Lecture 2 — Whether sight is made of fire

At present some inquire about the organs of the body in which these are actualized with reference to the elements of bodies. But not being able to adapt them to four, since there are five, they are concerned about the fifth.

But all of them have sight be made of fire, because they do not know the cause of a certain affection: when the eye is squeezed and moved, fire seems to shine. This happens in darkness, or when the eyelids are lowered, which also makes it dark.

But this presents another difficulty. For if something visible cannot escape the notice of one who is sensing and seeing, it will be necessary that the eye always be seeing fire. Why then does it not happen when it is at rest?

Now the cause of this—both for the objection and for the view that sight is made of fire—is to be understood as follows. Smooth things naturally shine in darkness, but they do not produce light; and the so-called black part and center of the eye is smooth. And what appears, appears to an eye that is moved because what happens is that what is one becomes as though it were two, and the speed of the movement makes it seem that what sees and what is seen are different. Hence it does not happen unless quickly.

This happens in darkness, for a smooth thing in darkness naturally shines, as do certain heads of fish, and the ink of the cuttle-fish. And when the eye is gently moved, it does not happen that what sees and what is seen seem simultaneously to be one and two. But in the other case, the eye itself sees itself, as it does in refraction.

If it were fire, as Empedocles says, and as is written in the Timaeus, (Empedocles, Fragment 84; Plato, Timaeus 45b4–c2, 67e4–68b1.) and seeing took place by a light going out, the way it does from a lamp, why does sight not also see in darkness?

To say that in going out it is “extinguished” in darkness is completely foolish.

For what is “extinction” of light? What is hot and dry is extinguished by either moisture or cold, as can be seen in the case of the embers of a fire, and flame. But neither appears to be the case with light.

If it were the case, but the light escapes our notice because of “weakness,” it would have to be extinguished by day, and in water, and it would be more darkened where there is ice, for flame and burning bodies are affected in this way. But no such thing happens in this case.

Empedocles seems to think, as was said before, that seeing takes place by light going out. For he says: as when someone contemplating going out on a winter night prepares a lamp, he kindles a light of burning fire in such a way as to block the force of all winds, for he deflects the breath of blowing winds. But the light breaks out: however farther it expands it illuminates with rays subdued by a covering—likewise ancient light guarded in membranes—fine linens—pours out around in a circle through the pupil, which will reveal a depth of water flowing around. But the light comes out, however farther it expands. Sometimes, then, he says that seeing is like this, but sometimes that it takes place by emanations from what is seen.

33. After the Philosopher has summarized what is necessary for the present consideration of sensitive powers themselves, now he proceeds to his principal proposal in this book by applying the consideration of sense powers to what is bodily.

First, with respect to sense-organs.

Second, with respect to sensible objects, where he says: concerning sensible objects (439a6; [77]).

On the first point, he does two things.

He assigns sense-organs to elements, first disproving arguments of others;

second, determining what might more probably be the case, where he says: if, then, what happens in these cases (438b16; [66]).

On the first point, he does two things.

First, he touches in general on the way in which the ancients assigned sense-organs to elements.

Second, he focuses specifically on the organ of sight, about which many were mistaken, where he says: but all of them have sight be made of fire (437a22; [35]).

Accordingly, he first says that previous philosophers asked, with reference to the elements of bodies, what are the kinds of bodily organs in which, and by which, the operations of the sense powers are exercised?

They did so because, as was said in On the Soul 1, they held that like is known by like, and hence they held that the soul itself has the same nature as the principles of things, so that thereby it might know all things, being as it were conformed to all things, since all things share in the principles. (Aristotle, On the Soul 1.2, 404b17–18, 405b15–17; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 1, lect. 4, nn. 43–47; lect. 5, nn. 65–66.) For the same reason they assigned sense-organs to elements of bodies, because all bodily things are known through sense-organs.

34. But immediately one difficulty occurred to them: there are five senses and four elements. And so they looked for something to which they could assign the organ of the fifth sense.

Now between air and water there is an intermediary, denser than air but finer than water, which is called “smoke” or “vapor,” and some held that it is also a first principle, to which they assigned the organ of smell, since odor is perceived by means of a smoky evaporation. And they assigned the other four senses to the four elements: touch to earth, taste to water (since flavor is perceived by means of moisture), hearing to air, and sight to fire.

35. Then, when he says: but all of them have sight be made of fire (437a22), he proceeds specifically to the organ of sight, which they assigned to fire.

First, he disproves the cause they gave for their position.

Second, he disproves the position itself, where he says: if it were fire (437b10; [41]).

On the first point, he does three things.

First, he presents the cause that moved some to assign the organ of sight to fire.

Second, he raises a difficulty, where he says: but this presents another difficulty (437a26; [36]).

Third, he determines the truth about both points, where he says: now the cause of this (437a30; [37]).

Accordingly, he first says that all who assign the organ of sight to fire do so because they do not know the cause of a certain affection that occurs in the eye: if the eye is pressed and forcefully moved, it seems that fire shines. If the eyelids are open, this happens only when the surroundings are dark; it also happens when the surroundings are bright if the eyelids are first closed, because thus one makes it dark to the closed eye. They thought that this affection is a clear sign that the organ of sight pertains to fire.

