Book 9
1
Since some of these principles are inherent in inanimate things, and others in animate things and in the soul and in the rational part of the soul, it is clear that some of the potencies also will be irrational and some rational. Hence all arts, i.e. the productive sciences, are potencies; because they are principles of change in another thing, or in the artist himself qua other.
2
Every rational potency admits equally of contrary results, but irrational potencies admit of one result only. E.g., heat can only produce heat, but medical science can produce disease and health. The reason of this is that science is a rational account, and the same account explains both the thing and its privation, though not in the same way; and in one sense it applies to both, and in another sense rather to the actual fact.
3
Therefore such sciences must treat of contraries—essentially of the one, and non-essentially of the other; for the rational account also applies essentially to the one, but to the other in a kind of accidental way, since it is by negation and removal that it throws light on the contrary. For the contrary is the primary privation,Cf. Aristot. Met. 10.4.7. and this is the removal of that to which it is contrary.Literally of the other, i.e. the positive term.
4
And since contrary attributes cannot be induced in the same subject, and science is a potency which depends upon the possession of a rational formula, and the soul contains a principle of motion, it follows that whereas the salutary can only produce health, and the calefactory only heat, and the frigorific only cold, the scientific man can produce both contrary results.
