Lecture 14 — The object of sight

That of which there is sight is the visible; and the visible is color and also something which, though it has no name, we can state descriptively. It will be evident what we mean when we have gone further into the matter.

For the visible is color, and it is this which is visible in itself, not, however, by its account, but because it has in itself the cause of being visible. For every color is able to move what is actually transparent: this is its very nature. Hence nothing is visible without light; but by light each and every color is visible. Wherefore, we must first say what light is.

There is, accordingly, something transparent. By “transparent” I mean that which is, indeed, visible, yet not of itself, as if to speak without qualification, but by virtue of concomitant color. Air and water and many solids are such. But transparency does not belong to either air or water as such, but because there is the same nature in each of them, and in eternal body above as well.

Light is the act of this transparent thing as transparent; but it is potentially in what is also darkness. Now, light is like the color of the transparent, insofar as it is actually transparent due to fire or some such thing, as in the upper body, which contains indeed something of one; for one and the same thing is present in this.

We have, then, indicated what the transparent is and what light is; for light is not fire or any bodily thing, nor any emanation from a body—for then it would be a sort of body, and so be fire or the presence of something similar in the transparent.

But it is impossible for two bodies to exist in the same place at the same time.

Light seems to be the contrary of darkness; and darkness is the privation of this sort of thing had by the transparent. So it is also plain that the presence of this is light.

Empedocles (or anyone else who may have said the same) did not speak correctly when he said that light is borne along and extended between the earth and its container, but does so unperceived by us. For this deviates both from sound reasoning and from how things appear. For such a thing might escape our observation over a small space, but that it should remain unnoticed running from the east to the west asks quite a great deal.

399. That of which there is sight (418a26). Having distinguished the proper sense-objects from the common and from those that are sensible accidentally, the Philosopher now treats of the proper object of each sense:

first, of the proper object of sight;

second, at now let us start (419b4; [439]), of the proper object of hearing;

third, at it is not so easy (421a7; [479]), of the proper object of smell;

fourth, at the tasteable (422a8; [501]), of the proper object of taste;

and fifth, at the same reasoning holds (422b17; [517]), of the proper object of touch.

As to the first, he discusses two things:

first, he determines about the visible;

second, at at present what is clear (419a7; [431]), how the visible comes to be seen.

Regarding the first, he does two things.

First, he determines what the visible is, dividing it into two.

Second, he deals with both visibles, at for the visible is color (418a29; [400]).

Thus, first he says that since the proper sense-object is that which each sense properly perceives, the sense-object of which sight is properly receptive is the visible. (Lect. 13, nn. 384–85.) Now, under the “visible,” two things are included; for both the visible is color and also something else that can be designated descriptively, but has no proper name given it. This visible belongs to things that can be seen at night, such as glow-worms and certain fungi on oak-trees and the like, concerning which things will be evident as the treatise progresses, after we gain a deeper understanding of the visible from the knowledge of color, which is more manifestly visible. (Lect. 15, nn. 429–30.)

400. Then, at for the visible (418a29), he begins to define both objects of sight:

first, color;

and then, at not all visible things (419a1; [429]), that of which he says that it has no proper name.

About the first, he does two things:

first, he shows what color has to do with visibility;

second, at there is, accordingly, something transparent, he determines what is required for color to be seen.

First of all, then, he says that, since color is something visible, to be visible belongs to it in itself, for color inasmuch as it is color is visible.

401. But “in itself” or per se is said in two ways. (See Aristotle, Posterior Analytics 1.4.)

In one way, a proposition is per se when the predicate falls within the definition of the subject, for example, man is an animal; for animal enters into the definition of man. And since that which is within the definition of anything is in some way the cause of it, in things that are called per se in this way, the predicate is the cause of the subject.

In another way, a proposition is per se in a contrary way, when the subject is placed in the definition of the predicate, as when it is said that nose is snub, or number is even; for the snub is nothing but the curved nose, and the even nothing but the number that has a middle; and in these cases the subject is a cause of the predicate.

402. Therefore, one should understand that color is visible in itself in this second manner, not in the first; for visibility is a certain passion of color, just as being snub is a passion of a nose. And this is why he says that color is visible in itself but not by its account, that is to say, not such that visibility is placed in its definition, but because it has in itself the cause that it should be visible, as a subject possesses in itself the cause of its own proper passion.