36. Then, when he says: but this presents another difficulty (437a26), he raises a difficulty about the foregoing.

For it is clear that a sense power apprehends a sensible thing that is present, and hence that sight apprehends a visible thing that is present; and fire, because of its light, is something visible. Therefore, if fire is always present to sight, that is, to the organ of sight, since it exists in it, it seems that sight should always be seeing fire.

But this does not follow from the principles that Aristotle has established, for he holds that a sense power is in potentiality to a sensible thing, and that it must be altered by the thing through a medium. According to him, then, a sensible thing placed on top of a sense power is not perceived, as is said in On the Soul 2, (Aristotle, On the Soul 2.7, 419a11–13, 28–30; 2.11, 423b20–22; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 15, nn. 432, 437; lect. 23, n. 545.) so that if the organ of sight were made of fire, for this very reason sight would not see fire.

But according to the other philosophers sight and the other senses perceive sensible things inasmuch as they actually are of the same kind as—that is, like—the sensible things, because they have the same nature as the principles of things, as was said.

But then there remains the difficulty that Aristotle here introduces: why does an eye at rest not see fire, as does an eye that is moved?

37. Then, when he says: now the cause of this (437a30), he gives the cause of the above-mentioned appearance, by which the difficulty raised is solved, and it is shown how foolish of them it was to think that sight is made of fire.

On this point it must be understood that smooth—that is, polished and clean—bodies have, from a property of their nature, a certain shine, one that does not occur in rough and uneven bodies because some parts rise above others and overshadow the latter. And although such bodies in a way shine in themselves, they do not have enough shine to be able to make a medium bright in actuality, as do the sun and such bodies.

Now it is clear that the center of the eye, which is called the black part of the eye, is as it were smooth and polished. Hence it has a shine by reason of its smoothness, not from the nature of fire, as the others thought. This already removes any need to assign the organ of sight to fire, since the cause of the brightness that appears can be attributed to something other than fire.

But whether it is caused by fire or the pupil’s smoothness, there remains a difficulty common to both positions: why an eye that is moved sees this shining, but an eye at rest does not.

38. So he gives the cause of this, and says that shining of this kind appears to an eye that is moved because what happens through the movement of the eye is that what is one becomes as though it were two. For the shining and seeing pupil is one and the same in subject: but inasmuch as it shines, it projects its shine outward; and inasmuch as it sees, it apprehends the shining by, as it were, receiving it from without.

When the eye is still, therefore, it emits the shine outward, and so sight does not receive the shine in such a way as to see it. But when it is quickly moved, its black part is brought, before the brightness fades, to the “external” place to which the pupil emitted its brightness. Thus the pupil, having been quickly brought to the second place, receives its own brightness as if from without, and so it seems that what sees and what is seen are different, although they are the same in subject. So this appearance of the shining does not happen unless the eye is quickly moved, because if it is moved slowly, the impression of the shining will fade from the “external” place the shining went to before the pupil gets there.

39. But it seems that no speed of movement would suffice for this to happen. For however rapid local movement may be, it must be in time. But emission of shine at the presence of a shining body, and its cessation at the absence of it, occur instantaneously. Therefore it does not seem possible, however quickly the eye is moved, for the pupil to reach the “external” place before the shining, which came there from the pupil existing in the other place, ceases.

According to Alexander in the commentary, we must say in response that the pupil is a body, divisible into parts. Hence if the eye is quickly moved, when part of the pupil begins to reach the other place, the shining is still coming there from the rest of the body of the pupil that has not yet gotten to that place. This is why the pupil begins to see a shining that is, as it were, shining from elsewhere. A sign of this is that the shining does not seem to fade: rather, when it passes, the sight of it ceases abruptly. (Alexander of Aphrodisias, On Aristotle’s “On Sense Perception,” trans. Alan Towey, pp. 30–31.)

40. He also gives the reason why this kind of operation happens in darkness (437b5) and not in light, namely because the shining of smooth bodies is so small that it is obscured by great brightness, although it is seen in darkness. This also happens with other things that have a small amount of light, and for this reason are seen in darkness but not in light, for instance certain heads of fish, and the dark fluid of the fish called the cuttlefish. He adds that if the eye is gently—that is, slowly—moved, the abovementioned appearance does not happen by what sees and what is seen seeming simultaneously to be one and two, as was said. But in the other case—that is, when the eye is quickly moved—then the eye sees itself, being as it were affected by itself in its other position. This also happens in refraction—that is, reflection—for instance when an eye sees itself in a mirror from which, as from without, the form of the eye returns to the eye itself by way of reflection, just as, in the above-mentioned appearance the shining of the eye returns to the eye itself, as was said.

41. Then when he says: if it were fire (437b10), he proceeds to disprove the position itself:

first, inasmuch as those philosophers assigned sight to fire;

second, inasmuch as they held that sight sees by extromission, where he says: it is altogether irrational (438a25; [55]).

