2. When, then, we see any corporeal object, these three things, as is most easy to do, are to be considered and distinguished: First, the object itself which we see; whether a stone, or flame, or any other thing that can be seen by the eyes; and this certainly might exist also already before it was seen; next, vision or the act of seeing, which did not exist before we perceived the object itself which is presented to the sense; in the third place, that which keeps the sense of the eye in the object seen, so long as it is seen, viz. the attention of the mind. In these three, then, not only is there an evident distinction, but also a diverse nature. For, first, that visible body is of a far different nature from the sense of the eyes, through the incidence of which sense upon it vision arises. And what plainly is vision itself other than perception informed by that thing which is perceived? Although there is no vision if the visible object be withdrawn, nor could there be any vision of the kind at all if there were no body that could be seen; yet the body by which the sense of the eyes is informed, when that body is seen, and the form itself which is imprinted by it upon the sense, which is called vision, are by no means of the same substance. For the body that is seen is, in its own nature, separable; but the sense, which was already in the living subject, even before it saw what it was able to see, when it fell in with something visible, -- or the vision which comes to be in the sense from the visible body when now brought into connection with it and seen, -- the sense, then, I say, or the vision, that is, the sense informed from without, belongs to the nature of the living subject, which is altogether other than that body which we perceive by seeing, and by which the sense is not so formed as to be sense, but as to be vision. For unless the sense were also in us before the presentation to us of the sensible object, we should not differ from the blind, at times when we are seeing nothing, whether in darkness, or when our eyes are closed. But we differ from them in this, that there is in us, even when we are not seeing, that whereby we are able to see, which is called the sense; whereas this is not in them, nor are they called blind for any other reason than because they have it not. Further also, that attention of the mind which keeps the sense in that thing which we see, and connects both, not only differs from that visible thing in its nature; in that the one is mind, and the other body; but also from the sense and the vision itself: since this attention is the act of the mind alone; but the sense of the eyes is called a bodily sense, for no other reason than because the eyes themselves also are members of the body; and although an inanimate body does not perceive, yet the soul commingled with the body perceives through a corporeal instrument, and that instrument is called sense. And this sense, too, is cut off and extinguished by suffering on the part of the body, when any one is blinded; while the mind remains the same; and its attention, since the eyes are lost, has not, indeed, the sense of the body which it may join, by seeing, to the body without it, and so fix its look thereupon and see it, yet by the very effort shows that, although the bodily sense be taken away, itself can neither perish nor be diminished. For there remains unimpaired a desire [appetitus] of seeing, whether it can be carried into effect or not. These three, then, the body that is seen, and vision itself, and the attention of mind which joins both together, are manifestly distinguishable, not only on account of the properties of each, but also on account of the difference of their natures.
3. And since, in this case, the sensation does not proceed from that body which is seen, but from the body of the living being that perceives, with which the soul is tempered together in some wonderful way of its own; yet vision is produced, that is, the sense itself is informed, by the body which is seen; so that now, not only is there the power of sense, which can exist also unimpaired even in darkness, provided the eyes are sound, but also a sense actually informed, which is called vision. Vision, then, is produced from a thing that is visible; but not from that alone, unless there be present also one who sees. Therefore vision is produced from a thing that is visible, together with one who sees; in such way that, on the part of him who sees, there is the sense of seeing and the intention of looking and gazing at the object; while yet that information of the sense, which is called vision, is imprinted only by the body which is seen, that is, by some visible thing; which being taken away, that form remains no more which was in the sense so long as that which was seen was present: yet the sense itself remains, which existed also before anything was perceived; just as the trace of a thing in water remains so long as the body itself, which is impressed on it, is in the water; but if this has been taken away, there will no longer be any such trace, although the water remains, which existed also before it took the form of that body. And therefore we cannot, indeed, say that a visible thing produces the sense; yet it produces the form, which is, as it were, its own likeness, which comes to be in the sense, when we perceive anything by seeing. But we do not distinguish, through the same sense, the form of the body which we see, from the form which is produced by it in the sense of him who sees; since the union of the two is so close that there is no room for distinguishing them. But we rationally infer that we could not have sensation at all, unless some similitude of the body seen was wrought in our own sense. For when a ring is imprinted on wax, it does not follow that no image is produced, because we cannot discern it unless when it has been separated. But since, after the wax is separated, what was made remains, so that it can be seen; we are on that account easily persuaded that there was already also in the wax a form impressed from the ring before it was separated from it. But if the ring were imprinted upon a fluid, no image at all would appear when it was withdrawn; and yet none the less for this ought the reason to discern that there was in that fluid before the ring was withdrawn a form of the ring produced from the ring, which is to be distinguished from that form which is in the ring, whence that form was produced which ceases to be when the ring is withdrawn, although that in the ring remains, whence the other was produced. And so the [sensuous] perception of the eyes may not be supposed to contain no image of the body, which is seen as long as it is seen, [merely] because when that is withdrawn the image does not remain. And hence it is very difficult to persuade men of duller mind that an image of the visible thing is formed in our sense, when we see it, and that this same form is vision.
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.