If then we introduce into the context of natural science the concept of God in order to explain the purposiveness in nature, and subsequently use this purposiveness to prove that there is a God, there is no internal consistency in either science [_i.e._ either in natural science or theology]; and a delusive circle brings them both into uncertainty, because they have allowed their boundaries to overlap.
The expression, a purpose of nature, already sufficiently prevents the confusion of mixing up natural science and the occasion that it gives for judging _teleologically_ of its objects, with the consideration of God, and so of a _theological_ derivation of them. We must not regard it as insignificant, if one interchanges this expression with that of a divine purpose in the ordering of nature, or gives out the latter as more suitable and proper for a pious soul, because it must come in the end to deriving these purposive forms in nature from a wise author of the world. On the contrary, we must carefully and modestly limit ourselves to the expression, a purpose of nature, which asserts exactly as much as we know. Before we ask after the cause of nature itself, we find in nature, and in the course of its development, products of the same kind which are developed in it according to known empirical laws, in accordance with which natural science must judge of its objects, and, consequently, must seek in nature their causality according to the rule of purposes. So then it must not transgress its bounds in order to introduce into itself as a domestic principle that, to whose concept no experience can be commensurate, upon which we are only entitled to venture after the completion of natural science.
Natural characteristics which demonstrate themselves _a priori_, and consequently admit of insight into their possibility from universal principles without any admixture of experience, although they carry with them a technical purposiveness, yet cannot, because they are absolutely necessary, be referred to the Teleology of nature, as to a method belonging to Physic for solving its problems. Arithmetical or geometrical analogies, as well as universal mechanical laws,--however strange and admirable may seem to us the union of different rules, quite independent of one another according to all appearance, in a single principle,--possess on that account no claim to be teleological grounds of explanation in Physic. Even if they deserve to be brought into consideration in the universal theory of the purposiveness of things of nature, yet they belong to another [science], _i.e._ Metaphysic, and constitute no internal principle of natural science; as with the empirical laws of natural purposes in organised beings, it is not only permissible but unavoidable to use the teleological _mode of judging_ as a principle of the doctrine of nature in regard to a particular class of its objects.