On the first point, he does three things.

He presents first the opinion of Plato;

second the opinion of Empedocles, where he says: Empedocles seems to think (437b23; [46]);

and third the opinion of Democritus, where he says: Democritus says correctly (438a5; [48]).

On the first point, he does two things.

First, he raises an objection against Plato.

Second, he disproves Plato’s response, where he says: to say that in going out (437b14; [42]).

Concerning the first point it must be known that Empedocles, and Plato in the Timaeus, agreed on two things:

one is that the organ of sight pertains to fire;

the second is that vision occurs by a light going out from the eye as from a lamp.

From these two positions the Philosopher concludes that sight should see in darkness, just as it does in light: for light can be emitted from a lamp to light up the medium in the dark; and so, if the eye saw by emission of light, it would follow that the eye should also be able to see in the dark.

42. Then, when he says: to say that in going out (437b14), he disproves the response of Plato, who says in the Timaeus that when light goes out from the eye, if it encounters light in the medium it is preserved by it as by something like itself, and because of this vision occurs; but if it does not encounter light, but darkness, then because of the unlikeness of darkness to light, the light going out from the eye is extinguished, and so the eye does not see.

43. But Aristotle says that to give this cause is foolish. And he proves this where he says: for what is extinction of light? (437b15)

For no reason can be given why the light of the eye is extinguished by darkness. The Platonists said that there are three species of fire, namely light, flame, and ember. (See Aristotle, Topics 5.5, 134b28–135a8; 5.8, 138b19–21.) Since fire is naturally hot and dry, it is extinguished by either cold or moisture, and this clearly happens in the case of embers and flame. But neither is a cause of extinction in the case of light, for it is destroyed neither by cold nor by moisture. Therefore it is incorrect to say that light is extinguished in the way fire is.

But Alexander says in the commentary that there is another reading, as follows: as is seen in fire in embers and flame. But neither seems to belong to darkness. For it is neither moisture nor cold by which the extinction occurs. (Alexander of Aphrodisias, On Aristotle’s “On Sense Perception,” trans. Alan Towey, p. 32.) And on this reading Aristotle’s argument seems to be more to the point: the fire-light that appears in embers and flames is found to be extinguished by cold and moisture; but darkness is neither something cold nor something moist; therefore the fire-light that goes out from the eye cannot be extinguished by darkness.

44. Someone might say that the fire-light going out from the eye is not extinguished in darkness, but that, because it is not strengthened by external light, and so is weak, it escapes our notice (437b19), and that this is why vision does not occur.

45. But Aristotle disproves this where he says: if it were the case.

On this point it must be known that fire-light is extinguished or darkened in two ways: in one way with respect to a property of light, namely that a little light is extinguished by the presence of a greater light; in another way with respect to a property of fire, which is extinguished in water.

Accordingly, if the weak light going out from the eye were made of fire, it would have to be extinguished in daylight, because of the greater brightness; and in water, because of its contrariety to fire; and the light of the visual power would be even more darkened where there is ice, for we see that this is what happens in the case of flame and burning bodies. But it does not happen in the case of sight, and so it is clear that the abovementioned response is foolish.

46. Then, when he says: Empedocles seems to think (437b23), he relates the opinion of Empedocles, the disproof of which has already been touched on.

He says that Empedocles seems to think, as was said, that vision occurs by light going out.

And he gives the words of Empedocles, which were composed in meter. Empedocles said that what happens in vision is like when someone thinking of going out along a road on a winter night—that is, when wind is blowing—prepares a lamp. He kindles a light of burning fire, and to block the force of all winds he puts the light kindled in a lantern, and by this means he deflects the blowing of breathing winds—that is, he prevents their blowing from reaching the light of the fire. But the light contained within goes out, and however farther outward it expands, it illuminates the air more, but in such a way that the rays going out are subdued—that is, weakened—by a covering on the lamp, for instance skin or some such thing; for the air is not as brightly illuminated by the lamp as it would be by unshielded fire.

And he says that something similar happens in the eye, in which ancient light—that is, light there since the eye’s formation—is guarded—that is, safely preserved—for sensing, in membranes—that is, corneas of the eye—through which, as if through fine linens, light pours out around in all directions through the pupil. These corneas reveal, by the rays emitted through them, a depth of water flowing around the fire kindled in the pupil, water for nourishing, or rather tempering, the fire gathered in the depth. And so the light reaches out, however farther it expands, starting from inside.

Alternatively, his mention of around in a circle should be understood with reference to the roundness of the pupil.

47. It should be noted that he said, significantly, with rays subdued by a covering, to indicate the reason why a thing is not seen in darkness, namely that the light going out is weakened by passing through the above-mentioned coverings, so that it cannot completely illuminate the air.

Having presented the words of Empedocles, he adds that sometimes he said that vision occurs by emission of light, as was said, but sometimes he said that vision occurs by certain bodies emanating from visible things and coming to sight. And perhaps his opinion was that the two are united for vision.