Physics   

matter, the form, the mover, 'that for the sake of which'. The last
three often coincide; for the 'what' and 'that for the sake of which'
are one, while the primary source of motion is the same in species as
these (for man generates man), and so too, in general, are all things
which cause movement by being themselves moved; and 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).
Part 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