We shall now explain the difference obtaining between a part and a faculty of the soul. One part differs from another by the characteristics of its genus (or, kind); while different faculties may relate to a common genus. That is why Aristotle did not allow that the soul contained parts, though granting that it contained faculties. Indeed, the introduction of a new part changes the nature of the subject, while the diversity of faculties does not alter its unity. Longinus did not allow in the animal (or, living being) for several parts, but only for several faculties. In this respect, he followed the doctrine of Plato, according to whom the soul, in herself indivisible, is divided within bodies. Besides, that the soul does not have several parts does not necessarily imply that she has only a single faculty; for that which has no parts may still possess several faculties.
To conclude this confused discussion, we shall have to lay down a principle of definition which will help to determine the essential differences and resemblances that exist either between the parts of a same subject, or between its faculties, or between its parts and its faculties. This will clearly reveal whether in the organism the soul really has several parts, or merely several faculties, and what opinion about them should be adopted. (For there are two special types of these.) The one attributes to man a single soul, genuinely composed of several parts, either by itself, or in relation to the body. The other one sees in man a union of several souls, looking on the man as on a choir, the harmony of whose parts constitutes its unity, so that we find several essentially different parts contributing to the formation of a single being.
First we shall have to study within the soul the differentials between the part, the faculty and the disposition. A part always differs from another by the substrate, genus, and function. A disposition in a special aptitude of some one part to carry out the part assigned to it by nature. A faculty is the habit of a disposition, the power inherent in some part to do the thing for which it has a disposition. There was no great inconvenience in confusing faculty and disposition; but there is an essential difference between part and faculty. Whatever the number of faculties, they can exist within a single “being,” or nature, without occupying any particular point in the extension of the substrate, while the parts somewhat participate in its extension, occupying therein a particular point. Thus all the properties of an apple are gathered within a single substrate, but the different parts that compose it are separate from each other. The notion of a part implies the idea of quantity in respect to the totality of the subject. On the contrary, the notion of a faculty implies the idea of totality. That is why the faculties remain indivisible, because they penetrate the whole substrate, while the parts are separate from each other because they have a quantity.
How then may we say that a soul is indivisible, while having three parts? For when we hear it asserted that she contains three parts in respect to quantity, it is reasonable to ask how the soul can simultaneously be indivisible, and yet have three parts. This difficulty may be solved as follows: the soul is indivisible in so far as she is considered within her “being,” and in herself; and that she has three parts in so far as she is united to a divisible body, and that she exercises her different faculties in the different parts of the body. Indeed, it is not the same faculty that resides in the head, in the breast, or in the liver; (the seats of reason, of anger and appetite). Therefore, when the soul has been divided into several parts, it is in this sense that her different functions are exercised within different parts of the body.
Nicholas (of Damascus), in his book “On the Soul,” used to say that the division of the soul was not founded on quantity, but on quality, like the division of an art or a science. Indeed, when we consider an extension, we see that the whole is a sum of its parts, and that it increases or diminishes according as a part is added or subtracted. Now it is not in this sense that we attribute parts to the soul; she is not the sum of her parts, because she is neither an extension nor a multitude. The parts of the soul resemble those of an art. There is, however, this difference, that an art is incomplete or imperfect if it lack some part, while every soul is perfect, and while every organism that has not achieved the goal of its nature is an imperfect being.
Thus by parts of the soul Nicholas means the different faculties of the organism. Indeed, the organism, and, in general, the animated being, by the mere fact of possessing a soul, possesses several faculties, such as life, feeling, movement, thought, desire, and the cause and principle of all of them is the soul. Those, therefore, who distinguish parts in the soul thereby mean the faculties by which the animated being can produce actualizations, or experience affections. While the soul herself is said to be indivisible, nothing hinders her functions from being divided. The organism, therefore, is divisible, if we introduce within the notion of the soul that of the body; for the vital functions by the soul communicated to the body must thereby necessarily be divided by the diversity of the organs, and it is this division of vital functions that has caused parts to be ascribed to the soul herself. As the soul can be conceived of in two different conditions, according as she lives within herself, or as she declines towards the body, it is only when she declines towards the body that she splits up into parts. When a seed of corn is sowed, and produces an ear, we see in this ear of corn the appearance of parts, though the whole it forms be indivisible, and these indivisible parts themselves later return to an indivisible unity; likewise, when the soul, which by herself is indivisible, finds herself united to the body, parts are seen to appear.
We must still examine which are the faculties that the soul develops by herself (intelligence and discursive reason), and which the soul develops by the animal (sensation). This will be the true means of illustrating the difference between these two natures (“beings”), and the necessity of reducing to the soul herself those parts of her “being” which have been enclosed within the parts of the body.
(KSGP IV)