403. Which he proves by this, that every color is able to move what is transparent actually. Now, the diaphanous is the same thing as the transparent (for example, air or water), and color has it in its own nature that it can move what is transparent in act. And that which moves the actually transparent is visible; whence it follows that color is visible according to its own nature. And since the transparent is brought to act only through light, it follows that color is not visible without light. And therefore, before explaining how color is seen, we must say what light is.

404. Then, at there is, accordingly (418b4), he discusses those things without which color cannot be seen, namely, the transparent and light; and this is divided into three sections.

First, he explains the transparent.

Second, at light is (418b9; [405]), he treats of the transparent’s actuality, that is, light.

Third, he shows how the transparent is receptive of color, at now, that only can receive color (418b26; [427]).

To begin with, therefore, he says that if color is that which of its own nature moves the transparent, the transparent must be something. Now, the transparent is what does not have its own proper color, such that it could be seen in virtue of it, but it is receptive of color from without, in virtue of which it is, in a way, visible. Examples of such are air and water and many solid bodies, such as certain jewels and glass. Now, whereas other accidents, which belong to the elements or to bodies constituted from them, belong to them on account of the nature of those elements (such as heat and cold, weight and lightness, etc.), transparency does not belong to them due to the nature of air or water as such; rather, it is consequent upon some nature common not only to air and water, which are corruptible bodies, but also to the celestial body, which is perpetual and incorruptible. For at least some of the celestial bodies are manifestly transparent. For we should not be able to see the fixed stars of the eighth sphere unless the lower spheres of the planets were transparent or diaphanous. Hence it is evident that to be transparent is not a property consequent on the nature of air or of water, but of some more common nature, in which the cause of transparency is to be found, as we shall see later.

405. Next, at light is the act (418b9), he shows what light is:

first, stating the truth;

and then dismissing an error, at we have, then (418b13; [406].

He says, to begin with, that light is the act of this transparent thing as transparent. For it is evident that neither air nor water nor anything of that sort is actually transparent unless it is illuminated. Of itself, the transparent is in potency to both light and darkness (the latter being a privation of light), as primary matter is in potency both to form and privation. Now, light is to the transparent as color is to a body of definite dimensions, since each is the act and form of that which receives it. And on this account, he says that light is, as it were, a certain color of the transparent, insofar as the transparent is made actually so by some luminous body, such as fire, or some such thing, or by a celestial body. For to be actually luminous and able to illuminate is common to fire and to a certain celestial body, just as to be transparent is common to air and water and the celestial body.

406. Then, at we have, then, indicated (418b13), he rejects a false opinion on light; and about this he does two things.

First, he shows that light is not a body;

second, he refutes an objection brought against the arguments which prove that light is not a body, at Empedocles was wrong (418b20; [409]).

As to the first point, he does three things.

First, he states his own view, saying that, once it is clear what the transparent is, and what light is, it is evident that light is not fire (as some have said, positing three kinds of fire: the combustible, flame, and light); or any bodily thing; nor anything flowing from a body, as Democritus supposed, asserting that light consisted of atomic particles emanating from luminous bodies. For if there were these emanations from bodies, they would themselves be bodies, or a sort of body, and so be nothing other than fire, or something similar in the transparent, present in the transparent, which is the same as to say that light is a body and an emanation from a body.

407. Second, at but it is impossible (418b17), he proves his own hypothesis thus. It is impossible for two bodies to exist in the same place at the same time. If therefore light were a body, it could not co-exist with a transparent body; but this is false; therefore light is not a body.

408. Third, at light seems (418b18), he shows that light does co-exist with the transparent body. For contraries belong to the same subject. But light and darkness are contraries in the manner in which a possession and its privation are contraries, as is stated in Metaphysics 10. (Aristotle, Metaphysics 10.4, 1055a33; cf. Aquinas, Commentary on Metaphysics, bk. 10, lect. 6, nn. 2036–58.) Obviously, darkness is the privation of this sort of thing had, that is, of light, by the transparent—and in this way the transparent is the subject of darkness. Hence, too, the presence of this possession mentioned (that is, of the luminous) is light. Therefore it co-exists with the transparent.

