The Complete Aristotle (eng.)

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.
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9
    As regards what is ‘of necessity’, we must ask whether the
necessity is ‘hypothetical’, or ‘simple’ as well. The current view
places what is of necessity in the process of production, just as
if one were to suppose that the wall of a house necessarily comes
to be because what is heavy is naturally carried downwards and what
is light to the top, wherefore the stones and foundations take