The Principles of Nature

It is impossible that there should be one principle only because contraries are not one, but there cannot be infinite principles. There is also one contouriety 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 Ampedocles does, then from infinite principles. But some assume many principles from which they constitute the nature of things. To preserve both assertions it is necessary to introduce a certain third thing as a subject to contourys.