Physics   

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.
Part 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 the lowest place, with earth
above because it is lighter, and wood at the top of all as being the
lightest. Whereas, though the wall does not come to be without these,
it is not due to these, except as its material cause: it comes to be
for the sake of sheltering and guarding certain things. Similarly in
all other things which involve production for an end; the product
cannot come to be without things which have a necessary nature, but it
is not due to these (except as its material); it comes to be for an
end. For instance, why is a saw such as it is? To effect so-and-so and
for the sake of so-and-so. This end, however, cannot be realized
unless the saw is made of iron. It is, therefore, necessary for it to
be of iron, it we are to have a saw and perform the operation of
sawing. What is necessary then, is necessary on a hypothesis; it is
not a result necessarily determined by antecedents. Necessity is in
the matter, while 'that for the sake of which' is in the definition.
Necessity in mathematics is in a way similar to necessity in things
which come to be through the operation of nature. Since a straight
line is what it is, it is necessary that the angles of a triangle
should equal two right angles. But not conversely; though if the
angles are not equal to two right angles, then the straight line is
not what it is either. But in things which come to be for an end, the
reverse is true. If the end is to exist or does exist, that also which
precedes it will exist or does exist; otherwise just as there, if-the
conclusion is not true, the premiss will not be true, so here the end
or 'that for the sake of which' will not exist. For this too is itself
a starting-point, but of the reasoning, not of the action; while in
mathematics the starting-point is the starting-point of the reasoning
only, as there is no action. If then there is to be a house,
such-and-such things must be made or be there already or exist, or
generally the matter relative to the end, bricks and stones if it is a
house. But the end is not due to these except as the matter, nor will
it come to exist because of them. Yet if they do not exist at all,
neither will the house, or the saw-the former in the absence of
stones, the latter in the absence of iron-just as in the other case
the premisses will not be true, if the angles of the triangle are not
equal to two right angles.
The necessary in nature, then, is plainly what we call by the name of
matter, and the changes in it. Both causes must be stated by the
physicist, but especially the end; for that is the cause of the
matter, not vice versa; and the end is 'that for the sake of which',
and the beginning starts from the definition or essence; as in
artificial products, since a house is of such-and-such a kind, certain
things must necessarily come to be or be there already, or since
health is this, these things must necessarily come to be or be there
already. Similarly if man is this, then these; if these, then those.
Perhaps the necessary is present also in the definition. For if one
defines the operation of sawing as being a certain kind of dividing,
then this cannot come about unless the saw has teeth of a certain
kind; and these cannot be unless it is of iron. For in the definition
too there are some parts that are, as it were, its matter.