We must explain then (1) that Nature belongs to the class of
causes which act for the sake of something; (2) about the necessary
and its place in physical problems, for all writers ascribe things
to this cause, arguing that since the hot and the cold, &c., are of
such and such a kind, therefore certain things necessarily are and
come to be-and if they mention any other cause (one his "friendship
and strife", another his "mind"), it is only to touch on it, and
then good-bye to it.

    A difficulty presents itself: why should not nature work, not for
the sake of something, nor because it is better so, but just as the
sky rains, not in order to make the corn grow, but of necessity?
What is drawn up must cool, and what has been cooled must become water
and descend, the result of this being that the corn grows. Similarly
if a man's crop is spoiled on the threshing-floor, the rain did not
fall for the sake of this-in order that the crop might be
spoiled-but that result just followed. Why then should it not be the
same with the parts in nature, e.g. that our teeth should come up of
necessity-the front teeth sharp, fitted for tearing, the molars
broad and useful for grinding down the food-since they did not arise
for this end, but it was merely a coincident result; and so with all
other parts in which we suppose that there is purpose? Wherever then
all the parts came about just what they would have been if they had
come be for an end, such things survived, being organized
spontaneously in a fitting way; whereas those which grew otherwise
perished and continue to perish, as Empedocles says his "man-faced
ox-progeny" did.

    Such are the arguments (and others of the kind) which may cause
difficulty on this point. Yet it is impossible that this should be the
true view. For teeth and all other natural things either invariably or
normally come about in a given way; but of not one of the results of
chance or spontaneity is this true. We do not ascribe to chance or
mere coincidence the frequency of rain in winter, but frequent rain in
summer we do; nor heat in the dog-days, but only if we have it in
winter. If then, it is agreed that things are either the result of
coincidence or for an end, and these cannot be the result of
coincidence or spontaneity, it follows that they must be for an end;
and that such things are all due to nature even the champions of the
theory which is before us would agree. Therefore action for an end
is present in things which come to be and are by nature.

    Further, where a series has a completion, all the preceding steps
are for the sake of that. Now surely as in intelligent action, so in
nature; and as in nature, so it is in each action, if nothing
interferes. Now intelligent action is for the sake of an end;
therefore the nature of things also is so. Thus if a house, e.g. had
been a thing made by nature, it would have been made in the same way
as it is now by art; and if things made by nature were made also by
art, they would come to be in the same way as by nature. Each step
then in the series is for the sake of the next; and generally art
partly completes what nature cannot bring to a finish, and partly
imitates her. If, therefore, artificial products are for the sake of
an end, so clearly also are natural products. The relation of the
later to the earlier terms of the series is the same in both. This
is most obvious in the animals other than man: they make things
neither by art nor after inquiry or deliberation. Wherefore people
discuss whether it is by intelligence or by some other faculty that
these creatures work,spiders, ants, and the like. By gradual advance
in this direction we come to see clearly that in plants too that is
produced which is conducive to the end-leaves, e.g. grow to provide
shade for the fruit. If then it is both by nature and for an end
that the swallow makes its nest and the spider its web, and plants
grow leaves for the sake of the fruit and send their roots down (not
up) for the sake of nourishment, it is plain that this kind of cause
is operative in things which come to be and are by nature. And since
"nature" means two things, the matter and the form, of which the
latter is the end, and since all the rest is for the sake of the
end, the form must be the cause in the sense of "that for the sake
of which".

    Now mistakes come to pass even in the operations of art: the
grammarian makes a mistake in writing and the doctor pours out the
wrong dose. Hence clearly mistakes are possible in the operations of
nature also. If then in art there are cases in which what is rightly
produced serves a purpose, and if where mistakes occur there was a
purpose in what was attempted, only it was not attained, so must it be
also in natural products, and monstrosities will be failures in the
purposive effort. Thus in the original combinations the "ox-progeny"
if they failed to reach a determinate end must have arisen through the
corruption of some principle corresponding to what is now the seed.

    Further, seed must have come into being first, and not straightway
the animals: the words "whole-natured first..." must have meant seed.

    Again, in plants too we find the relation of means to end, though
the degree of organization is less. Were there then in plants also
"olive-headed vine-progeny", like the "man-headed ox-progeny", or not?
An absurd suggestion; yet there must have been, if there were such
things among animals.

    Moreover, among the seeds anything must have come to be at random.
But the person who asserts this entirely does away with "nature" and
what exists "by nature". For those things are natural which, by a
continuous movement originated from an internal principle, arrive at
some completion: the same completion is not reached from every
principle; nor any chance completion, but always the tendency in
each is towards the same end, if there is no impediment.

    The end and the means towards it may come about by chance. We say,
for instance, that a stranger has come by chance, paid the ransom, and
gone away, when he does so as if he had come for that purpose,
though it was not for that that he came. This is incidental, for
chance is an incidental cause, as I remarked before. But when an event
takes place always or for the most part, it is not incidental or by
chance. In natural products the sequence is invariable, if there is no
impediment.

    It is absurd to suppose that purpose is not present because we do
not observe the agent deliberating. Art does not deliberate. If the
ship-building art were in the wood, it would produce the same
results by nature. If, therefore, purpose is present in art, it is
present also in nature. The best illustration is a doctor doctoring
himself: nature is like that.

    It is plain then that nature is a cause, a cause that operates for a
purpose.