Physics

For the universal is knowable by reason, but the particular by sense. For reason concerns the universal, but sense concerns the particular; for example, great and small are known by reason, but dense and rare by sense. That the principles must be opposites, therefore, is clear.

It would be next to say whether they are two, or three, or more. For it is not possible for them to be one, because the opposite is not one. Nor can they be infinite, because being would not be knowable. Also, there is one contrariety in every single genus, and substance is one genus. And it is possible for them to be from a finite number. It is better to have finite principles, as Empedocles does, than infinite ones. For he thinks he provides everything that Anaxagoras provides from the infinite. Furthermore, some opposites are prior to others, and some come from others, like sweet and bitter, and white and black, but principles must always remain. That they are neither one nor infinite is clear from this. Since they are finite, not making them only two has some reason. For one might ask how density is naturally suited to make something from rarity, or vice versa. Similarly with any other contrariety. For love does not gather strife and make something from it, nor does strife make something from love; rather, both make some third thing. Some also take more things from which they construct the nature of existing things. In addition to this, one might also raise this problem if one does not posit a different nature for the opposites; for we do not see the substance of any existing thing as the opposites themselves. And the principle must not be predicated of any subject. For there would be a principle of the principle; for the subject is a principle, and seems to be prior to the predicate. Furthermore, we say that no substance is opposite to substance. How, then, could substance come from non-substances? Or how could a non-substance be prior to substance? Therefore, if one considers both the previous argument and this one to be true, it is necessary, if one is to save both, to posit a third thing, just as those who say the whole is one nature do, such as water or fire or the intermediate between them.