Book Two — Creation, Chapter 73 — Summa Contra Gentiles
[1] On the basis of what has already been said it can be clearly demonstrated that there is not one possible intellect of all present, future and past men, as Averroes imagined.
[2] For it has been proved that the substance of the intellect is united to the human body as its form. But one form cannot possibly exist in more than one matter, because the proper act comes to be in the proper potentiality, since they are proportioned to one another. Therefore, there is not one intellect of all men.
[3] Moreover, every mover ought to have its proper instruments; the flute-player uses one kind of instrument, the builder another. Now, the intellect is related to the body as its mover, as Aristotle shows in De anima III [10]. So, just as it is impossible for the builder to use a flute-player’s instruments, so is it impossible for the intellect of one man to be the intellect of another.
[4] Again, Aristotle in De anima I [3] takes the ancients to task for discussing the soul without saying anything about its proper recipient, “as if it were possible, as in the Pythagorean fables, that any soul might put on any body.” It is, then, impossible for the soul of a dog to enter the body of a wolf, or for a man’s soul to enter any body other than a man’s. But the proportion between man’s soul and man’s body is the same as between this man’s soul and this man’s body. Therefore, the soul of this man cannot possibly enter a body other than his own. But it is this man’s soul by which this man understands: man understands by his soul, as Aristotle puts it in De anima I [4]. Hence, this man and that man have not the same intellect.
[5] Then, too, a thing owes its being and its unity to the same principle, for unity and being are consequent upon one another. But every thing has being through its form. Therefore, a thing’s unity follows upon the unity of its form. Hence, there cannot possibly be one form of diverse individual things. But the form of this particular man is his intellective soul. Therefore, it is impossible that there should be one intellect for all men.
[6] Now, if it be said that this man’s sensitive soul is distinct from that man’s, and that to this extent there is not one man although there is one intellect-such an argument cannot stand. For each thing’s proper operation is a consequence and a manifestation of its species. Now, just as the proper operation of an animal is sensation, so the operation proper to man is understanding, as Aristotle says in Ethics I [7]. It is therefore necessary that just as this individual is an animal because it possesses the power of sensation, as Aristotle remarks in De anima II [2], so is be a man in virtue of that by which he understands. But “that whereby the soul, or man through the soul, understands,” is the possible intellect, as the same philosopher says in De anima III [4]. This individual, then, is a man through the possible intellect. Now, suppose that this man has a distinct sensitive soul from that man’s, and yet not a distinct possible intellect but one and the same possible intellect. The consequence is obviously impossible, namely, that this man and that man will be two animals, but not two men. Therefore, there is not one possible intellect of all men.
[7] Now, the Commentator Averroes replies to these arguments by saying that the possible intellect comes into contact with us through its form, that is, by the intelligible species, whose single subject is the phantasm existing in us and which is distinct in distinct subjects. Thus, the possible intellect is particularized in diverse subjects, not by reason of its substance but of its form.
[8] It is clear from what has been said above” that this reply is worthless. For, if the possible intellect makes contact with us only in that way, man’s understanding is rendered impossible, as we have shown.”
[9] But, even if we supposed that the contact in question sufficed to account for man’s knowing, Averroes’ reply still fails to solve the arguments we adduced. For in the Averroistic theory under consideration nothing pertaining to the intellect save only the phantasm will be particularized in accordance with the number of men. Nor will this phantasm itself be particularized so far as it is actually understood, because in this state it exists in the possible intellect, being abstracted from material conditions by the agent intellect. But the phantasm, as understood potentially, is not above the level of being of the sensitive soul; so that this man will still remain indistinguishable from that man, except as concerns the sensitive soul; and there will follow the incongruity previously noted, namely, that this and that man are not several men.
[10] Moreover, a thing derives its species, not from that which is in potentiality, but from that which is in act. Yet the phantasm, as particularized, has only a potentially intelligible being. Therefore, it is not to the phantasm as particularized that this individual owes the specific character of intellective animal, which is the nature of man. And so we have the same result as before, namely, that the thing from which man’s specific nature is derived is not particularized in diverse subjects.