Summa contra Gentiles2.98 Chapter XCVIII. How one separate substance understands another

If separate substances understand things that are by themselves intelligible, as we have proved; and if separate substances are by themselves intelligible, since freedom from matter makes a thing intelligible by itself, as appears from the foregoing, it follows that separate substances understand separate substances as their proper objects. Wherefore each of them knows itself and others.

Each one indeed knows itself otherwise than the possible intellect knows itself. For the possible intellect is in potentiality in intelligible being, and is made actual by the intelligible species, even as primary matter is made actual in sensible being by a natural form. Now nothing is known according as it is only in potentiality, but a thing is known according as it is in act; hence the form is the principle whereby we know the thing which is made actual thereby, and in like manner the cognitive power is made actually cognizant by some species. Accordingly our intellect does not know itself except by the species whereby it is made actual in intelligible being; for which reason Aristotle (3 De AnimaY says that it is knowable in the same way as other things, namely by species derived from phantasms, as by their proper forms. On the other hand separate substances, by their nature, exist actually in intelligible being. Wherefore each one of them knows itself by its essence, and not by the species of another thing. Since, however, all knowledge is according as the image of the thing known is in the knower; and since one separate substance is like another as regards the common generic nature, while they differ the one from the other in regard to the species, as appears from the foregoing; * it would seem to follow that the one does not know the other, as regards its proper specific nature, but only as regards the common nature of their genus.

Accordingly some say that one separate substance is the efficient cause of another. Now in every efficient cause there must be the image of its effect, and likewise in every effect there must be the likeness of its cause: because every agent produces its like. Hence in the higher separate substance there exists the likeness of the lower, as in the cause there is the likeness of its effect; while in the lower there is the likeness of the higher, as in the effect there is the likeness of its cause. Now if we consider non-univocal causes, the likeness of the effect exists in the cause in a more eminent manner, and the likeness of the cause is in its effect in a less eminent manner. And the higher separate substances must needs be causes of this kind with respect to the lower separate substances: because they are placed in various degrees which are not of one species. Therefore a lower separate substance knows a higher according to the mode of the substance knowing, and not according to the mode of the substance known, but in a lower manner: whereas the higher knows the lower in a more eminent way. This is the meaning of the statement in De Causis, that an intelligence knows what is below it, and what is above it, according to the mode of its substance: because the one is the cause of the other.

But since we have shown above that intellectual separate substances are not composed of matter and form, they cannot be caused except by way of creation. Now to create belongs to God alone, as we proved above. Therefore one separate substance cannot be the cause of another.

Further. It has been proved that the principal parts of the universe are all created immediately by God. Therefore one of them is not caused by another. Now each of the separate substances is a principal part of the universe, much more than the sun or moon: since each of them has its proper species, which is also more noble than any species of things corporeal. Therefore one of them is not caused by another, but all are produced immediately by God.

Hence, according to the foregoing, each of the separate substances knows God by its natural knowledge, according to the mode of its substance, whereby they are like God as their cause. But God knows them as their proper cause, having in Himself the likeness of them all. Yet one separate substance is unable to know another in this way, since one is not the cause of another.

We must, therefore, observe that, since none of these substances according to its essence is an adequate principle of the knowledge of all other things, it is necessary for each of them, in addition to its own substance, to have some intelligible images, whereby each of them is enabled to know another in its proper nature.

This can be made clear in the following manner. The proper object of an intellect is an intelligible being: and this includes all possible differences and species of being: because whatever can be, is intelligible. Now, since all knowledge is caused by some kind of likeness, the intellect is unable to know its object wholly, unless it has in itself the likeness of all being and of all its differences. But such a likeness of all being can only be an infinite nature, which is not confined to any species or genus of being, but is the universal principle and active force of all being: and this is the divine nature alone, as we proved in the First Book. And every other nature, since it is confined to some genus or species of being, cannot be a universal likeness of all being. It follows, therefore, that God alone, by His essence, knows all things; while every separate substance, by its nature, knows its own species alone with a perfect knowledge: whereas the possible intellect does not know itself at all thus, but by its intelligible species, as stated above.

Now from the very fact that a particular substance is intellectual, it is capable of understanding all being. Wherefore, as a separate substance is not, by its nature, made actually to understand all being, that substance, considered in itself, is in potentiality, as it were, to the intelligible images whereby all being is known, and these images will be its acts, according as it is intellectual. But it is not possible that these images be otherwise than several: for it has been already proved that the perfect image of the whole universal being cannot but be infinite; and just as the nature of a separate substance is not infinite, but limited, so an intelligible image existing therein cannot be infinite, but is confined to some species or genus of being; wherefore several such images are requisite for the comprehension of all being. Now, the higher a separate substance is, the more is its nature similar to the divine; and consequently it is less limited, as approaching nearer to the perfection and goodness of the universal being, and for this reason it has a more universal participation of goodness and being. Consequently the intelligible images that are in the higher substance are less numerous and more universal. This agrees with the statement of Dionysius {Coel. Hier. xii.) that the higher angels have a more universal knowledge: and it is said in De Causis that the higher intelligences have more universal forms. Now, the highest point of this universality is in God, Who knows all things by one, namely His essence: whereas the lowest is in the human intellect, which, for each intelligible object, requires an appropriate intelligible species commensurate with that object.

