Lecture 15 — How color is seen

Now, that only can receive color which itself is colorless, as only that which is soundless can receive sound. What is without color is the transparent and the invisible, or what is barely seen, being dark. The transparent is of this nature when it is not in act but in potency. For the same nature is sometimes dark, sometimes light.

Not all visible things, however, are visible in light, but only the color proper to each. There are certain things that are, indeed, not seen in light, but which produce a sensation in darkness, such as those which burn and are luminous. These are not called by any one term. Such are the fungi of certain trees, horn, fish-heads, scales, and eyes. But the color proper to each of these is not seen. Why these things are thus seen is a matter for another inquiry.

At present what is clear is that what is seen in light is color; therefore it is not seen without light. For to be color is to be able to move the transparent in act; and this act of the transparent is light. A manifest sign of this is that if one places on the sight itself a colored object, it is not seen. But color moves the transparent medium (say, air); and the sensitive organ is moved by this extended continuum.

For Democritus did not speak well when he put forward the opinion that if the medium were a vacuum, perception would be everywhere exact, even if an ant were in the sky. This is, however, impossible; for only when the sensitive power is affected does vision occur. This cannot, however, be brought about by the color itself, which is seen. It must, therefore, be due to the medium, so there must be a medium. If there were a vacuum, a thing, so far from being perceived exactly, would not be seen at all. We have stated, then, why it is necessary that color be seen in light.

But fire is seen in both darkness and light: necessarily so, for the transparent is made luminous by it.

The same account holds for both sound and smell. For no sensation is produced when either of these touches the sensitive organ, but a medium is moved by the sound and smell, and the sense organ of one or the other sense by the medium. But if one places an object that sounds or smells upon the sense organ itself, no sensation occurs. The same holds good of touch and taste, although this is not obvious. The reason for this will be made clear later.

The medium of sound is air, but that of smell has no special name. For a certain passion is common from air and water, and as the transparent is for color, so it is for what has smell, which is present in each of these. For it seems that aquatic animals possess a sense of smell. But man, and whatever of things walking on feet breathe, are unable to smell except when breathing. The cause of this will be dealt with later.

427. Now, that only can receive color (418b26). After showing what color, the transparent, and light are, the Philosopher now proceeds to explain how the transparent is related to color. It is clear from the foregoing that the transparent is receptive of color; for color, we have seen, moves the transparent. (Lect. 14, nn. 423–26.) Now, what can receive color must itself be colorless, just as what receives sound must be soundless; for nothing receives what it already has. The transparent must therefore be without color.

428. But since bodies are visible by their colors, it follows that the transparent in virtue of itself is invisible. Yet since one and the same power knows opposites, it follows that sight, which knows light, also knows darkness. Hence, although the transparent medium of itself lacks color and light, being receptive of both, and is thus not of itself visible in the way that luminous or colored things are visible, it can still be called “visible” in the way that things in shadows, which are barely seen, are so called. For the transparent is of this nature, that is, dark, so long as it is not in act but only in potency; for the same nature is sometimes underlying darkness, but sometimes light. Thus, the transparent, while it lacks light, which occurs when it is potentially transparent, must be in a state of darkness.

429. Then, at not all visible things (419a1), having determined color, which is seen in light, he determines the other visible object that he said above has no proper name. (Lect. 14, n. 399.) He observes that not all visible things are visible in light, but only the color proper to each body is visible in light. Some things—for example, certain things that appear to burn and are luminous in the dark—are not seen in the light, but only in darkness. There are many such things, including the fungi of certain trees, the horn of certain beasts, the heads of certain fish, and some animals’ scales, and eyes. But, even though these things are visible in the dark, still the color proper to each is not seen in the dark. Such things, then, are seen both in light and in darkness, but in darkness only as luminous, in light as colored.

430. Why these things are seen as luminous in the darkness is a matter for another inquiry. Aristotle only mentions the fact accidentally, in order to show the relation of the visible to light.

This, however, seems to be the reason for their being visible in the dark, namely, that such things have in their own constitution something of luminosity, inasmuch as the luminosity of fire and the transparency of air and water is not entirely smothered in them by the opacity of earth. But because they have only a small amount of luminosity, their luminosity is obscured in the presence of a greater light. Hence, in the light they appear not as luminous but only as colored. However, their luminosity is so weak that it is unable perfectly to actualize the transparent insofar as it is naturally apt to be moved by their color. Hence, under their luminosity neither their own color nor that of other things is seen, but only their luminosity. For luminosity, since it is more effective for moving the transparent than is color, and in itself more visible, can be seen with less modification of the transparent.

