St. Thomas Aquinas, Commentary of Boethius's De Trinitate, Q.6, a.3, corp.

We know a thing in two ways: in one way when we know that it is, and in another way when we know what it is. Now in order to know what anything is, our intellect must penetrate its quiddity or essence either directly or by means of other things that adequately reveal its quiddity. But in this life our intellect cannot directly penetrate the essence of God or other separate essences, because it directly extends to images, to which it bears the same relation as sight does to color, as the De Anima says. So the intellect can directly conceive the quiddity of a sensible reality but not of an intelligible reality. Thus Dionysius says, “According to our way of knowing, we cannot immediately attain to the contemplation of the invisible.” There are some invisible things, however, whose quiddity or nature is perfectly revealed by the known quiddities of sensible things; and we can also know what these intelligible objects are, although indirectly. For instance, from the fact that we know what man and animal are, we come to know adequately the relation of one to the other, and from this we know what a genus and a species are.

But the sensible natures known to us do not adequately reveal the divine essence or even other separate essences, since naturally considered they do not belong to one genus; and quiddity and all such terms predicated almost equivocally of sensible things and of these substances. That is why Dionysius calls the likenesses of sensible things, transferred to immaterial substances, “unlike likenesses, which intellectual beings participate in one way and sensible beings in another.” Consequently, we cannot have adequate knowledge of the former from the latter by way of likeness or even by way of causality, because the effects of those substances found in lower beings do not measure up to their powers so that we can come to know the essence of their cause in this way.

Accordingly, in the present life it is absolutely impossible to know the essence of immaterial substances, not only (~) by natural knowledge but also (~) by revelation; for, as Dionysius say, the light of divine revelation comes to us adapted to our condition. Thus even though revelation elevates us to know something of which we should otherwise be ignorant, it does not elevate us to know in any other way than through sensible things. Thus Dionysius says: “It is impossible for the divine light to illumine us from above unless it be hidden within the covering of many sacred veils.” Now knowledge by way of the sensible is inadequate to enable us to know the essences of immaterial substances. So we conclude that we do not know what immaterial forms are, but only that they are, whether by natural reason based upon created effects or even by revelation, by means of likenesses taken from sensible things.

It should be noticed, however, that we cannot know that a thing is without knowing in some way what it is, either perfectly or at least confusedly, as the Philosopher says we know things defined before we know the parts of their definition. For if a person knows that man exists and wants to find out what man is by definition, he must know the meaning of the term “man.” And this is possible only if he somehow forms a concept of what he knows to exist, even though he does not know its definition. That is to say, he forms a concept of man by knowing a proximate or remote genus and accidental characteristics which reveal him externally. For our knowledge of definitions, like that of demonstrations, must begin with some previous knowledge.

Similarly, therefore, we cannot know that God and other immaterial substances exist unless we know somehow, in some confused way, what they are. (~) Now we cannot do this by knowing a proximate or remote genus, for God is in no genus, since his essence is not distinct from his being; a condition required in all genera, as Avicenna says. Created immaterial substances, however, are indeed in a genus; but even though from the viewpoint of logic they share the same remote genus of substance with sensible substances, from the viewpoint of physics they do not belong to the same genus, as neither do heavenly and terrestrial bodies. For the corruptible and the incorruptible do not belong to the same genus, as the Metaphysics says. For the logician considers concepts in themselves; and from this point of view nothing prevents the immaterial and the material, or the incorruptible and the corruptible, from having something in common. But the philosopher of nature and the metaphysician treat of essences as existing in reality; and therefore they say that there are different genera wherever they find diverse modes of potency and act, and consequently diverse modes of being. Neither has God any accidental characteristics, as we will prove later. If other immaterial substances have such characteristics, we do not know them. Accordingly, we cannot say that we know immaterial substances obscurely by knowing their genus and observable accidents.

Instead of knowing the genus of these substances, we know them (+) by negations; for example, by understanding that they are immaterial, incorporeal, without shapes, and so on. The more negations we know of them the less vaguely we understand them, for subsequent negations limit and determine a previous negation as differences do a remote genus. Our knowledge of the heavenly bodies is also negative for the most part, because they belong to a different genus from that of inferior bodies. We know, for instance, that they are not light or heavy, or hot or cold. And instead of accidental characteristics in these substances we have their connections with sensible ones, either with regard to (+) the relationship of cause to effect or with regard to (+) the relationship of transcendence.

We conclude, then, that in the case of immaterial forms we know that they exist; and instead of knowing what they are we have knowledge of them by way of negation, by way of causality, and by way of transcendence. These are the same ways Dionysius proposes in his Divine Names; and this is how Boethius understands that we can know the divine form by removing all images, and not that we know that it is. The solution of the opposing arguments is clear from what has been said: for the first arguments are based on perfect knowledge of what a thing is, the others on imperfect knowledge of the sort described.

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Return to Lesson 1: What Would It Mean to ‘Prove’ God Exists?