Book Two — Creation, Chapter 60 — Summa Contra Gentiles
[1] These arguments are countered by others in keeping with the doctrine considered above. For Averroes says that man differs specifically from the brutes by the intellect which Aristotle calls passive and which is the same as the cogitative power that is proper to man, in place of which the other animals have a certain natural estimative power. Now, it is the function of this cogitative power to distinguish individual intentions and to compare them with one another, even as the intellect which is separate and unmixed compares and distinguishes universal intentions. And by this cogitative power, together with the imagination and memory, the phantasms are prepared to receive the action of the agent intellect, whereby they are made intelligible in act, just as there are certain arts which prepare the matter for the master artificer. Accordingly, this power is given the name of intellect or reason, which physicians declare to be seated in the middle cell of the head. And according to the disposition of this power, one man differs from another in genius and in other qualities pertaining to understanding. And by the use and exercise of this power a man acquires the habit [habituss] of science. Hence, the habits of the sciences are in this passive intellect as their subject. Moreover, this passive intellect is in the child from the beginning, and through it the child receives its specific nature as a human being, before it actually understands.
[2] But it is quite obvious that these notions are false and involve an abuse of terms. For the vital operations are compared to the soul as second acts to the first act, as Aristotle makes clear in De anima II [1]. Now, in the same thing first act precedes the second in time, just as knowledge precedes reflection, Consequently, in whatever thing we find a vital operation we must place a part of the soul which will be related to that operation as first act to second act. But man has a proper operation higher than the other animals, namely, understanding and reasoning, which is the operation of man as man, as Aristotle says in Ethics I [7]. Hence, we must attribute to man a principle that properly gives him his specific nature and is related to the act of understanding as first act to second act. Now, this principle cannot be the aforesaid passive intellect, because the principle of man’s proper operation must be impassible and not mixed with the body, as Aristotle proves [ De anima III, 4]; whereas, the contrary is clearly true of the passive intellect. Therefore, it is impossible that man’s specific nature, whereby he is distinguished from the other animals, should be given him by the cogitative power, which is called the passive intellect.
[3] Furthermore, an affection of the sensitive part of a thing cannot place it in a higher kind of life than the sensitive, just as an affection of the nutritive soul does not place it in a higher kind of life than the nutritive. Now, it is clear that the imagination, and like powers consequent upon it, such as the memory and so on, are affections of the sensitive part, as Aristotle proves in the De memoria [I]. Hence, an animal cannot be placed by these powers or by any one of them in a higher category of life than the sensitive. But man’s life is of a higher kind—a point clearly explained in De anima II [2], where Aristotle, in distinguishing the kinds of life, places the intellective, which he attributes to man, above the sensitive, which he ascribes to all animals in general. Therefore, it is not by virtue of the aforesaid cogitative power that man is a living being with a life proper to himself.
[4] Then, too, every self-mover is composed of mover and moved, as Aristotle proves in Physics VIII [5]. Now, man, in common with the other animals, is a self-mover. Therefore, mover and moved are parts of him. And the first mover in man is the intellect, since the intellect by its intelligible object moves the will. Nor can it be said that the passive intellect alone is the mover, because the passive intellect has to do with particulars only, whereas, actual movement involves both the universal judgment, which belongs to the possible intellect, and the particular judgment, which can belong to the passive intellect, as Aristotle explains in De anima III [11], and in Ethics VII [3]. Therefore, the possible intellect is a part of man. And it is the most noble and most formal thing in him. Hence, man derives his specific nature from it, and not from the passive intellect.
[5] The possible intellect, moreover, is demonstrably not the act of any body, because it is cognizant of all sensible forms universally. Therefore, no power whose operation can extend to the universals of all sensible forms can be the act of a body. Now, such a power is the will, for our will can reach out to all the things that we can understand, at least our will to know them. And the act of the will is clearly directed to the universal; as Aristotle says in the Rhetoric [II, 4], “we hate robbers in general, but are angry only with individual ones.” Therefore, the will cannot be the act of any part of the body, nor can it follow upon a power that is an act of the body. Now, every part of the soul is an act of the body, with the single exception of the intellect properly so called. Therefore, the will is in the intellective part; and that is why Aristotle says in De anima in: “Will is in the reason, but the irascible and concupiscible appetite are in the sensitive part.” So it is that acts of concupiscence and irascibility involve passion, but not the act of the will, which involves choice. Now, man’s will is not outside him, as though it resided in some separate substance, but is within him. Otherwise, man would not be master of his own actions, since he would then be acted upon by the will of a separate substance, and in him there would be only the appetitive powers functioning in association with passion, namely, the irascible and concupiscible powers, which are in the sensitive part, as in other animals that are acted upon rather than act themselves. But this is impossible and would destroy all moral philosophy and sociality. It follows that there must exist in us the possible intellect, so that by it we differ from brute animals, and not only in terms of the passive intellect.