Physics   

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.