Book Two — Creation, Chapter 98 — Summa Contra Gentiles
[1] Now, if separate substances understand those things which are intelligible through themselves, as was shown, and if separate substances are intelligible through themselves, since, as we have also seen, freedom from matter makes a thing intelligible through itself, then it follows that separate substances have separate substances as the proper objects of their understanding. Each of them, therefore, knows both itself and others.
[2] Indeed, each separate substance knows itself otherwise than the possible intellect knows itself. For the possible intellect exists as in potency in intelligible being, and becomes in act through the intelligible species, just as prime matter is actualized in sensible being by a natural form. Now, nothing is known, so far as it is only in potentiality, but so far as it is in act. That is why the form is the principle of the knowledge of the thing which becomes in act through the form. And the cognitive power likewise is rendered actually cognitive through some species. Thus, our possible intellect knows itself, thanks only to the intelligible species whereby it becomes in act in intelligible being; and for this reason Aristotle says in De anima III [4] that the human intellect “is itself knowable in the same way as other things are,” namely, through species derived from phantasms, as through proper forms. But separate substances by their very nature enjoy intelligible being actually; so that each of them knows itself through its essence, and not through the species of another thing.
[3] Now, as the likeness of the thing known is in the knower, so in every case is the knowledge. But, one separate substance is like another as regards the nature of the genus that such substances have in common, while they differ from each other in species, as was made clear above. It would then seem to follow that the one separate substance knows the other, not according to the proper nature of the species, but only as regards the common nature of the genus.
[4] Some therefore sayr, that one separate substance is the efficient cause of another. Now, in every efficient cause there must be the likeness of its effect, and, similarly, in every effect the likeness of its cause must be present; for every agent produces its like. Thus, in the higher separate substance there exists the likeness of the lower, as in the cause resides the likeness of the effect; and in the lower is the likeness of the higher, as in the effect dwells the likeness of its cause. Now, in non-univocal causes the likeness of the effect exists in the cause in a higher mode, while the likeness of the cause is in the effect in a lower mode. But the higher separate substances must be non-univocal causes of the lower ones, since the former, placed in diverse grades, are not of one species. Therefore, a lower separate substance knows a higher substance in a lower way, according to the mode of the substance knowing and not of the substance known; whereas the higher knows the lower in a higher way. This is expressed as follows in the work On Causes [VIII]: “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.”
[5] But, since it was shown above that separate intellectual substances are not composed of matter and form, they cannot be caused except by way of creation. We have also proved that to create belongs to God alone. One separate substance, therefore, could not be the cause of another.
[6] It has been demonstrated, moreover, that all the principal parts of the universe are created immediately by God. Hence, 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 the moon; since each of them has the nature of a species all its own, which is nobler than that of any corporeal things. Therefore, one separate substance is not caused by another, but all are immediately from God.
[7] So, a6cording to what was said above, each of the separate substances knows God, by its natural knowledge, after the manner of its substance; and through this knowledge they are like God as their cause. But God knows them as their proper cause, possessing in Himself the likeness of them all. Not in this way, however, could one separate substance know another, since one is not the cause of another.
[8] We must, therefore, consider that, since none of these substances is by its essence a sufficient principle of the knowledge of all other things, there must accrue to each of them, over and above its own substance, certain intelligible likenesses, whereby each of them is enabled to know another in its proper nature.
[9] Now, this can be made clear as follows. The proper object of intellect is intelligible being, which includes all possible differences and species of being, since whatever can be, can be known. Now, since all knowledge is brought about by way of likeness, the intellect cannot 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 be nothing other than an infinite nature: a nature not determined to some species or genus of being, but the universal principle of all being and the power productive of all being; and this, as was shown in Book I, is the divine nature alone. Indeed, no other nature can be the universal likeness of all being, since every nature except God is limited to some genus and species of being. It therefore remains that God alone, by His essence, knows all things. Every separate substance, on the other hand, is by its nature possessed of a perfect knowledge only of its own species; while the possible intellect knows itself not at all in this way, but through the intelligible species, as we remarked already in this chapter.