The Complete Aristotle (eng.)

‘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.)
    Some identify the nature or substance of a natural object with
that immediate constituent of it which taken by itself is without
arrangement, e.g. the wood is the ‘nature’ of the bed, and the
bronze the ‘nature’ of the statue.
    As an indication of this Antiphon points out that if you planted
a bed and the rotting wood acquired the power of sending up a
shoot, it would not be a bed that would come up, but wood-which
shows that the arrangement in accordance with the rules of the art
is merely an incidental attribute, whereas the real nature is the
other, which, further, persists continuously through the process of
making.
    But if the material of each of these objects has itself the same
relation to something else, say bronze (or gold) to water, bones
(or wood) to earth and so on, that (they say) would be their nature
and essence. Consequently some assert earth, others fire or air or
water or some or all of these, to be the nature of the things that
are. For whatever any one of them supposed to have this
character-whether one thing or more than one thing-this or these he
declared to be the whole of substance, all else being its
affections, states, or dispositions. Every such thing they held to
be eternal (for it could not pass into anything else), but other
things to come into being and cease to be times without number.
    This then is one account of ‘nature’, namely that it is the
immediate material substratum of things which have in themselves a
principle of motion or change.
    Another account is that ‘nature’ is the shape or form which is
specified in the definition of the thing.
    For the word ‘nature’ is applied to what is according to nature
and the natural in the same way as ‘art’ is applied to what is
artistic or a work of art. We should not say in the latter case
that there is anything artistic about a thing, if it is a bed only
potentially, not yet having the form of a bed; nor should we call
it a work of art. The same is true of natural compounds. What is
potentially flesh or bone has not yet its own ‘nature’, and does
not exist until it receives the form specified in the definition,
which we name in defining what flesh or bone is. Thus in the second
sense of ‘nature’ it would be the shape or form (not separable
except in statement) of things which have in themselves a source of
motion. (The combination of the two, e.g. man, is not ‘nature’ but
‘by nature’ or ‘natural’.)
    The form indeed is ‘nature’ rather than the matter; for a thing
is more properly said to be what it is when it has attained to
fulfilment than when it exists potentially. Again man is born from
man, but not bed from