In the first place, then, we must assign the reason why nature ranks among the number of causes which subsist for the sake of something; and in the next place we must show how necessity subsists in physical concerns: for all natural causes are referred to this cause. Thus because that which is hot is naturally a thing of this kind, also that which is cold, and all such particulars; hence these things necessarily are, and are generated, and possess natural aptitudes: for those they assign another cause, yet as they merely mention it, they dismiss it; one of these, indeed, assigning as causes friendship and strife, but another intellect. But it is dubious what should hinder nature from operating for the sake of something, and because it is better so to operate; and not in the same manner as Jupiter rains, not that the corn may be increased, but from necessity: for that which is elevated must necessarily be refrigerated; and that which is refrigerated when it becomes water must descend: but it happens that in consequence of this taking place the corn is increased. In like manner, if the corn on the ground of anyone is destroyed, it does not rain for the sake of this, that it may be destroyed, but this is an accidental circumstance. What then hinders but that the parts in nature may thus subsist? For instance that the teeth should arise from necessity; those in front, indeed, sharp, and adapted to divide; but the grinders broad, and adapted to breaking the food in pieces: for it may be said that they were not made for the sake of this, but that it so happened: and in like manner with respect to the other parts of the body, to which a subsistence for the sake of something appears to belong. Where, therefore, all things happened as if they were made for the sake of something, these were preserved, being aptly composed from chance; but such things as were not thus made, these were lost and still perish, according to what Empedocles says concerning the bull species with human heads. This, therefore, and a similar reason, if there be any such, may lead someone to doubt on this subject. It is, however, impossible that these things should subsist after this manner: for these, and everything which is produced by nature, are either always, or for the most part thus produced; but this is not the case with anything which is produced from fortune and chance: for neither does it appear to be from fortune or chance that it frequently rains in the winter, but if this should be the case when the sun is under the dog star, it might be said to be the effect of these two. Nor when there is heat under the dog star is it a casual event, but when this is the case in winter. If, therefore, these things appear to be either from chance, or from the sake of something; and if it is not possible that they can be from chance, they must be for the sake of something. All such things, however, subsist from nature, as is even acknowledged by those who make these assertions. There is, therefore, a subsistence for the sake of something in things which are produced by and exist from nature.
Again, in things in which there is a certain end, that which is prior and that which has a consequent subsistence are effected for the sake of this end. As, therefore, it is effected, so is it naturally adapted to be effected; and according to the natural aptitude of anything, such also, unless there is some impediment, is it effected. But it is effected for the sake of something: and hence it is naturally adapted to be effected for the sake of this: for instance, if a house ranked in the class of things which are produced by nature, it would be produced by nature in such a manner as it is now by art. But if things which are the offspring of nature were not only produced by nature, but also by art, they would in a similar manner be produced according to their natural aptitude. The one, therefore, would be for the sake of the other; and, in short, art partly perfects what nature is incapable of bringing to perfection, and partly imitates nature. If, therefore, things artificial are produced for the sake of something, it is evident that this is also the case with things which have a natural subsistence. Four things posterior with respect to prior are similarly affected towards each other, both in artificial and natural productions. This, however, is especially manifest in other animals, whose productions are neither the effect of art, nor investigation, nor consultation. Hence, it is doubted by some, whether spiders, and ants, and animals of this kind, operate from intellect, or something else. But to him who thus gradually precedes, even in plants things which contribute to the end will be apparent; as, for instance, leaves are produced for the sake of covering the fruit. Hence if the swallow forms her nest, and thus spider her web, naturally, end for the sake of something; and if plants produce leaves for the sake of the fruit, and the roots do not tend upward but downward for the sake of nutriment; it is evident that a cause of this kind belongs to the class of things which are produced by and subsist from nature. Since, also, nature is twofold, one kind being as matter, but another as form, which last is an end, and the rest are for the sake of the end; this will be a cause for the sake of which natural productions subsist. Error also takes place in things artificial: for a grammarian has written incorrectly; and a physician has improperly administered the medicine. So that it is evident that this also happens in natural productions. If, therefore, there are certain artificial things, in which rectitude subsists for the sake of something; but in things erroneous, art is attempted for the sake of something, but deviates from the end, the like also takes place in the productions of nature; So that monsters will be the errors of that which operates for the sake of something. In the first constitution of things, therefore, the bowl species with human heads, if there were such animals, must have been produced from the corruption of a certain principle, as things are now produced from the corruption of seed.
Further still, it is necessary that seed should have been first produced, and not immediately animals; and that soft mass which first subsisted was seed. In plants also there is that which subsists for the sake of something, but is less distinct. Whether, therefore, or not, as there were the bull species with human heads, so also in plants there were the vine species with the anterior part resembling olives? This, indeed, is absurd, yet it would be necessary, since it also happened in animals. Again, it would be necessary that there should be a casual generation in the seeds of things. And, in short, he who asserts this subverts both that which subsists from nature, and nature itself: for those things subsist from nature, which being continually moved from a certain principle contained in themselves, arrive at a certain end. From each principle, however, neither the same thing is produced in each individual, nor that which is casual; but it always arrives at the same end unless something impedes. But that for the sake of which, and that which is for the sake of this, may subsist from fortune. Thus we say that a stranger came fortuitously, and having redeemed a certain person departed, when he so acted as if he came for the sake of this, and yet did not come on this account; and when he did this from accident. For fortune belongs to causes which subsist according to accident, as we have before said. But when this happens either always or for the most part, then it is neither from accident, nor fortune. In natural productions, however, it is always so, unless something impedes. It is also absurd to fancy that things are not produced for the sake of anything, unless that which moves is seen to have deliberated; though art also does not deliberate: for if the shipbuilding art were in the timber, it would make similar to nature. So that if there is a subsistence for the sake of something in art, there will also be this in nature. But this is especially evident when anyone heals himself: for to this man nature is similar. That nature, therefore, is a cause, and in such a manner as to operate for the sake of something, is evident.
