Lecture 3 — Sight constituted of water

Democritus says correctly that it is water, but not correctly that he thought that the appearance is the seeing itself. For this occurs because the eye is smooth. And it is not in that, but in what is doing the seeing. For the affection is a reflection. But it seems that it was not yet clear to him about appearances and reflection.

But it is inconsistent that it also did not occur to him to wonder why only the eye sees, and none of the other things in which “idols” appear.

For it is indeed true that sight is made of water, but seeing does not occur according as it is water, but according as it is transparent, which is something common to it and air. But water is more preservable than air and denser, which is why the pupil and eye are made of water.

This is manifested in the very workings. For when eyes are destroyed, water can be seen flowing out. And in completely new-formed ones, there is extreme cold and brightness. And in those that have blood, the white of the eye is fat and thick, to keep the moisture unfrozen; and so the eye is the part of the body that feels cold least, for no-one has ever felt cold inside the eyelids. The eyes of bloodless animals are made of hard skin, and this provides protection.

It is altogether irrational that sight should see by something going out, whether it extends all the way to the stars, or, as some say, it goes only so far and “coalesces.”

For in the latter case it is better for it to be united at the beginning, that is, in the eye.

But this is also foolish. For what is it for light to be “united” to light? Or how is it possible, since not just anything is united with anything? And how is the inner light united with the outer, since the membrane is between?

48. Democritus says correctly that it is water (438a5). After the opinions of Plato and Empedocles, here, in the third place, the Philosopher follows up the opinion of Democritus. On this point, he does three things.

First, he shows what Democritus said correctly and what he said incorrectly.

Second, he follows up what he said incorrectly, where he says: but it is inconsistent (438a10; [52]).

Third, he follows up what he said correctly, where he says: for it is indeed true that sight is made of water (438a12; [53]).

Accordingly, he first says that Democritus spoke correctly in assigning sight to water, but he spoke incorrectly in saying he thought that vision is nothing but the appearance in the pupil of the thing seen. For such an appearance occurs in the pupil because of the bodily disposition of the eye, that is, because the eye is smooth, that is, polished and clean, as it were. So it is clear that seeing itself is not located in the appearance of this form in the eye, but in what is doing the seeing, that is, in what has the power of sight: for the eye is a seeing thing not because it is smooth, but because it has the power of sight. For that affection—the appearing of the form of the thing seen in the eye—is a reflection; that is, it is caused by the reflection or rebound of the form from a polished body.

We see something like this happen in a mirror also. For when the alteration of the transparent caused by a visible body reaches a body that is not transparent, the alteration can go no further, but is somehow turned back, like a ball thrown at a wall and bounced back, and because of this rebound the form of the thing seen goes back in the opposite direction. Thus in a mirror one can see oneself, or even some other thing not directly presented to one’s sight.

49. This takes place only if two things coincide. One is that the body be smooth on its surface, and therefore somewhat bright, as was explained above; (1.2, 437a31–32; cf. ch. 2, nn. 37–38.) the form reflected in it is manifested by this moderate brightness. The other is that the body be terminated within at some point so that the above-mentioned alteration does not go beyond; thus we see that this kind of appearance does not occur, unless glass is covered with lead or something similar to block its transparency, so that the alteration does not go beyond.

Now both of these coincide in the eye: it is moderately bright because of its smoothness, as was established above, (1.2, 437a32–b1; cf. ch. 2, nn. 37–38.) and it has something in its depth that terminates its transparency. So it is clear that this event of the form of the thing seen appearing in the eye happens merely because of reflection, which is a bodily affection caused by the determinate disposition of a body.

50. But it was not yet clear to Democritus about these reflections and about the forms that appear in mirroring bodies because of reflection. Vision itself, according to the truth of the matter, is not a bodily affection: rather, its principal cause is a power of soul. But Democritus held that soul is something bodily, and so it is no wonder that he called an operation of soul nothing but a bodily affection.

51. It should be known, however, that the above-mentioned appearance is a cause of vision with respect to initial reception of the form. For vision is an act of soul only through a bodily organ, and so it is no wonder that it has a cause from the point of view of a bodily affection, but in such a way that the bodily affection is not the same thing as the vision: rather, it is a cause of it with respect to the initial “impact,” if I may so call it, of the visible form on the eye. The subsequent reflection contributes nothing to the eye’s seeing of the thing seen through the form appearing in it, but rather contributes to the form’s being able to appear to someone else; thus the eye that sees the thing by means of the form that appears in it does not see this form itself.

52. Then, when he says: but it is inconsistent (438a10), he follows up what Democritus said incorrectly.

He says that since Democritus held that vision is nothing but the above-mentioned appearance, it seems very inconsistent that the following difficulty did not occur to him: why do other bodies in which forms—which he called idols—of visible things appear as in a mirror not see, but only the eye? From this it is clear that the above-mentioned appearing is not the whole essence (ratio) of vision, but that there is something else in the eye that causes vision, namely the power of sight.

53. Then, when he says: for it is indeed true that sight is made of water (438a12), he follows up what Democritus said correctly.

First, he presents the truth.

Second, he clarifies it by signs, where he says: this is manifested (438a17; [54]).

Accordingly, he first says that what Democritus said in assigning the organ of sight to water is true. However, it must be known that vision is assigned to water not according as it is water, but by reason of transparency, which water and air have in common: for a visible object is something that moves the transparent, as is said in the book On the Soul. (Aristotle, On the Soul 2.7, 418a31–b1; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 14–15, nn. 399–438.) However, vision is assigned to water rather than to air for two reasons.

