Thomas Aquinas, Commentary on Aristotle's Physics

53. After the Philosopher has disproved the opinion concerning principles of those who did not speak of nature as natural philosophers, he here pursues the opinions of those who, not disregarding motion, spoke of the principles of nature as natural philosophers. And he calls these men physicists, i.e., natural philosophers.

Concerning this he makes two points. First he sets forth the diversity of their opinions. Secondly he examines one of these opinions, where he says, 'The theory of Anaxagoras . . .' (187 a 28).

54. He says first that according to the opinion of the natural philosophers there are two ways in which things are generated from principles. One of the opinions was advanced by the natural philosophers who held that there is only one material principle. This principle would be either one of three elements, i.e., fire, air, and water (for no one made earth alone the principle, as was said above or else some intermediate between them, for example, that which would be more dense than fire and more subtle than air. They then said that all other things were generated from this one principle by rarity and density. For example, those who made air to be the principle said that fire was generated from air by rarefaction, and water by condensation. However, the dense and the rare are contraries and are reduced to excess and defect as to something more universal. For the dense is what has much matter, whereas the rare has little.

55. And thus they agreed in a certain respect with Plato who held that the great and the small are principles which also pertain to excess and defect. But they differed from Plato as follows. Plato held that the great and the small are on the side of matter, because he posited one formal principle which is a certain idea participated in by different things according to a diversity of matter; the ancient natural philosophers, on the other hand, maintained a contrariety on the part of form, because they held that the first principle is one matter from which many things were constituted in being according to different forms.

56. Other natural philosophers, however, held that things come to be from principles in such a way that contraries themselves and different things are drawn forth from one thing in which they already existed, as it were, mixed and confused.

But they differed as follows. Anaximander held that the principle is one confused state in which there are not many things mixed together. Thus he held one principle only. But Empedocles and Anaxagoras held rather that the principles are the very things which are mixed together in that one confused state. And so they held many principles, although they also held that this one confused state is in some way a principle.

57. But Anaxagoras and Empedocles differed on two points. First, Empedocles held that there is a certain cycle of mixing and separating. For he held that the world has been made and corrupted many times; that is to say, when the world has been corrupted by friendship gathering all into one, the world is then generated again by strife separating and distinguishing. And thus the distinction of things follows upon their being confused, and vice versa. But Anaxagoras held that the world was made only once, such that from the beginning all things were mixed into one. But mind, which began to draw out and to distinguish, will never cease to do this, so that all things never will be mixed into one.

They also differed in another way. Anaxagoras held that the principles are infinite parts which are alike and contrary. Thus there are infinite parts of flesh which are like each other and infinite parts of bone and other things which have similar parts, yet each has a contrariety to the others. Thus the contrariety of the parts of bone to the parts of blood is that of the dry to the moist. But Empedocles held as principles only those four things which are commonly called elements, i.e., fire, air, water, and earth.