431. Next, at at present what is clear (419a7), he explains how color reaches sight. And about this, he does two things.

First, he points out what is necessary for sight to be moved by color.

And second, at the same account holds (419a25; [437]), he indicates a similar necessary condition in the case of the other senses.

About the first, he does two things. First, he shows how color is seen; second, how luminous things are seen, at but fire is seen. Regarding the first, he does two things:

First, he determines the truth of the matter.

And second, at Democritus did not speak well (419a15; [433]), he sets aside an error.

First, then, he says that what is clear is that what is seen in light is color, and that color cannot be seen without light; and this is because, as has been explained, it belongs to the account of color that it is able to move the transparent in act, and it does this in virtue of light, which is the act of the transparent. (Lect. 14, n. 424.) Hence, light is necessary if color is to be seen.

432. A sign of this is the fact that if someone places a colored body upon the organ of sight, it cannot be seen; for then there remains no actually transparent medium to be moved by the color. For although the pupil of the eye is a certain transparent thing, still it will not be actually transparent so long as the colored body remains placed upon it. Color, however, must move what is actually transparent, say, air or something of the kind, and the sensitive organ, that is, the organ of sight, is moved by this, as by a body continuous with itself. For bodies only affect one another through contact.

433. Then, at Democritus did not speak well (419a15), he sets aside an erroneous view. Democritus, he says, did not speak well when he put forward the opinion that if the medium between the eye and the thing seen were a vacuum, any object, however small, would be visible at any distance, for example, if an ant were in the sky. But this cannot be, for if anything is to be seen, it must actually affect the organ of sight.

Now, it has been shown that it cannot be acted upon by an immediate object, since the visible placed on the eye is not seen. So it remains that the organ of sight must be acted upon by the visible through some medium: so there must be a medium between the visible object and the sight. But if there is a vacuum, there is no medium that can change or be changed. Hence, if it were a vacuum, it would not be seen at all.

434. Democritus went wrong because he thought that the reason why distance impedes the sight of something is that the medium is of itself an impediment to the action of the visible object. But it is false. For the transparent has no contrariety to light or color; rather, it is in the last disposition to their reception, a sign of which is that it is modified by light or color instantaneously. The real reason why distance impedes sight is that everything seen is seen within the angle of a sort of triangle, or rather, a pyramid, whose base is the object seen and whose apex is in the eye that sees.

435. Nor does it make a difference for this whether seeing takes place by a movement from the eye outwards—so that the lines enclosing the triangle or pyramid run from the eye to the object—or in the opposite way, so long as seeing does involve this triangular or pyramidal figure; and this is necessary, because, since the object seen is larger than the pupil of the eye, its effect upon the medium has to be scaled down gradually until it reaches the eye. And, obviously, the longer are the sides of a triangle or pyramid, the smaller is the angle at the apex, provided that the base remains the same. The further away, then, is the object seen, the less is it seen—until at a certain distance it cannot be seen at all.

436. Next, at but fire (419a23), he explains how fire and luminous bodies are seen. And he says that they are seen not only in light, like colored objects, but even in the dark. It is necessarily so, because fire contains enough light that it can make the transparent wholly in act, such that both itself and other things are seen. Nor is its light so weak that it is overshadowed in the presence of a greater light, as happens with the objects mentioned above.

437. Then, at the same account (419a25), he shows how the case of the other senses is similar to sight. And he says that the same account holds for both sound and smell as for color, for none of these is sensed if there is immediate contact with the organ in question. But a medium is moved by the sound and smell, and one or the other of the organs of hearing and smell by the medium. But when someone puts a body that sounds or smells upon the organ of the sense, it is not sensed. The same is true even of touch and taste, though, for a reason to be given later, this is less evident. (Lect. 21, nn. 507–8; lect. 23, n. 545.)

438. Finally, at the medium of sound (419a32), he states what is the medium in hearing and smelling. That of hearing is air, and that of smelling is something common to air and water—just as both of these is a medium that is moved by color, though each is moved by color insofar as it is transparent. There is indeed no name for the passion in air and water that is moved by the smell; for they are not moved by the smell insofar as they are transparent.

And that both air and water are moved by smell he shows from the fact that aquatic animals have a sense of smell, which manifests this. Man, however, and other animals that walk and breathe, only smell by breathing, which proves that air is the medium of smell. The cause of this will be dealt with later. (Lect. 20.)