If, then, the parts of a thing are not destructive of one another, and these things are destructive of one another, it follows that they are not parts of one another; and if they |208 are not parts of one another, they will not be parts of one and the same matter. But in fact they will not be matter at all, because that nothing is destructive of itself, as is the case with opposites. For nothing is opposed to itself, opposites being by nature opposed to things other than themselves; as for example, white is not the opposite of white, but is said to be opposed to black; light, too, is shown not to be opposed to itself, but to darkness, and similarly with countless other things. So then, if matter is a single substance, it cannot be its own opposite; and if this doctrine of opposites holds good, it appears that there is no such matter.