Causal combination as thought merely by the Understanding is a connexion constituting an ever-progressive series (of causes and effects); and things which as effects presuppose others as causes cannot be reciprocally at the same time causes of these. This sort of causal combination we call that of effective causes (nexus effectivus). But on the other hand, a causal combination according to a concept of Reason (of purposes) can also be thought, which regarded as a series would lead either forwards or backwards; in this the thing that has been called the effect may with equal propriety be termed the cause of that of which it is the effect. In the practical department of human art we easily find connexions such as this; e.g. a house, no doubt, is the cause of the money received for rent, but also conversely the representation of this possible income was the cause of building the house. Such a causal connexion we call that of final causes (nexus finalis). We may perhaps suitably name the first the connexion of real causes, the second of those which are ideal; because from this nomenclature it is at once comprehended that there can be no more than these two kinds of causality.
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.
In a watch one part is the instrument for moving the other parts, but the wheel is not the effective cause of the production of the others; no doubt one part is for the sake of the others, but it does not exist by their means. In this case the producing cause of the parts and of their form is not contained in the nature (of the material), but is external to it in a being which can produce effects according to Ideas of a whole possible by means of its causality. Hence a watch wheel does not produce other wheels, still less does one watch produce other watches, utilising (organising) foreign material for that purpose; hence it does not replace of itself parts of which it has been deprived, nor does it make good what is lacking in a first formation by the addition of the missing parts, nor if it has gone out of order does it repair itself—all of which, on the contrary, we may expect from organised nature.—An organised being is then not a mere machine, for that has merely moving power, but it possesses in itself formative power of a self-propagating kind which it communicates to its materials though they have it not of themselves; it organises them, in fact, and this cannot be explained by the mere mechanical faculty of motion.
We say of nature and its faculty in organised products far too little if we describe it as an analogon of art; for this suggests an artificer (a rational being) external to it. Much rather does it organise itself and its organised products in every species, no doubt after one general pattern but yet with suitable deviations, which self-preservation demands according to circumstances. We perhaps approach nearer to this inscrutable property, if we describe it as an analogon of life; but then we must either endow matter, as mere matter, with a property which contradicts its very being (hylozoism), or associate therewith an alien principle standing in communion with it (a soul). But in the latter case we must, if such a product is to be a natural product, either presuppose organised matter as the instrument of that soul, which does not make the soul a whit more comprehensible; or regard the soul as artificer of this structure and so remove the product from (corporeal) nature. To speak strictly, then, the organisation of nature has in it nothing analogous to any causality we know.[108] Beauty in nature can be rightly described as an analogon of art, because it is ascribed to objects only in reference to reflection upon their external aspect, and consequently only on account of the form of their external surface. But internal natural perfection, as it belongs to those things which are only possible as natural purposes, and are therefore called organised beings, is not analogous to any physical, i.e. natural, faculty known to us; nay even, regarding ourselves as, in the widest sense, belonging to nature, it is not even thinkable or explicable by means of any exactly fitting analogy to human art.
The concept of a thing as in itself a natural purpose is therefore no constitutive concept of Understanding or of Reason, but it can serve as a regulative concept for the reflective Judgement, to guide our investigation about objects of this kind by a distant analogy with our own causality according to purposes generally, and in our meditations upon their ultimate ground. This latter use, however, is not in reference to the knowledge of nature or of its original ground, but rather to our own practical faculty of Reason, in analogy with which we considered the cause of that purposiveness.
Organised beings are then the only beings in nature which, considered in themselves and apart from any relation to other things, can be thought as possible only as purposes of nature. Hence they first afford objective reality to the concept of a purpose of nature, as distinguished from a practical purpose; and so they give to the science of nature the basis for a teleology, i.e. a mode of judgement about natural Objects according to a special principle which otherwise we should in no way be justified in introducing (because we cannot see a priori the possibility of this kind of causality).