Chapter 22 That the Soul Never Thinks without an Image Thomas Aquinas
It is impossible for our intellect, in its present state of being joined to a body capable of receiving impressions, actually to understand anything without turning to sense images.
This is evident on two counts. First, because, since it is a faculty which does not use a corporeal organ, the intellect would be in no sense impeded by an injury to a corporeal organ if for its act another act of a faculty that does use a corporeal organ were not required. But the senses, the imagination, and the other faculties of the sense part of man do use corporeal organs. Hence it is obvious that, for the intellect actually to understand (not only in acquiring new knowledge but also in using knowledge already acquired), acts of the imagination and the other faculties are necessary.We see, in fact, that if acts of the imagination are impeded by an injury to its organ—for instance, in a seizure—or, similarly, if acts of sense memory are impeded— for instance, in coma—a man is impeded from actually understanding even things which he had known before.
The second count is this. As anyone can experience for himself, if he attempts to understand anything, he will form images for himself which serve as examples in which he can, as it were, look at what he is attempting to understand. This is the reason, indeed, why, when we want to help someone understand something, we propose examples to him so that he can form images for himself in order to understand.
The reason for all this is that cognitive faculties are proportioned to their objects. For instance, an angel's intellect, which is totally separate from corporeal reality, has as its proper object intelligible substances separate from corporeal reality, and it is by means of these intelligible objects that it knows material realities. The proper object of the human intellect, on the other hand, since it is joined to a body, is a nature or 'whatness' found in corporeal matter—the intellect, in fact, rises to the limited knowledge it has of invisible things by way of the nature of visible things.
But by definition a nature of this kind exists in an individual which has corporeal matter, for instance, it is of the nature of stone that it should exist in this or that particular stone, or of the nature of horse that it should exist in this or that particular horse, etc. Thus the nature of stone or any other material reality cannot be Imown truly and completely except in so far as it exists in a particular thing. Now we apprehend the particular through the senses and imagination. Therefore if it is actually to understand its proper object, then the intellect must needs turn to sense images in order to look at universal natures existing in particular things.Whereas if the proper object of our intellect were an immaterial form, or if the natures of sensible things subsisted apart from particulars, as the Platonists think, it would not be necessary for our intellect when understanding always to be turning to sense images. Hence:
1. Spedes stored up in the possible intellect remain there in a habitual way when the intellect is not actually understanding, as was said above. Thus, in order for us actually to understand, a mere storing of spedes is not sufficient; we must also use them, and indeed in accord with the things of which they are images, which are natures existing in particulars.
2. Since the sense image is itself a likeness of a particular thing, the imagination does not need a further likeness of a particular, as does the intellect.
3. We know incorporeal realities, which have no sense images, by analogy with sensible bodies, which do have images, just as we understand truth in the abstract by a consideration of things in which we see truth. God we know, according to Dionysius, as cause about which we ascribe the utmost perfection and negate any limit. Furthennore, we cannot, in our present state, know other incorporeal substances except negatively and by analogy with corporeal realities. Thus when we understand anything of these beings, we necessarily have to him to images of sensible bodies even though they do not themselves have such images.