The intellect is not the essence of the soul, but a faculty; for in God only, Whose operation is His Being, can intellect be the same as essence. Therefore faculty stands to operation, as essence to being; and understanding, in creatures, is a faculty of the being who understands.
Our intellect, indeed, is a passive power in respect of intelligible things, and at the first like a clean slate. For the human intellect is lowest in order of intelligence and furthest removed from the Intellect of God, as appears from the fact that at first it is only in potentia and afterwards is made actually intelligent.
The Divine Intellect stands towards universal being as the actuality of all, because all beings pre-exist therein as in their First Cause; and since no created being can be actualized in respect of all being, which would require it to be infinite, every created intellect is related to the things which are intelligible to it as the potential to the actual. The potential, however, bears a twofold relation to the actual, because some things are always perfectly actuated; others, such as matter, which is susceptible of generation and dissolution, are not so. Thus the angelic intellect is always perfectly actualized on account of its nearness to the First Intellect, which is Pure Actuality; but ours is passive according to the third mode of passivity, inasmuch as whatever is in potentia to anything either receives the same, or, not possessing it, remains subject to that thing.
It is necessary, moreover, to admit an intellectus agens, for nothing which is in potentia can be actualized except by some Being itself actual, and we have seen that our intellect is in potentia to intelligible things. Hence, according to Aristotle, it must be actualized by something which we call the "active intellect "; although it is not so according to Plato, who held the theory of " ideas," by participation with which he supposed both matter and souls to come into existence, the intelligible species being actualized by itself.
We see, also, that there must be some intellectus agens in the soul, because, although it participates in a higher intellect by which it is assisted, yet, owing to its discursive mode of understanding, and gradual conversion from the potential to the actual, it neither understands all things, nor perfectly. There must, therefore, be some virtue received from the Superior Intellect, inherent in the soul itself, by which things are made actually intelligible; as is the case with other perfect things. For besides causes which act universally there are infused virtues, proper to things, which are derived from the Superior Intellect, whereby the intellect of man is enabled to elucidate phantasms, elicit universal conditions from particular ones, and make things actually intelligible.
But every action belongs to things in virtue of some principle formally inherent in them; therefore the power which is the principle of such action is something in the soul itself. And since the active intellect is something in the soul itself, it follows that it must be multiplied according to the plurality of souls; for the same power cannot belong to several subjects. While, if it were something separate, the intellect of all men would be one; which has been disproved above.
Memory also belongs to the intellectual part, if it be understood as the power of preserving species. Understood as the power of recalling the past, it belongs rather to the sensitive part; for the sensitive soul apprehends particulars.
Nor can there be any difference of faculties in the intellect except that of possible and acting. For there is a difference between the active faculty, which renders an object actual; and the passive faculty, which is moved by an object already existing. Thus the active faculty, compared with its object, is as actual being to being in potentia; and the passive faculty vice versa: because the faculty which receives and retains is not distinct from the intellect itself.
Nor is reason a faculty distinct from the intellect. For to understand is simply to apprehend intelligible truth; and to reason is to proceed from one intellection to another in order to arrive at the knowledge of intelligible truth. Thus the angels, who possess such knowledge perfectly, do not require to go through the process of reasoning, since they have by simple intelligence what man acquires discursively. Reasoning, therefore, compared to understanding, is as movement to rest or acquiring to possessing; which processes are the work of the same faculty, and differ only as the imperfect differs from the perfect.
Neither are the superior and inferior reason different faculties; they are distinguished only by their operations and diversity of use; for the medium and the end both belong to the faculty of reason. The act of reasoning, indeed, somewhat resembles that kind of motion in which the moveable passing through the medium is the same as that which arrives at the end; and thus wisdom is attributed to the superior reason and knowledge to the inferior.
Nor is the speculative intellect, which regards only the apprehension of truth, a different faculty from the practical intelligence which directs the work; they only differ as to their end. For that which is accidental to the conception of the object does not diversify the faculty; and something is apprehended by the intellect, whether it be ordered to the work or not: hence all are apprehended by the same faculty.
Synderesis is not a faculty, but a natural habit, by which we are impelled to good and indisposed towards evil. For as speculative principles are implanted in us, so also are operative ones. Both are natural, not pertaining to any special faculty, but to a particular use of the intellect; which in speculative things is called the faculty of understanding, and in practical things, synderesis.
Conscience, likewise, is an act, not a faculty. For the order of knowledge requires that somewhat be said to testify and judge as to that which should be done, or left undone; to deter or instigate; to judge of right and wrong; to accuse, or excuse, or to cause remorse. But inasmuch as the habit is the principle of the act, the name " conscience" is sometimes applied to the first natural habit of the mind, viz. synderesis; for, by a common figure of speech, cause and effect are taken the one for the other.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni