Selection from St. Thomas Aquinas, Commentary on the Physics, trans. Richard J. Blackwell, Richard J. Spath & W. Edmund Thirlkel, (New Haven, CT: Yale University Press, 1963), bk. 2, lect. 1, n. 145.

Next where [Aristotle] says, ‘... nature is...’ (192 b 22), he concludes from the above the definition of nature in the following manner.

Natural things differ from the non-natural insofar as they have a nature. But they differ from the non-natural only insofar as they have in themselves a principle of motion. Therefore, nature is nothing other than a principle of motion and rest in that in which it is primarily and per se and not per accidens.

Now ‘principle’ is placed in the definition of nature as its genus, and not as something absolute, for the name ‘nature’ involves a relation to a principle. For those things are said to be born which are generated after having been joined to a generator, as is clear in plants and animals, thus the principle of generation or motion is called nature. Hence they are to be laughed at who, wishing to correct the definition of Aristotle, tried to define nature by something absolute, saying that nature is a power seated in things or something of this sort.

Moreover, nature is called a principle and cause in order to point out that in that which is moved nature is not a principle of all motions in the same way, but in different ways, as was said above.

Moreover, he says that nature is a principle ‘of motion and rest’. For those things which are naturally moved to a place, also or even more naturally rest in that place. Because of this, fire is naturally moved upward, since it is natural for it to be there. And for the same reason everything can be said to be moved naturally and to rest naturally in its place. . . .

Further he says ‘in which it is’ in order to differentiate nature from artificial things in which there is motion only per accidens.

Then he adds ‘Primarily’ because even though nature is a principle of the motion of composite things, nevertheless it is not such primarily. Hence that an animal is moved downwards is not because of the nature of animal insofar as it is animal, but because of the nature of the dominant element.

He explains why he says ‘per se and not per accidens’ where he says, ‘I say “not in virtue of...”’ (192 b 24).

It sometimes happens that a doctor is the cause of his own health, and so the principle of his own coming to health is in him, but per accidens. Hence nature is not the principle of his coming to health. For it is not insofar as he is cured that he has the art of medicine, but insofar as he is a doctor. Hence the same being happens to be a doctor and to be cured, and he is cured insofar as he is sick. And so because these things are joined per accidens, they are also at times separated per accidens. For it is one thing to be a doctor who cures, and another thing to be a sick person who is cured. But the principle of a natural motion is in the natural body which is moved insofar as it is moved. For insofar as fire has lightness, it is carried upward. And these two things are not divided from each other so that the lightness is different than the body which is moved upward. Rather they are always one and the same. And all artificial things are like the doctor who cures. For none of them has in itself the principle of its own making. Rather some of them come to be from something outside, as a house and other things which are carved by hand, while others come to be through an intrinsic principle, but per accidens, as was said. And so it has been stated what nature is.