It follows that with the higher substances knowledge through more universal forms is not more imperfect, as it is with us. For through the image of animal, whereby we know something in its genus only, we have a more imperfect knowledge than through the image of man, whereby we know the complete species: since to know a thing as to its genus only, is to know it imperfectly and potentially as it were, whereas to know a thing as to its species, is to know it perfectly and actually. Now our intellect, since it obtains the lowest place in intellectual substances, requires images particularized to that extent that to each proper object of its knowledge there must needs correspond a proper image in it: wherefore, by the image of animal it knows not rational, and consequently neither does it know man, except in a certain respect. On the other hand the intelligible image that is in a separate substance is of more universal virtue, and suffices to represent more things. Consequently it argues not a more imperfect but a more perfect knowledge: because it is virtually universal, like the active form in a universal cause which, the more universal it is, the greater the extent of its efficiency, and the more efficacious its production. Therefore by one image it knows both animal and the differences of animal: or again it knows them in a more universal or more limited way according to the order of the aforesaid substances.

Hence we may take examples of this, as we have stated, in the two extremes, namely in the divine and human intellects. For God knows all things by one, namely His essence: while man requires different likenesses to know different things. Moreover the higher his intellect, the more things is he able to know through fewer: wherefore we need to give particular examples to those who are slow of intelligence, in order that they may acquire knowledge about things.

Now since a separate substance, considered in its nature, is in potentiality to the images by which all being is known, we must not think that it is devoid of all such images: for such is the disposition of the possible intellect before it understands, as stated in 3 De Animad Nor again must we think that it has some of them actually, and others potentially only: even as primary matter in the lower bodies has one form actually and others potentially; and as our possible intellect, when we are already possessed of knowledge, is in act in respect of some intelligibles, and in potentiality in respect of others. For since these separate substances are not moved, neither per se nor accidentally, as we have proved, whatever is potential in them, must be actual; else they would pass from potentiality to act, and thus they would be moved per se or accidentally. There is, therefore, in them potentiality and act as regards intelligible being, just as there is in the heavenly bodies as regards natural being. For the matter of a heavenly body is so perfected by its form, that it does not remain in potentiality to other forms: and in like manner the intellect of a separate substance is wholly perfected by intelligible forms, with respect to its natural knowledge. On the other hand our possible intellect is proportionate to the corruptible bodies to which it is united as a form: because it is made to have certain intelligible forms actually in such a way that it remains in potentiality to others. For this reason it is stated in De Causis that an intelligence is full of forms, since, to wit, the whole potentiality of its intellect is perfected by intelligible forms. And thus one separate substance is able to understand another through these intelligible forms.

Someone, however, may think that, since a separate substance is intelligible by its nature, there is no need to assert that one is understood by another through intelligible species, but that they understand one another by the very essence of the substance understood. For it would seem that the fact of a substance being understood through an intelligible species is accidental to material substances, from their not being actually intelligible through their essence: wherefore it is necessary for them to be understood through abstract intentions. Moreover this seems in accord with the statement of the Philosopher who says (ii Metaph.y that in separate substances there is no distinction between matter, intellect, the act of understanding, and the thing understood.

And yet if this be granted it involves not a few difficulties. First, because the intellect in act is the thing understood in act according to the teaching of Aristotle and it is difficult to see how one separate substance is identified with another when it understands it.

to which its operation corresponds, as heating corresponds to heat; wherefore we see the thing whose species informs the sight. Yet it does not seem possible for one separate substance to be the form of another, since each has its being separate from the other. Therefore it is seemingly impossible for the one to be seen by the other through its essence.

Moreover. The thing understood is the perfection of the one who understands. Now a lower substance cannot be a perfection of a higher. It would follow, therefore, that the higher would not understand the lower, if each were understood through its essence and not by another species.

Further. The intelligible object is within the intellect as to that whereby it is understood. Now no substance enters into the mind save God alone. Who is in all things by His essence, presence, and power. Therefore it is seemingly impossible for a separate substance to be understood by another through its essence, and not through its image in that other.

This must indeed be true according to the opinion of Aristotle who says that understanding takes place through the fact that the thing understood in act is one with the intellect in act. Wherefore a separate substance, although it is by itself actually intelligible, is nevertheless not understood in itself except by an intellect with which it is one. And it is thus that a separate substance understands itself by its essence: so that according to this the intellect, the thing understood, and the act of understanding are the same thing.

But according to the opinion of Plato, understanding takes place through contact of the intellect with the thing understood. So that, in consequence, one separate substance can understand another through its essence, when it is in spiritual contact with it; the higher understanding the lower, by enclosing and containing it by its power as it were, and the lower understanding the higher, as though it grasped it as its own perfection. Hence Dionysius says {Div. Nom. iv.) that the higher substances are intelligible as the food of the lower substances.