Physics   

to be. For if it came to be, something must have existed as a primary
substratum from which it should come and which should persist in it;
but this is its own special nature, so that it will be before coming
to be. (For my definition of matter is just this-the primary
substratum of each thing, from which it comes to be without
qualification, and which persists in the result.) And if it ceases to
be it will pass into that at the last, so it will have ceased to be
before ceasing to be.
The accurate determination of the first principle in respect of form,
whether it is one or many and what it is or what they are, is the
province of the primary type of science; so these questions may stand
over till then. But of the natural, i.e. perishable, forms we shall
speak in the expositions which follow.
The above, then, may be taken as sufficient to establish that there
are principles and what they are and how many there are. Now let us
make a fresh start and proceed.
Physics
By Aristotle
Written 350 B.C.E Part 1
Of things that exist, some exist by nature, some from other causes.
'By nature' the animals and their parts exist, and the plants and the
simple bodies (earth, fire, air, water)-for we say that these and the
like exist 'by nature'.
All the things mentioned present a feature in which they differ from
things which are not constituted by nature. Each of them has within
itself a principle of motion and of stationariness (in respect of
place, or of growth and decrease, or by way of alteration). On the
other hand, a bed and a coat and anything else of that sort, qua
receiving these designations i.e. in so far as they are products of
art-have no innate impulse to change. But in so far as they happen to
be composed of stone or of earth or of a mixture of the two, they do
have such an impulse, and just to that extent which seems to indicate
that nature is a source or cause of being moved and of being at rest
in that to which it belongs primarily, in virtue of itself and not in
virtue of a concomitant attribute.
I say 'not in virtue of a concomitant attribute', because (for
instance) a man who is a doctor might cure himself. Nevertheless it is
not in so far as he is a patient that he possesses the art of
medicine: it merely has happened that the same man is doctor and
patient-and that is why these attributes are not always found
together. So it is with all other artificial products. None of them
has in itself the source of its own production. But while in some
cases (for instance houses and the other products of manual labour)
that principle is in something else external to the thing, in others
those which may cause a change in themselves in virtue of a
concomitant attribute-it lies in the things themselves (but not in
virtue of what they are).
'Nature' then is what has been stated. Things 'have a nature'which
have a principle of this kind. Each of them is a substance; for it is
a subject, and nature always implies a subject in which it inheres.
The term 'according to nature' is applied to all these things and also
to the attributes which belong to them in virtue of what they are, for
instance the property of fire to be carried upwards-which is not a
'nature' nor 'has a nature' but is 'by nature' or 'according to
nature'.
What nature is, then, and the meaning of the terms 'by nature' and
'according to nature', has been stated. That nature exists, it would
be absurd to try to prove; for it is obvious that there are many
things of this kind, and to prove what is obvious by what is not is
the mark of a man who is unable to distinguish what is self-evident
from what is not. (This state of mind is clearly possible. A man blind
from birth might reason about colours. Presumably therefore such
persons must be talking about words without any thought to
correspond.)