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Aristotle, Physics. Book I, Chapter 7

After this, it follows that we should see whether there are two or three or more principles. For it is impossible that there should be one principle only, because contraries are not one. But there cannot be infinite principles, because if this were admitted, being would not be an object of scientific knowledge. There’s also one contrariety in every one genus, but essence is one certain genus. The causes also of all natural things may be better assigned from finite principles, as Empedocles does, than from infinite principles. For Empedocles thinks that he unfolds all things from finite in the same manner as Anaxagoras from infinite principles. Farther still, some countries are prior to others and some are generated from others as, for instance, the sweet and the bitter, the white and the black. But it is necessary that principles should always remain. From these things, therefore, it is evident that there is neither one principle nor infinite principles. But since they are finite, there is some reason why they should not be considered as two alone. Someone, however, may doubt how density is naturally adapted to produce rarity, or rarity, density, and in like manner with respect to any other contrariety whatever. For friendship does not conjoin strife or produce anything from it, nor does strife make anything from friendship, but both produce a certain other third thing. But some assume many principles from which they constitute the nature of things, to which may be added that the following doubt may be urged by someone unless another nature is introduced as a subject to contraries. For, we see that contraries are not the essence of anything, and principle, ought not to be predicated of any subject, since if it were there would be a principle of a principle. For subject appears to be the principle, and to be prior to that which is predicated of it. Farther still, we say that essence is not contrary to essence. How, therefore, will essence consist from things which are not essences? Or how will non-essence be prior to essence? Hence, if someone should think that what is before asserted is true and should also admit the truth of what is now said, it is necessary, if he wishes to preserve both assertions, that he should introduce a certain third thing as a subject to contraries, just as they say, who affirm that the universe is one certain nature, such as water or fire, or that which subsists between these. But it appears that the subject is rather that which subsists between the elements, for fire and earth, air and water are complicated with contrarieties. Hence they do not act absurdly who make the subject to be different from these. But of the rest they think more rightly who make air to be the subject, for air has less sensible differences than the rest, and water follows air. All however give form to this one thing from contraries. As, for instance, from density and rarity and from the more and the less. And these things in short, are excess and defect as was before said. This opinion, too, appears to be ancient, I mean, that the one excess and defect are the principles of things, except that the ancients did not consider these principles after the same manner as the moderns. For the Ancients asserted that two of these principles are active, but that the one is passive. But certain of the moderns assert, on the contrary, that the one is active, but that the two are rather passive. The assertion, therefore, that there are three elements may appear to have some weight, as we have before observed to those who, from these and other speculations of the same kind, consider the affair. It is not, however, probable that there are more than three principles, for one principle is sufficient for the purpose of being passive. But if there are four principles, there will be two contraries, and it will be necessary that a certain other middle nature should subsist as a subject separate from both. If two, they’re able to generate from each other, the other of the contrarieties will be superfluous. At the same time, however, it is impossible that there should be many first contrarieties. For essence is one certain genus of being, so that principles will differ from each other in prior and posterior alone, but not in genus. For in one genus there is always one contrariety and all contrarieties appear to be referred to one. That there is neither one element there for nor more than two or three is evident. But which of these assertions is the more true is, as we have said, attended with much doubt.

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