Of things some are (exist) by nature, others from other causes (e.g. art, chance, etc.).
By nature are the animals and their parts, the plants and the simple bodies, i.e. earth, fire, air, water – for we say that these and the like exist by nature. All these seem to be different from those which are not constituted by nature.
For each of them has within itself a principle of motion and of standstill, some in virtue of place, others in virtue of increase and of decrease, and others in virtue of alteration. On the other hand, a bed or a cloak, or anything else of that sort, in as the above attributes are referred to them, and they exist by art, have no innate impulse to change, but in so far as they happen to be composed of stone or of earth, or compounded of these, they do have, and to that extent, as nature is a certain principle or cause for motion and for stillness within that to which it belongs, primarily, just by itself and not accidentally.
I say ‘not accidentally’ in that a man might cure himself if he is a doctor. However, it is not because he cures himself that he possesses the art of medicine, but it so happened that the same man is both doctor and he who cures himself. And this is why sometimes the one parts from the other (for were it to cure himself unconditionally, the one who is ill, it would never happen to part what cures from what is ill; here, however, the one parts from the other, since whoever is ill does not necessarily cure himself).
Similarly with all other artificial products. For none of these has in itself the source of its own production, but some have that principle in others and externally, such as a house or what is made of manual labour, others, which are capable of being causes in themselves accidentally (e.g. a doctor), do have that one in themselves but none of them unconditionally.
Has a nature
Nature then is what has been said. Something ‘has a nature’ when it has a principle of this kind. And each of them is a substance; for nature is a subject and is always in a subject (what is accidental is not capable of being a subject, for none of them is a subject whereas what is not accidental is a subject; hence what has a nature is a substance).
According to nature
Now all these things which have a nature as well as those which belong to them unconditionally – as being carried upwards belongs to fire – are ‘according to nature’; for that (upward carrying) is not a ‘nature’ nor ‘has a nature’ but is ‘by nature’ and ‘according to nature’.
Bibliography: Aristotle Physics (192b.8)
Translation – text editing: George Kotsalis