Book Two — Creation, Chapter 75 — Summa Contra Gentiles
[1] We must now show the inefficacy of the arguments put forward with the object of proving the unit of the possible intellect.
[2] For it seems that every form which is one specifically and many in number is individuated by matter; because things one in species and many in number agree in form and differ in matter. Therefore, if the possible intellect is multiplied numerically in different men, while being specifically one, then it must be individuated in this and that man by matter. But this individuation is not brought about by matter which is a part of the intellect itself, since in that case the intellect’s receptivity would be of the same genus as that of prime matter, and it would receive individual forms; which is contrary to the nature of intellect. It remains that the intellect is individuated by that matter which is the human body and of which the intellect is held to be the form. But every form individuated by matter of which that form is the act is a material form. For the being of a thing must stem from that to which it owes its individuation; since just as common principles belong to the essence of the species, so individuating principles belong to the essence of this individual thing. It therefore follows that the possible intellect is a material form, and, consequently, that it neither receives anything nor operates without a bodily organ. And this, too, is contrary to the nature of the possible intellect. Therefore, the possible intellect is not multiplied in different men, but is one for them all.
[3] Also, if the possible intellect in this and that man were distinct, then the species understood would be numerically distinct in this and that man, though one in species. For the possible intellect is the proper subject of species actually understood, so that, with a multiplication of possible intellects, the intelligible species must be multiplied numerically in those diverse intellects. Now, species or forms which are specifically the same and numerically diverse are individual forms. And these cannot be intelligible forms, because intelligibles are universal, not particular. Therefore, the possible intellect cannot be multiplied in diverse human individuals; it must be one in all.
[4] And again, the master imparts the knowledge that he possesses to his disciple. Hence, either he conveys numerically the same knowledge or a knowledge numerically, but not specifically, diverse. The latter seems impossible, because in that case the master would cause his own knowledge to exist in his disciple, even as he causes his own form to exist in something else by begetting one specifically like to himself; and this seems to apply to material agents. It follows that the master causes numerically the same knowledge to exist in the disciple. But, unless there were one possible intellect for both persons, this would be impossible. So, the existence of one possible intellect for all men seems to be a necessary conclusion.
[5] Nevertheless, just as this doctrine is devoid of truth, as we have shown, so the arguments put forward to confirm it are easy of solution.
[6] As to the first argument adduced above, we admit that the possible intellect is specifically one in different men and yet is numerically many; though this is not to be taken so as to emphasize the fact that man’s parts are not ascribed to his generic or specific essence as such, but only as principles of the whole man. Nor does it follow that the possible intellect is a material form dependent on the body for its being. For just as it belongs to the human soul by its specific nature to be united to a particular species of body, so this particular soul differs only numerically from that one as the result of having a relationship to a numerically different body. In this way are human souls individuated in relation to bodies, and not as though their individuation were caused by bodies; and so the possible intellect, which is a power of the soul, is individuated likewise.
[7] Averroes’ second argument fails because it does not distinguish between that by which one understands and that which is understood. The species received into the possible intellect is not that which is understood; for, since all arts and sciences have to do with things understood, it would follow that all sciences are about species existing in the possible intellect. And this is patently false, because no science, except logic and metaphysics, is concerned with such things. And yet, in all the sciences, whatever is known is known through those species. Consequently, in the act of understanding, the intelligible species received into the possible intellect functions as the thing by which one understands, and not as that which is understood, even as the species of color in the eye is not that which is seen, but that by which we see. And that which is understood is the very intelligible essence of things existing outside the soul, just as things outside the soul are seen by corporeal sight. For arts and sciences were discovered for the purpose of knowing things as existing in their own natures.