Unity of the Possible Intellect

An objector may say: the intellect is indeed incorruptible, but there is only one intellect in all men; and so what remains after the corruption of all men is but one. That there is only one intellect for all men, the objector may continue, can be established on many grounds.

First, on the part of the intelligible species. If I have one intellect and you have another, there will have to be one intelligible species in me and another in you, and consequently there will be one object that I understand and another that you understand. Hence the intelligible species will be multiplied according to the number of individuals, and so it will not be universal but individual. The conclusion would then seem to follow that it is understood not in act, but only in potency; for individual species are intelligible in potency, not in act.

Moreover, since the intellect, as we have seen, is a substance subsisting in its own being, and since intellectual substances that are numerically many do not belong to one species, as we have also seen, it follows that if I have one intellect and you have another that is numerically different, the two must differ specifically. And so you and I would not belong to the same species.

Furthermore, since all individuals share in one specific nature, there must be something besides specific nature whereby individuals may be distinguished from one another. Accordingly, if there is one specific intellect in all men, but many intellects that are numerically distinct, something must be found that will make one intellect differ numerically from another. This cannot be anything pertaining to the substance of the intellect, since the intellect is not composed of matter and form. Consequently any difference that might be admitted, on the part of the substance of the intellect, would be a formal difference that would cause diversity in the species. The only possibility left is that the intellect of one man cannot differ numerically from the intellect of another man except by reason of the diversity of their bodies. Therefore, when the various bodies corrupt, it seems that only one intellect, and not a plurality of intellects, would remain.

The absurdity of this whole position is easily perceived. To make this clear, let us proceed as one would proceed against those who deny fundamental principles. That is, let us establish a truth that simply cannot be denied. Let us suppose that this man, for example, Socrates or Plato, understands. Our adversary could not deny that the man understands, unless he knew that it ought to be denied. By denying he affirms, for affirmation and denial are intelligent actions. If, then, the man in question understands, that whereby he formally understands must be his form, since nothing acts unless it is in act. Hence that whereby an agent acts, is his act; just as the heat by which a heated body causes warmth, is its act. Therefore the intellect whereby a man understands is the form of this man, and the same is true of another man. But the same numerical form cannot belong to numerically different individuals, for numerically different individuals do not possess the same existence; and yet everything has existence by reason of its form. Accordingly the intellect whereby a man understands cannot be but one in all men.

Perceiving the force of this difficulty, some endeavor to find a way of escaping it. They say that the possible intellect, of which there was question above, receives the intelligible species by which it is reduced to act. These intelligible species are, in some way, in the phantasms. Hence the possible intellect is continuous and is joined to us so far as the intelligible species is both in the possible intellect and in the phantasms that are in us. It is thus that we are able to understand through the agency of the possible intellect.

Unfortunately for this solution, it is utterly valueless. In the first place, the intelligible species, as it exists in the phantasms, is a concept only in potency; and as it exists in the possible intellect, it is a concept in act. As existing in the possible intellect, it is not in the phantasms, but rather is abstracted from the phantasms. Hence no union of the possible intellect with us remains. Secondly, even granting that there may be some sort of union, this would not suffice to enable us to understand. The presence of the species of some object in the intellect does not entail the consequence that the object understands itself, but only that it is understood; a stone does not understand, even though a species of it may be in the possible intellect. Hence, from the fact that species of phantasms present in us are in the possible intellect, it does not follow that we thereupon understand. It only follows that we ourselves, or rather the phantasms in us, are understood.

This will appear more clearly if we examine the comparison proposed by Aristotle in Book III of De anima [7, 431 a 14], where he says that the intellect is to phantasm what sight is to color. Manifestly, the fact that the species of colors on a wall are in our vision does not cause the wall to see, but to be seen. Likewise, the fact that the species of the phantasms in us come to be in the intellect, does not cause us to understand, but to be understood. Further, if we understand formally through the intellect, the intellectual action of the intellect must be the intellectual action of the man, just as the heating action of fire and of heat are the same. Therefore, if intellect is numerically the same in me and in you, it follows that, with respect to the same intelligible object, my action of understanding must be the same as yours, provided, of course, both of us understand the same thing at the same time. But this is impossible, for different agents cannot perform one and the same numerical operation. Therefore it is impossible for all men to have but a single intellect. Consequently, if the intellect is incorruptible, as has been demonstrated many intellects, corresponding to the number of men, will survive the destruction of their bodies.

The arguments advanced to support the contrary view are easily answered.

The first argument has many defects. First of all, we concede that the same thing may be understood by all. By the thing understood I mean that which is the object of the intellect. However, the object of the intellect is not the intelligible species, but the quiddity of the thing. The intellectual sciences are all concerned with the natures of things, not with intelligible species; just as the object of sight is color, not the species of color in the eye. Hence, although there may be many intellects belonging to different men, the thing understood by all may be but one; just as a colored object which many look at is but one. Secondly, the consequence does not necessarily follow that, if a thing is individual, it is understood in potency and not in act. This is true only of things that are individuated by matter. Of course, what is understood in act must be immaterial. Accordingly immaterial substances, even though they may be individuals existing by themselves, are understood in act. The same holds for intelligible species, which are immaterial; although they differ numerically in me and in you, they do not on that account lose their property of being intelligible in act. The intellect that understands its objects by means of them reflects upon itself, thereby understanding its very action of understanding as well as the species whereby it understands. Moreover, we should realize that, even if we admit but one intellect for all men, the difficulty is still the same. There would still remain many intellects, because there are many separate substances endowed with intelligence. And so it would follow, pursuing our adversaries’ line of reasoning, that the objects understood would be numerically different, hence individual and not understood in first act. Obviously, therefore, if the objection under discussion had any cogency, it would do away with a plurality of intellects simply as such, and not merely in men. Since this conclusion is false, the argument manifestly does not conclude with necessity.

The second argument is readily answered, if we but consider the difference between an intellectual soul and separate substances. In virtue of its specific nature, the intellectual soul is meant to be united to some body as the latter’s form; the body even enters into the definition of the soul. For this reason, souls are numerically differentiated according to the relation they have to different bodies; which is not the case with separate substances.

This also indicates how the third argument is to be answered. In virtue of its specific nature, the intellectual soul does not possess the body as a part of itself, but has only an aptitude for union with the body. Therefore it is numerically differentiated by its capacity for union with different bodies. And this remains the case with souls even after their bodies have been destroyed: they retain a capacity for union with different bodies even when they are not actually united to their respective bodies.


St. Thomas Aquinas. (1265-1274). Compendium Theologiae: Unity of the Possible Intellect, trans. by Cyril Vollert. St. Louis & London: B. Herder Book Co., 1947

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