transcendental predicates). Both principles are, however, not empirical, but _a priori_; because for the combination of the predicate with the empirical concept of the subject of their judgements no further experience is needed, but it can be apprehended completely _a priori_. That the concept of a purposiveness of nature belongs to transcendental principles can be sufficiently seen from the maxims of the Judgement, which lie at the basis of the investigation of nature _a priori_, and yet do not go further than the possibility of experience, and consequently of the cognition of nature--not indeed nature in general, but nature as determined through a variety of particular laws. These maxims present themselves in the course of this science often enough, though in a scattered way, as sentences of metaphysical wisdom, whose necessity we cannot demonstrate from concepts. “Nature takes the shortest way (_lex parsimoniae_); at the same time it makes no leaps, either in the course of its changes or in the juxtaposition of specifically different forms (_lex continui in natura_); its great variety in empirical laws is yet unity under a few principles (_principia praeter necessitatem non sunt multiplicanda_),” etc.