Lecture 4 — The matter of each sense-organ

That it is impossible to see without light was said elsewhere. But whether it is light or air that is between the thing seen and the eye, the movement through it causes seeing.

It is reasonable that what is inside be made of water, for water is transparent. And it seems that, as what is outside is not without light, so also what is inside. Therefore it must be transparent. Therefore it is necessarily water, because it is not air.

For the soul, or the sensitive part of soul, is not at the limit of the eye, but clearly inside. Hence the inside of the eye needs to be transparent and receptive of light.

This is also clear from what happens. For when some are wounded in war about the temples in such a way that the passages of the eye are cut off, they experience a darkening as when a lamp is extinguished, because the transparent thing called the pupil is, like a torch, cut off.

If, then, what happens in these cases is as we have said, it is clear that, if one must, following this method, attribute and assign each one of the sensitive parts to one of the elements, one should think that eyesight is of water, what is perceptive of sounds is of air, and smelling is of fire.

For what smelling is in actuality, the olfactory part is in potentiality. For a sensible object makes a sense power act, and so the latter necessarily exists first in potentiality. But odor is smoky evaporation. But smoky evaporation is from fire.

For this reason the proper sensitive part of smell is in a place around the brain: for the potentially hot is the matter of what is cold.

And the generation of the eye also has the same mode: for it stems from the brain, and the brain is coldest and moistest of all parts of the body.

But the tactile part is of earth, and the tasting part is a kind of touch.

And so the sensitive part for these, namely taste and touch, is near the heart: for the heart is opposite the brain, and is the warmest of the parts.

Let it be determined in this way about the sensitive parts of the body.

59. That it is impossible to see without light (438b2), etc. After the Philosopher has disproved the opinion of those who hold that vision occurs by extromission, here he determines the truth. On this point, he does three things.

First, he makes clear how vision occurs according to his own thought.

Second, on this basis, he gives the cause of something mentioned above concerning the organ of sight, where he says it is reasonable (438b5; [63]).

Third, he shows the cause by a sign, where he says this is also clear (438b11; [65]).

Accordingly, he first takes up something that was said in the book On the Soul, that it is impossible to see without light. (Aristotle, On the Soul 2.7, 418a26–b3; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 14, nn. 423–26.) For because vision occurs through a medium that is transparent, vision requires light, which makes a body be transparent in actuality, as was said in the book On the Soul. (Aristotle, On the Soul 2.7, 418b9–13; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 15, nn. 431–32.) And so, whether the medium that is between the thing seen and the eye is air that is illuminated in actuality, or whether it is light—light existing not in itself, since it is not a body, but in something else that is a body, such as glass or water—the movement that occurs through this medium causes vision.

60. This movement should not be taken to be local movement, as if, as Democritus and Empedocles held, it were a movement of bodies emanating from the thing seen to the eye, because then it would follow that the bodies that are seen would be reduced by this emanation until they were totally worn away. It would also follow that the eye would be injured by the continuous striking of these bodies. Again, it would not be possible for a whole body to be seen by anyone, but only as much as could be taken in by the pupil.

61. Rather, this is “movement” according to alteration: the alteration is a movement towards a form that is a quality of the thing seen. Inasmuch as it is bright in actuality, the medium is in potentiality to this form; the medium is an unbounded transparent (color is a quality of a bounded transparent, as will be said below), (1.3, 439a25–b14; cf. ch. 5, n. 93.) and what is unbounded is related to what is bounded as potentiality to actuality, for form is a boundary of matter.

62. Now because of the different nature (ratio) of the transparent in a transparent medium, the medium receives the form of a color in a mode that is different from the mode in which it exists in the colored body, where there is a bounded transparent, as will be said below; for actualities are in receivers according to the mode of the latter. (1.3, 439b1–5; cf. ch. 5, nn. 87–92.) Thus color is in a colored body as a quality complete in its natural being, but it is in the medium incompletely, according to an intentional being. Otherwise something black and something white could not be seen through the same medium. For whiteness and blackness cannot simultaneously be in the same thing as forms complete in their natural being, but with respect to the above-mentioned incomplete being they do exist in the same thing: for this mode of being, because of its imperfection, approaches the mode by which something exists in something else in potentiality, and opposites are simultaneously in potentiality in the same thing.

