Lecture 2 — Rational and irrational potencies

And since some such principles are present in non-living things, and others in living things and in the soul, and in the soul having reason, it is evident that some potencies will be devoid of reason and others will be rational. And for this reason, all the arts and productive sciences are potencies, for they are principles of change in some other thing inasmuch as it is other.

And all those potencies which are rational are open to contrary determinations, and those which are irrational are each determined to one thing. For example, what is hot is capable of heating, whereas the medical art is concerned with both sickness and health.

And the reason for this is that science is a conception, and the same conception explains both a thing and its privation, though not in the same way. And in one sense, it is a conception of both, and in another it applies rather to the existent thing. Hence it is necessary that such sciences should deal with contraries, but with one directly and with the other indirectly. For the conception applies to one essentially, but to the other in a kind of accidental way, because it explains the contrary by negation and removal. For the contrary is the primary privation, and this is the removal of the other term.

Also, since contraries do not exist in the same subject, and since science is a potency in what has reason, and the soul has a principle of motion, it follows that while what is healthful produces only health, and what is capable of heating produces heat, and what is capable of cooling produces cold, one who has a science may thus have both contraries.

For reason extends to both, but not in the same manner, and it exists in a soul that possesses a principle of motion. Hence the soul will initiate both by the same principle by joining both to the same rational plan.

And for this reason, those things whose potency is rational produce effects contrary to those whose potency is irrational, for one principle of contrary determinations is contained in the rational plan.

It is also evident that a potency for doing something well involves the potency of merely doing something or undergoing some change. But the latter does not always involve the former, for he who does a thing well must do it, but he who does something need not do it well.

1786. Having explained the different senses in which the term “potency” is used, here the Philosopher establishes the truth about potency in relation to the things in which it is found. This is divided into two parts.

In the first (1046a36; [1786]), he shows how these potencies differ from each other on the basis of a difference in their subjects.

In the second (1046b29; [1795]), he shows how potency and act are simultaneous or not in a substance, at there are some.

In regard to the first, he does three things.

First, he shows how potencies differ on the basis of a difference in their subjects. He says that, since potencies are principles both for acting and being acted upon, some of these principles are in non-living things and some in living ones. And since living things are composed of body and soul, and the principles for acting and being acted upon which are present in the body of living things do not differ from those in non-living ones, he therefore adds, and in the soul, because the principles of action which are present in the soul clearly differ from those present in non-living things.

Subjects of potency

1787. Again, there are several kinds of souls, and many of these do not differ to any great extent, either in acting or in being acted upon, from non-living things that act by natural instinct, for the parts of the nutritive and sentient soul act by natural impulse. Now, only the rational part of the soul has dominion over its acts, and it is in this respect that it differs from non-living things. Therefore, having pointed out the difference between souls, he adds, and in the soul having reason, because those principles of living things that are found in the rational part of the soul differ specifically from those of non-living things. Hence it is evident that some powers of the soul are irrational and others rational.

1788. He explains what he means by those which are rational when he adds that all the arts and productive sciences—like the building and constructive arts, whose actions pass over into external matter, and all sciences which do not perform actions that pass over into external matter, like the moral and logical sciences—all arts of this kind, I say, are powers. And this is concluded from the fact that they are principles of change in some other thing inasmuch as it is other. This is the definition of active power, as is clear from what was said above.

1789. And all those (1046b4). Second, he gives the difference between the above-mentioned potencies. He says that the same rational potencies are open to contrary determinations, just as the art of medicine, which is a potency, as has been explained [1404–7], can produce both health and sickness. But irrational potencies are not open to contrary determinations, but properly speaking, each is determined to one thing. For example, the heat of the sun has as its proper effect to heat, although it can be the cause of coldness inasmuch as, by opening the pores, it causes the loss of internal heat, or by absorbing the matter of a hot humor, it destroys the heat, and thereby cools.

1790. And the reason (1046b7). Then the Philosopher gives the reason for the aforesaid difference, and it is as follows. A science, which is a rational potency, is a conception of the thing known existing in the mind. Now, the same conception explains both the thing and its privation, although not in the same way, because it first makes known the existing thing and subsequently its privation. For example, the power of sight itself is known properly by means of the notion of sight, and then blindness is known, which is nothing but the very lack of sight in a thing naturally disposed to have it. Hence, if science is a conception of the thing known existing in the mind, the same science must deal with contraries, with one primarily and properly, and with the other secondarily. For example, the art of medicine is cognitive and productive primarily of health and secondarily of sickness, because, as has been pointed out, this art has to do with the conception of the thing known in the mind, and this conception is of one of the contraries directly and of the other indirectly.

1791. And since the remarks which the Philosopher had made above about privation he afterward transferred to contraries, he shows that the same conception applies to a contrary and to a privation. For just as a privation is explained by negation and removal (for example, the removal of sight explains blindness), in a similar fashion, a contrary is explained by negation and removal, because privation, which is merely the removal of some attribute, is a sort of first principle among contraries. For in the case of all contraries, one stands as something perfect and the other as something imperfect and the privation of the former. Black, for example, is the privation of white, and cold is the privation of heat. Thus, it is evident that the same science extends to contraries.

1792. Also, since (1046b15). He next develops this point, and he begins to give the reason for the aforesaid difference. For it is clear that natural things act by reason of the forms present in them. But contrary forms cannot exist in the same subject. Therefore, it is impossible for the same natural thing to produce contrary effects. But science is a potency for acting and a principle of motion, because a person has an idea of the thing to be made and this principle of motion is in the mind. And since this is so, it follows that natural things produce only one effect. For example, what is healthful produces only health, and what is capable of heating produces only heat, and what is capable of cooling produces only cold. But one who acts by science may be occupied with both contraries, because the conception of both contained in the soul is the same. For the soul possesses the principle of such motion, although not in the same way, as has been explained.

1793. Therefore, just as a natural action proceeds to bring about its effect as though it were united to its form, which is the principle of action whose likeness remains in the effect, in a similar fashion, the soul proceeds by its action to bring about both opposites by the same principle, that is, by the conception which is one for the two opposites, uniting both motions to this principle and causing both to terminate in it, inasmuch as the likeness of this principle is verified in both of the opposites brought into being. Therefore, it is evident that rational powers produce an effect opposite to irrational powers, because a rational power produces contrary effects, whereas an irrational power produces only one effect. The reason is that a single principle of contrary effects is contained in the conception belonging to a science, as has been explained.

1794. It is also evident (1046b24). He explains the relationship of some of the senses of “potency” mentioned above to those which come under them. For it was stated above that a thing is said to have active or passive potency, sometimes only because it can act or be acted upon, and sometimes because it can act or be acted upon well. Therefore, he says that the potency for acting or being acted upon well involves the potency for acting or being acted upon, but not the reverse. For it follows that someone acts if he acts well, but the opposite of this is not true.