First, water can be preserved better than air (air is easily dispersed), and so it is more suitable for the preservation of sight than is air, and nature always does what is better.

Second, water is denser than air, and by reason of its density it allows the form of a thing seen to appear in it by reflection, which is proper to the organ of sight.

What is proper to the medium of sight is to be transparent, which is common to air and water. So he concludes that the eye and pupil are to be assigned to water rather than to air.

The heavenly body is also transparent, but because it does not enter into the composition of the human body, he passes over it here.

54. Then, when he says: this is manifested (438a17), he shows that the organ of sight is made of water by three signs that are manifested in the very workings.

The first is that if the eyes are destroyed, there visibly appears water flowing out of them.

The second is that the new-formed eyes of embryos—eyes that, as it were, still retain much of the power (virtus) of their origin—have abundant cold and brightness, both of which are connatural to water.

The third sign is that in animals that have blood, in which there is the possibility of generating, so to speak, fat from the blood, the pupil is surrounded by the white of the eye, which has fatness and oiliness so that its heat will keep the water-moisture of the pupil from freezing, which would diminish the transparency of the water, and thus impede vision. And so, by reason of the above-mentioned fat, the eye, because of its heat, does not freeze: for no-one has ever suffered cold in the whole of what is contained inside the eyelids.

In animals that are bloodless, in which there is no fat, nature made eyes of hard skin to protect the water-moisture that is in the pupil.

55. Then, when he says: it is altogether irrational (438a25), he goes on to disprove what some held, namely that vision occurs by extromission, which was the reason for their assigning sight to fire. Thus, once the latter is eliminated, the former will be too. On this point, he does two things.

First, he presents two opinions of those who held that we see by extromission.

Second, he disproves the second opinion, where he says: for in the latter case (438a27; [57]).

Accordingly, he first says that it seems irrational that sight should see by something going out from it, which was held to occur in two ways.

In one way such that what goes out from the eye extends all the way to the thing seen, from which it would follow that even when we see the stars, what goes out from sight extends all the way to the stars.

This involves an obvious impossibility. Since “going out” pertains only to bodies, it would follow that some body going out from the eye would reach all the way to the stars, which is clearly illogical, for many reasons.

First, because it would follow that there are several bodies in the same place, both because what goes out from the eye would be in the same place as the air, and because these things going out from eyes would have to be multiplied in the same medium according to the multiplicity of those who are seeing through the same medium.

Second, because any projection of a body is stronger at the beginning but weaker at the end, which is why flame proceeding from a burning body tends towards an apex. But here the contrary happens, for the mathematicians, whose position this is, say that the apex of the body going out from the eye is inside the eye and the base at the thing seen.

Third, because the size of an eye is insufficient for a body going out from it to be big enough to reach all the way to the stars, however much the body might be rarefied, for there is a limit to the rarefaction of natural bodies; besides, the more rarefied it became, the more easily it would be destroyed.

Again, the body emitted from the eye would have to be either air or fire. There is no need for air to be emitted from the eye because it is abundant outside the eye. And if it were fire we would also see at night; on the other hand, we would not be able to see through the medium of water; and we would only be able to see upwards, where the movement of fire tends.

But it cannot be said that the bodily thing that goes out from the eye is light, because light is not a body, as was proved in the book On the Soul. (Aristotle, On the Soul 2.7, 418b13–26; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 14, nn. 406–8, 413–14.)

56. The other opinion is that of Plato, who held that light going out from the eye does not go all the way to the thing seen, but only so far, that is, some determinate distance, namely to where it coalesces with the external light; and that vision occurs by reason of this coalescence, as was said before. (Cf. ch. 2, n. 41.)

57. Then, when he says for in the latter case (438a27), he passes over the first opinion as clearly unreasonable, and disproves the second in two ways.

First, because it posits something uselessly and unnecessarily, which is just what he says: that it would be better to say that the inner light is united with the outer at the surface of the eye itself rather than outside at some distance. This is because if there is no outer light in the intermediate space, the inner light will be extinguished by darkness according to Plato’s opinion, as was said above. (Cf. ch. 2, nn. 42–45.) But if light reaches all the way to the eye, it is better for the two to be united immediately, because what can occur without a medium is better than what occurs through a medium, since it is better for something to occur by means of fewer things than more.

58. Second, where he says but this is also foolish (438a29), he disproves the union of inner light with outer even on the supposition that it occurs at the beginning, i.e., in the eye itself. He does this in three ways.

First, because “being united and separated” is proper to bodies, each of which has subsistence per se, but not to qualities, which exist only in a subject. Hence, since light is not a body but an accident, it is meaningless to say that light is “united” to light, unless what is meant is that a luminous body is united to a luminous body. However, it is possible for light to be intensified in the air by multiplication of luminous bodies, as heat is intensified by increase in the cause of heat, although this is not by addition, as is clear from Physics 4. (Aristotle, Physics 4.9, 217a26–b11; cf. Aquinas, Commentary on Physics, bk. 4, lect. 14.)

Second, he disproves the point as follows: even granted that both lights are bodies, it would still not be possible for them to be united, since they are not of the same nature. For not just any body is naturally united with just any body, but only those that are in some way homogenous.

Third, since an intervening body, namely the membrane—that is, the cornea of the eye—comes between the inner and the outer light, there cannot be a union of the two lights.