A Certain Trinity in the Sight. That There are Three Things in Sight, Which Differ in Their Own Nature. In What Manner from a Visible Thing Vision is Produced, or the Image of that Thing Which is Seen. The Matter is Shown More Clearly by an Example. How These Three Combine in One
4 But if any perhaps attend to what I am about to mention, they will find no such trouble in this inquiry. Commonly, when we have looked for some little time at a light, and then shut our eyes, there seem to play before our eyes certain bright colors variously changing themselves, and shining less and less until they wholly cease; and these we must understand to be the remains of that form which was wrought in the sense, while the shining body was seen, and that these variations take place in them as they slowly and step by step fade away. For the lattices, too, of windows, should we happen to be gazing at them, appear often in these colors; so that it is evident that our sense is affected by such impressions from that thing which is seen. That form therefore existed also while we were seeing, and at that time it was more clear and express. But it was then closely joined with the species of that thing which was being perceived, so that it could not be at all distinguished from it; and this was vision itself. Why, even when the little flame of a lamp is in some way, as it were, doubled by the divergent rays of the eyes, a twofold vision comes to pass, although the thing which is seen is one. For the same rays, as they shoot forth each from its own eye, are affected severally, in that they are not allowed to meet evenly and conjointly, in regarding that corporeal thing, so that one combined view might be formed from both. And so, if we shut one eye, we shall not see two flames, but one as it really is. But why, if we shut the left eye, that appearance ceases to be seen, which was on the right; and if, in turn, we shut the right eye, that drops out of existence which was on the left, is a matter both tedious in itself, and not necessary at all to our present subject to inquire and discuss. For it is enough for the business in hand to consider, that unless some image, precisely like the thing we perceive, were produced in our sense, the appearance of the flame would not be doubled according to the number of the eyes; since a certain way of perceiving has been employed, which could separate the union of rays. Certainly nothing that is really single can be seen as if it were double by one eye, draw it down, or press, or distort it as you please, if the other is shut.
5. The case then being so, let us remember how these three things, although diverse in nature, are tempered together into a kind of unity; that is, the form of the body which is seen, and the image of it impressed on the sense, which is vision or sense informed, and the will of the mind which applies the sense to the sensible thing, and retains the vision itself in it. The first of these, that is, the visible thing itself, does not belong to the nature of the living being, except when we discern our own body. But the second belongs to that nature to this extent, that it is wrought in the body, and through the body in the soul; for it is wrought in the sense, which is neither without the body nor without the soul. But the third is of the soul alone, because it is the will. Although then the substances of these three are so different, yet they coalesce into such a unity that the two former can scarcely be distinguished, even with the intervention of the reason as judge, namely the form of the body which is seen, and the image of it which is wrought in the sense, that is, vision. And the will so powerfully combines these two, as both to apply the sense, in order to be informed, to that thing which is perceived, and to retain it when informed in that thing. And if it is so vehement that it can be called love, or desire, or lust, it vehemently affects also the rest of the body of the living being; and where a duller and harder matter does not resist, changes it into like shape and color. One may see the little body of a chameleon vary with ready change, according to the colors which it sees. And in the case of other animals, since their grossness of flesh does not easily admit change, the offspring, for the most part, betray the particular fancies of the mothers, whatever it is that they have beheld with special delight. For the more tender, and so to say, the more formable, are the primary seeds, the more effectually and capably they follow the bent of the soul of the mother, and the phantasy that is wrought in it through that body, which it has greedily beheld. Abundant instances might be adduced, but one is sufficient, taken from the most trustworthy books; viz. what Jacob did, that the sheep and goats might give birth to offspring of various colors, by placing variegated rods before them in the troughs of water for them to look at as they drank, at the time they had conceived.
Source: On the Holy Trinity (New Advent)