409. Then, at Empedocles was wrong (418b20), he refutes an answer to one argument which might be urged against those who hold that light is a body. For it is possible to argue thus against them: if light were a body, illumination ought to be a local motion of light passing through the transparent, but no local movement of any body can be sudden or instantaneous; therefore, illumination would be not instantaneous but successive, according to this view.

410. Of which the contrary is a fact of experience; for in the very instant in which a luminous body becomes present, the transparent body is illuminated all at once, not part after part.

So Empedocles did not speak correctly, nor did anyone else, in saying that light was borne along by local motion, as a body is; and that it is extended successively through the space that is the medium between the earth and its container, that is, the heavens; and that this successive motion is unperceived by us, so that the whole thing seems to us to be illuminated simultaneously and suddenly.

411. For this statement is contrary to the truth that can be perceived in sound reasoning, for nothing is required for the illumination of the transparent other than the placing of a luminous body over against the one to be illumined, with no intervening obstacle.

412. Again, it contradicts appearances. For one might allow that successive local motion over a small space might escape our observation, but that a successive movement of light from the east all the way to the western horizon should escape our notice is an enormous request, one both difficult or even quite impossible.

413. But as the subject matter under discussion is threefold, that is, of the nature of light, and of the transparent, and of the necessity of light for seeing, we must take these three questions one by one.

On the nature of light various opinions have been held. Some, as we have seen, held that light was a body (as is said in the text); they were led to this by certain expressions used in speaking of light. For instance, we are accustomed to say that a ray “passes through” the air, that it is “thrown back,” that rays “intersect,” and so forth; which all seem to imply something bodily.

414. But this opinion cannot stand, as the arguments here adduced by Aristotle show, to which others might easily be added. For it is hard to see how such a body could be suddenly multiplied over the whole hemisphere, or come into existence or vanish; nor how the mere intervention of an opaque body should cause the corruption of this body in some part of the transparent. To speak of the motion or rebounding of light is to use metaphors, as when we speak of heat “proceeding into” things that are being heated or being “thrown back” when it meets an obstacle.

415. Then there are those who maintain, on the contrary, that light is a certain spiritual nature. Otherwise, they say, why do we use the term “light” in speaking of intellectual realities? For we say that in the intellectual substances there is a certain intelligible light. But this also is inadmissible.

416. For it is impossible that any spiritual or intellectual nature should fall within the apprehension of the senses, whose power, being bodily, cannot acquire knowledge of any but bodily things. But if anyone should say that there is a spiritual light other than the light that the senses perceive, we need not quarrel with him, so long as he admits that the light that vision perceives is not the spiritual nature. For there is no reason why one name can be imposed on very different realities.

417. The reason, in fact, why we employ “light” and other words referring to vision in matters concerning the intellect comes from the nobility of the sense of sight; it is more spiritual and more subtle than any other sense. This is evident in two ways.

First, from the object of sight. For things fall under sight in virtue of properties that lower bodies have in common with the heavenly bodies. On the other hand, touch is perceptive of properties that are proper to the elements (such as heat and cold and the like); and taste and smell perceive properties that pertain to compound bodies, according to how these are variously compounded of the hot and the cold, the moist and the dry; sound, again, is caused by local movement, which is also common to lower and the heavenly bodies, although the species of motion that causes sound does not befit the heavenly bodies, according to the opinion of Aristotle. (Aristotle, On the Heavens 2.14, 290b30–291a26.) Hence, from the very nature of the object, it is apparent that sight is the highest of the senses, with hearing nearest to it, and the others still more remote from its dignity.

418. The spirituality of the sense of sight is apparent in a second way from the manner of its modification. For in every other sense, there is no spiritual modification without a natural one. I speak of a “natural modification” when a quality is received in what is being acted upon according to an existence of nature, as when something is cooled, or heated, or moved about in space.

But by a “spiritual modification,” I mean what happens when the species is received in the sense organ, or in the medium, through the mode of an intention and not through the mode of a natural form. For the species is not being received in the sense according to that existence that it had in the sensible reality. Now, in the case of touching and tasting (which is a kind of touching), it is clear that a natural alteration occurs: for the organ itself grows hot or cold by contact with a hot or cold object; there is not only a spiritual modification.

