For a thing to be a natural purpose in the _first_ place it is requisite that its parts (as regards their being and their form) are only possible through their reference to the whole. For the thing itself is a purpose and so is comprehended under a concept or an Idea which must determine _a priori_ all that is to be contained in it. But so far as a thing is only thought as possible in this way, it is a mere work of art; _i.e._ a product of one rational cause distinct from the matter (of the parts), whose causality (in the collection and combination of the parts) is determined through its Idea of a whole possible by their means (and consequently not through external nature). But if a thing as a natural product is to involve in itself and in its internal possibility a reference to purposes,--_i.e._ to be possible only as a natural purpose, and without the causality of the concepts of rational beings external to itself,--then it is requisite _secondly_ that its parts should so combine in the unity of a whole that they are reciprocally cause and effect of each other’s form. Only in this way can the Idea of the whole conversely (reciprocally) determine the form and combination of all the parts; not indeed as cause--for then it would be an artificial product--but as the ground of cognition, for him who is judging it, of the systematic unity and combination of all the manifold contained in the given material. For a body then which is to be judged in itself and its internal possibility as a natural purpose, it is requisite that its parts mutually depend upon each other both as to their form and their combination, and so produce a whole by their own causality; while conversely the concept of the whole may be regarded as its cause according to a principle (in a being possessing a causality according to concepts adequate to such a product). In this case then the connexion of _effective causes_ may be judged as an _effect through final causes_. In such a product of nature every part not only exists _by means of_ the other parts, but is thought as existing _for the sake of_ the others and the whole, that is as an (organic) instrument. Thus, however, it might be an artificial instrument, and so might be represented only as a purpose that is possible in general; but also its parts are all organs reciprocally _producing_ each other. This can never be the case with artificial instruments, but only with nature which supplies all the material for instruments (even for those of art). Only a product of such a kind can be called a _natural purpose_, and this because it is an _organised_ and _self-organising being_.