SUMMA THEOLOGICA. Thomas Aquinas
principles of the species; wherefore it is a medium between the essence and accident thus understood. In this sense the powers of the soul may be said to be a medium between substance and accident, as being natural properties of the soul. When Augustine says that knowledge and love are not in the soul as accidents in a subject, this must be understood in the sense given above, inasmuch as they are compared to the soul, not as loving and knowing, but as loved and known. His argument proceeds in this sense; for if love were in the soul loved as in a subject, it would follow that an accident transcends its subject, since even other things are loved through the soul.
Reply to Objection 6: Although the soul is not composed of matter and form, yet it has an admixture of potentiality, as we have said above ( Question [75], Article [5], ad 4); and for this reason it can be the subject of an accident. The statement quoted is verified in God, Who is the Pure Act; in treating of which subject Boethius employs that phrase (De Trin. i).
Reply to Objection 7: Rational and sensitive, as differences, are not taken from the powers of sense and reason, but from the sensitive and rational soul itself. But because substantial forms, which in themselves are unknown to us, are known by their accidents; nothing prevents us from sometimes substituting accidents for substantial differences.
Whether there are several powers of the soul?
Objection 1: It would seem that there are not several powers of the soul. For the intellectual soul approaches nearest to the likeness of God. But in God there is one simple power: and therefore also in the intellectual soul.
Objection 2: Further, the higher a power is, the more unified it is. But the intellectual soul excels all other forms in power. Therefore above all others it has one virtue or power.
Objection 3: Further, to operate belongs to what is in act. But by the one essence of the soul, man has actual existence in the different degrees of perfection, as we have seen above ( Question [76], Articles [3],4). Therefore by the one power of the soul he performs operations of various degrees.
On the contrary, The Philosopher places several powers in the soul (De Anima ii, 2,3).
I answer that, Of necessity we must place several powers in the soul. To make this evident, we observe that, as the Philosopher says (De Coelo ii, 12), the lowest order of things cannot acquire perfect goodness, but they acquire a certain imperfect goodness, by few movements; and those which belong to a higher order acquire perfect goodness by many movements; and those yet higher acquire perfect goodness by few movements; and the highest perfection is found in those things which acquire perfect goodness without any movement whatever. Thus he is least of all disposed of health, who can only acquire imperfect health by means of a few remedies; better disposed is he who can acquire perfect health by means of many remedies; and better still, he who can by few remedies; best of all is he who has perfect health without any remedies. We conclude, therefore, that things which are below man acquire a certain limited goodness; and so they have a few determinate operations and powers. But man can acquire universal and perfect goodness, because he can acquire beatitude. Yet he is in the last degree, according to his nature, of those to whom beatitude is possible; therefore the human soul requires many and various operations and powers. But to angels a smaller variety of powers is sufficient. In God there is no power or action beyond His own Essence.
There is yet another reason why the human soul abounds in a variety of powers---because it is on the confines of spiritual and corporeal creatures; and therefore the powers of both meet together in the soul.
Reply to Objection 1: The intellectual soul approaches to the Divine likeness, more than inferior creatures, in being able to acquire perfect goodness; although by many and various means; and in this it falls short of more perfect creatures.
Reply to Objection 2: A unified power is superior if it extends to equal things: but a multiform power is superior to it, if it is over many things.
Reply to Objection 3: One thing has one substantial existence, but may have several operations. So there is one essence of the soul, with several powers.
Whether the powers are distinguished by their acts and objects?
Objection 1: It would seem that the powers of the soul are not distinguished by acts and objects. For nothing is determined to its species by what is subsequent and extrinsic to it. But the act is subsequent to the power; and the object is extrinsic to it. Therefore the soul's powers are not specifically distinct by acts and objects.
Objection 2: Further, contraries are what differ most from each other. Therefore if the powers are distinguished by their objects, it follows that the same power could not have contrary objects. This is clearly false in almost all the powers; for the power of vision extends to white and black, and the power to taste to sweet and bitter.
Objection 3: Further, if the cause be removed, the effect is removed. Hence if the difference of powers came from the difference of objects, the same object would not come under different powers. This is clearly false; for the same thing is known by the cognitive power, and desired by the appetitive.
Objection 4: Further, that which of itself is the cause of anything, is the cause thereof, wherever it is. But various objects which belong to various powers, belong also to some one power; as sound and color belong to sight and hearing, which are different powers, yet they come under the one power of common sense. Therefore the powers are not distinguished according to the difference of their objects.
On the contrary, Things that are subsequent are distinguished by what precedes. But the Philosopher says (De Anima ii, 4) that "acts and operations precede the powers according to reason; and these again are preceded by their opposites," that is their objects. Therefore the powers are distinguished according to their acts and objects.
I answer that, A power as such is directed to an act. Wherefore we seek to know the nature of a power from the act to which it is directed, and consequently the nature of a power is diversified, as the nature of the act is diversified. Now the nature of an act is diversified according to the various natures of the objects. For every act is either of an active power or of a passive power. Now, the object is to the act of a passive power, as the principle and moving cause: for color is the principle of vision, inasmuch as it moves the sight. On the other hand, to the act of an active power the object is a term and end; as the object of the power of growth is perfect quantity, which is the end of growth. Now, from these two things an act receives its species, namely, from its principle, or from its end or term; for the act of heating differs from the act of cooling, in this, that the former proceeds from something hot, which is the active principle, to heat; the latter from something cold, which is the active principle, to cold. Therefore the powers are of necessity distinguished by their acts and objects.
Nevertheless, we must observe that things which are accidental do not change the species. For since to be colored is accidental to an animal, its species is not changed by a difference of color, but by a difference in that which belongs to the nature of an animal, that is to say, by a difference in the sensitive soul, which is sometimes rational, and sometimes otherwise. Hence "rational" and "irrational" are differences dividing animal, constituting its various species. In like manner therefore, not any variety of objects diversifies the powers of the soul, but a difference in that to which the power of its very nature is directed. Thus the senses of their very nature are directed to the passive quality which of itself is divided into color, sound, and the like, and therefore there is one sensitive power with regard to color, namely, the sight, and another with regard to sound, namely, hearing. But it is accidental to a passive quality, for instance, to something colored, to be a musician or a grammarian, great or small, a man or a stone. Therefore by reason of such differences the powers of the soul