Aristotle introduces the question of the potencies of the soul in the following manner.19 The soul, he says, is defined as the principle of many diverse activities, such as to sense, to desire, to think, and to move from place to place. Does a living being perform each of these activities through the soul in its entirety, or should we say that they correspond to different parts in the soul? Leaving aside the rather complicated answer presented in De Anima, we shall go directly to the Summa of St. Thomas, which contains a good summary of the whole matter.20
1. The Essence of the Soul Cannot Be Its Potencies
The question, as we have said, is whether one has to admit the existence of certain principles of operation that are distinct from the essence of the soul. St. Thomas has a whole series of arguments to prove that such distinct principles must be acknowledged.21
a) In any given order of being, act and potency must belong to the same highest genus within that order. But it is clear that the operations of the soul do not belong to the genus of substance; therefore the corresponding potencies cannot belong to this genus. It follows that they are accidents; and if accidents, they are really distinct from the essence of the soul.
b) From the standpoint of its essence the soul is in act. If, then, it were the immediate principle of operation, one should have to say that it is continuously performing its operations. But experience does not bear out such a statement. Hence, the soul is not the immediate principle of operation.
c) Being diverse, the activities of the soul cannot be traced to a single pi inciple, yet the soul obviously is a single principle. Consequently, there must be a plurality of potencies, distinct from the soul, to account for the diversity of activities performed.
d) Sonic potencies are the acts of definitely determined bodily organs, but others are not. Manifestly, the essence of the soul, being one, cannot be in this twofold situation at the same time. So, there are distinct potencies for such distinct modes of operation.
e) Some potencies act upon others; for example, reason acts upon the sense appetite, both in its concupiscible and irascible form. This is clearly impossible if one does not admit, in addition to the essence of the soul, a plurality of potencies.
These are some of the arguments to prove that the potencies are distinct from the essence of the soul. Apropos of these arguments, the following points should be kept in mind:
First, the distinction in question between the soul and its faculties is, plainly, nothing less than a real one.
Second, the faculties are to be understood as belonging to the genus "quality," in the second of the four species of quality.
Third, the potencies that are linked to a bodily organ exist in the composite, that is, they inhere in the whole living being as in their subject; whereas the potencies that do not need a bodily organ for their operations, inhere directly in the soul.
Fourth, the potencies are said to "emanate" or issue from the essence of the soul; in some way, therefore, the soul may be considered as the cause of the potencies.
2. Concerning the Specification of the Potencies of the Soul
To the question whether the potencies of the soul are one or many, the obvious answer is that they are many. The multiplicity and diversity of operations displayed by living beings, especially by the higher types, cannot be explained without recourse to a plurality of potencies. How, then, are potencies distinguished from one another? St. Thomas, following the general principles of his metaphysics, teaches that they are distinguished or specified by their acts and their objects:
potentiae animae distinguuntur per actus et objecta.22
This principle stems from the very nature of a potency, since potency always implies order or relation to act. It follows, then, that potencies are diversified by the acts to which they are ordered. Acts, in turn, are specified by their objects, whether they be acts of passive potencies, which are moved by their objects, or acts of active potencies, which tend toward their objects as toward an end. In every case, therefore, we must say that potencies are specified by their objects through the medium of their acts. Furthermore, it should be noted that the differences to be considered in the object are those to which the potencies are disposed or ordered by their proper nature. For example, the senses are diversified by different qualities in the sensible object considered precisely as sensible, such as color or sound, and not by any difference that is accidental to the sense quality. Thus, color is the object of sight, but something that is colored may also be a grammarian. Nevertheless, being a grammarian is accidental to its color considered as the formal object of sight.
This doctrine of the specification of potencies by their acts and objects is of the uttermost importance in St. Thomas. It guides the whole order and elaboration of his psychology, and forms the basis for the distinction of habits or virtues; thus, the whole development of his moral doctrine also depends on it. In particular, the treatise on the virtues in the Secunda Secundae, both so penetrating and thoroughgoing, is only a sustained application of this truth and principle.
3. The Kinds of Soul and the Division of Potencies
St. Thomas treats of the kinds of soul and the division of potencies in several places.23 For our purpose it will be sufficient to outline the main points of the corresponding article in the Summa, where he gives an excellent summarization of the whole matter, setting forth in order the division of the soul into its kinds, the distinction of potencies, and the grades of life.24
a) There are three kinds of soul. - This enumeration rests on the very basic principle that not all vital activity is of the same kind. The fact is that vital operations differ as to greater or lesser dependence on the body and its activities, and these differences of operation betoken different kinds of life.
Accordingly, we may successively note: first, the rational soul, whose operation does not require the exercise of a bodily organ; next, the sensitive soul, which acts only through the medium of an organ, yet, strictly speaking, does not require any contribution from the properties of the material elements, since the elements serve only to dispose the organ; lastly, the vegetative soul, which, in addition to the activity of appropriate organs, implies the activity of material elements. In beings having a higher degree of life, the higher soul assumes the functions of the lower. Thus, in man the one rational soul is at once the principle of intellectual life, of sensitive life, and of vegetative life.
b) There are five distinct genera of potencies. - This classification is based, in part, on the scope of the object encompassed by a potency, for the higher the potency, the more inclusive is the object it considers.
From this point of view, then, we can divide objects into three large genera: first, the individual body united to the soul; second, the whole aggregate of sensible bodies; and third, all being universally. Corresponding to these three classes, and following the order of increasing perfection, there are the following distinct potencies: first, the vegetative potencies, which act only on the body in which they exist; secondly, relative to the two oilier genera of objects, two further genera of potencies, one in the order of knowledge, including sense and intellect, and the other in the order of appetition, including appetite and the locomotive power. In man, therefore, there are five distinct genera of potencies or faculties, which St. Thomas designate; as follows:
vegetativum, sensitivum, intellectivum, appetitivum, motivum secundum locum.
These, moreover, may be subdivided into several species.
c) There are four grades or modes of life. - This last division is predicated on the hierarchy of perfection in living beings, the perfection increasing with the number and variety of powers or faculties possessed by such beings. In the lowest order are those beings having only vegetative powers; these are the plants. Next are those that have both sensory and vegetative powers, but not self-locomotion; these are the lower animals. Still higher are those beings which, together with the aforesaid powers, have the capacity to move themselves locally, making it possible to move about in search of the things they need to live. But highest of all in nature are those beings endowed both with all the foregoing powers and with intellect; these are men. Appetite as such does not constitute a distinct grade of life, since it is found analogically in all being.