So, too, the modification of smell involves a sort of vaporous exhalation; and that of sound involves local movement. But seeing involves only a spiritual modification—hence it is clear that sight is more spiritual than the other senses, and after it, hearing. These two senses are, therefore, the most spiritual and are the only ones pertaining to learning. Hence the use we make of what refers to them—and especially of what refers to sight—in speaking of intellectual objects.

419. Then again, some have simply identified light with the manifestation of color. But this is patently untrue in the case of things that shine by night while their color, nevertheless, remains obscure.

420. Others, on the other hand, have said that luminosity is the substantial form of the sun, and that the light proceeding from the light-source has an intentional existence, as does the species of colors in the air.

But both these propositions are false. The former, because no substantial form is sensible through itself but is graspable only by intellect. And if it is said that what the sense sees in the sun is not luminosity but its splendor, we need not dispute about names, provided only it be granted that what we call “luminous,” that is, what is apprehended by sight, is not a substantial form.

And the latter proposition too is false, because whatever has only intentional existence does not cause natural change, whereas the rays from the heavenly bodies do in fact change all lower nature.

Hence, our own conclusion is that, just as the bodily elements have certain active qualities through which they act, so too, luminosity is the active quality of the heavenly body through which it acts, and this is in the third species of quality, like heat.

421. But it differs from heat in this: that luminosity is a quality of the primary change-effecting body, which has no contrary: therefore luminosity has no contrary, whereas there is a contrary to heat. And because there is no contrary to luminosity, there is no place for a contrary disposition in its recipient, and therefore what is passive to it, that is, the transparent, is always in the final disposition to its form. That is why illumination occurs instantaneously, whereas what can become hot does not do this. Now, the very participation or effect of the luminous within the transparent is called “light,” and if it comes about in a straight line to the luminous body, it is called a “ray,” whereas if it is caused by the reflection of a ray in relation to a luminous body, it is called “splendor.” But “light” is commonly used for every effect of the luminous within the transparent.

422. So much being admitted as to the nature of light, we can easily understand the reason why certain bodies are actually luminous, while others are transparent, and others opaque. For because luminosity is a quality of the primary change-effecting body, which is the most perfect and formal of bodies, those bodies that are most formal and noble are actually luminous; and close to these are the ones receptive of light, the transparent; but those that are most material have neither light in their own nature nor are they receptive of light, but they are opaque. One may see this in the elements themselves: for fire has luminosity in its own nature, though its luminosity is not apparent to us except in a foreign nature, on account of its density there; whereas air and water, being less formal, are transparent; while earth, the most material of all, is opaque.

423. With regard to the third point, note that some have said that light is necessary for seeing even on the side of the color itself; for they say that color has no power to move the transparent medium except through light.

An indication of this is, they say, that one who stands in the shadow can see what is in the light, but one who stands in the light cannot see what is in shadow. The reason for this, they said, is that, as seeing is one thing, a thing is visible only through one account; and that would not be so if color were visible through itself—not by the power of light—and light also were visible through itself.

424. Now, this view is clearly contrary to what Aristotle says here: and has in itself the cause of being visible (418a29). Hence, following his opinion, I say that light is necessary for seeing, not on the side of color, in that it actualizes colors (which some say are only in potency so long as they are in darkness), but on the side of the transparent, inasmuch as light makes it be actual, as the text states.

425. And in proof of this, note that every form is, as such, a principle of effecting a likeness with itself; whence, since color is a form, it has of itself the power to cause its likeness within the medium. But note also that there is this difference between a complete power and an incomplete power: the form that has perfect power in agency can not only induce its likeness within what is receptive to it, but can even dispose what it acts upon so that it becomes its proper recipient. This is beyond the power of the incomplete power.

One should say, then, that the power of color in its agency is imperfect relative to the power of light; for color is nothing other than a certain luminosity that is somehow dimmed by the admixture of the opaque body. Hence it lacks the power to render the medium disposed so that it might receive color; but pure luminosity can do this.

426. Whence it is also clear that, as luminosity is, in a certain way, the substance of color, all visible objects are traced back to the same nature; nor does color need to be made visible by some other extrinsic light. However, that illuminated colors are seen by one standing in the shade is due to even the medium’s having been sufficiently illuminated for the modification of sight.