man is well, there will indeed be health, and not disease, and so also when all things are white, there will be whiteness, but not blackness. Besides, if "Socrates is well" be the contrary of "Socrates is ill," and both cannot possibly be inherent in the same subject, it follows, that when one of the contraries exists, the other cannot possibly exist, for "Socrates is well" existing, "Socrates is ill" cannot exist.
Contraries, however, evidently are, by their nature, adapted to subsist about the same thing, either in species or genus, since disease and health naturally subsist in the body of an animal, but whiteness and blackness simply in body, and justice and injustice in the soul of man.
Notwithstanding, it is requisite that all contraries be either in the same genus, or in contrary genera, or be genera themselves; for white and black are in the same genus, as "colour" is the genus of them; but justice and injustice in contrary genera, for "virtue" is the genus of one, but "vice" of the other; lastly, "good" and "bad" are not in a genus, but are themselves the genera of certain things.
Chapter 12
A thing is said to be prior to another in four respects: first and most properly, in respect of time, according to which, one is said to be older and more ancient than another, since it is called older and more ancient, because the time is longer. Next, when it does not reciprocate, according to the consequence of existence: thus one is prior to two, for two existing, it follows directly that one exists; but when one is, it is not necessary that two should be, hence the consequence of the remainder's existence does not reciprocate from the existence of the one; but such a thing appears to be prior, from which the consequence of existence does not reciprocate.
