[One of the company... said:... is not this the direct contrary of what we admitted before—that out of the greater came the less and out of the less the greater, and that opposites are simply generated from opposites; whereas now this seems to be utterly denied.]... then we were speaking of opposites in the concrete, and now of the essential opposite which, as is affirmed, neither in us nor in nature can ever be at variance with itself... these essetial opposites will never, as we maintain, admit of generation into or out of one another.

Plato - Phaedo