The Complete Aristotle (eng.)

such as are not of this kind are no longer inside the
province of physics, for they cause motion not by possessing motion
or a source of motion in themselves, but being themselves incapable
of motion. Hence there are three branches of study, one of things
which are incapable of motion, the second of things in motion, but
indestructible, the third of destructible things.
    The question ‘why’, then, is answered by reference to the
matter, to the form, and to the primary moving cause. For in
respect of coming to be it is mostly in this last way that causes
are investigated-’what comes to be after what? what was the primary
agent or patient?’ and so at each step of the series.
    Now the principles which cause motion in a physical way are two,
of which one is not physical, as it has no principle of motion in
itself. Of this kind is whatever causes movement, not being itself
moved, such as (1) that which is completely unchangeable, the
primary reality, and (2) the essence of that which is coming to be,
i.e. the form; for this is the end or ‘that for the sake of which’.
Hence since nature is for the sake of something, we must know this
cause also. We must explain the ‘why’ in all the senses of the
term, namely, (1) that from this that will necessarily result
(’from this’ either without qualification or in most cases); (2)
that ‘this must be so if that is to be so’ (as the conclusion
presupposes the premisses); (3) that this was the essence of the
thing; and (4) because it is better thus (not without
qualification, but with reference to the essential nature in each
case).
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8
    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