63. Then, when he says it is reasonable (438b5), in keeping with what was said he gives the cause of the necessity of assigning sight to water, which he showed above only by signs. (Cf. ch. 3, nn. 53–54.)

He says that, because what causes vision is the alteration of a medium made bright by the body seen, it is reasonable that what is inside the pupil, which is the organ of sight, be made of water: for water is one of the transparent things. But the external medium is a transparent thing that has been illuminated, and nothing can be seen without this illumination: so the light must also be inside the eye. And because there can be light only in what is transparent, there must also be something transparent inside the eye. It is not the heavenly body, because this does not enter into the composition of the human body: and so it is necessarily water, which is easier to preserve and thicker than air, as was said. (Cf. ch. 3, n. 53.)

64. Then, when he says for the soul (438b8), he makes clear why light within is required for seeing.

If the power of sight were on the outer surface of the eye, the light of the external transparent, through which the alteration by the color reaches the outer surface of the pupil, would alone suffice for seeing. But the soul, or the sensitive part of soul, is not on the outer surface of the eye, but inside.

It should be noted that he significantly adds or sensitive part of soul. Since the soul is the form both of the whole body and of its individual parts, it is necessarily in the whole body and in each of its parts, because a form is necessarily in that of which it is the form. (Cf. ST I.76.8.) Now the sensitive part of the soul is called the sensitive “power,” being the principle of sensitive operation; and the principle of an operation of soul that is exercised by means of the body must be in a determinate part of the body. Thus the principle of sight is within, near the brain, where two nerves coming from the eyes meet.

Therefore, inside the eye there must be something transparent, receptive of light, so that there is uniform alteration from the thing seen all the way to the principle of sight.

65. Then, when he says this is also clear (438b11), he makes clear what he said by means of a sign, namely something that happens in some who are wounded around the temples in battle: when the passages that connect the pupil to the principle of sight are cut off, a darkening suddenly occurs through loss of sight, as if a lamp were extinguished. The pupil is like a torch lit up by an external light; thus, when the passages connecting the pupils to the principle of sight are cut off, the light of this torch cannot reach all the way to the principle of sight, and so sight is darkened.

66. Then, when he says if, then, what happens in these cases (438b16), having eliminated false opinions of others, he proceeds to the principal proposal.

First, with respect to the organs of the non-necessary senses.

Second, with respect to the organs of the necessary senses, where he says but the tactile part (438b30; [73]).

On the first point, he does two things.

First, he coordinates sense-organs with elements.

Second, he clarifies what he said, where he says for what smelling is in actuality (438b21; [68]).

On the first point it must be considered that it was not in keeping with the thought of Aristotle to assign the sense-organs to elements, as is clear in the book On the Soul. (Aristotle, On the Soul 3.1, 425a5–7; 3.13, 435a14–15; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 25, nn. 571–76.) But because other philosophers did assign the sense-organs to the four elements, therefore, condescending to them, as it were, in this matter, he says that, presupposing what was said about sight, if one must, following what others say, assign each of the sensitive parts—that is, sense-organs—to one of the elements, as others do, one should think that eyesight is to be assigned to water, what is perceptive of sounds to air, and smelling to fire.

67. But this seems to be contrary to what was said in On the Soul: the pupil is of water, hearing of air, and smell is of either of these; but fire either belongs to none or is common to all. (Aristotle, On the Soul 3.1, 425a4–6.)

In response to this it must be said that what the sense of smell is can be taken in two ways:

in one way according to potentiality, and thus the organ itself of smell is made either of air or of water, as is said in On the Soul 3; (Aristotle, On the Soul 3.1, 425a5; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 3, lect. 14, nn. 491–95.)

in another way according to actuality, and thus what is said here is true, as he himself will prove. Thus it is significant that he did not say that the sense of smell is made of fire, as he did say that the part perceptive of sounds is made of air and eyesight is made of water. Rather he says that smelling is made of fire. For sense of smell refers to the potentiality, but smelling to the actuality.

68. Then, when he says for what smelling is in actuality (438b21), he proves what he has said about the organ of smell. On this point, he does three things.

First, he shows that the act of smelling in actuality is related to fire.

Second, he concludes to what should be the quality and place of the organ of smell, which is the act of smelling in potentiality, where he says for this reason (438b25; [69]).

Third, he shows a resemblance of the organ of smell to the organ of sight, where he says and the generation of the eye (438b27; [70]).

Accordingly, he first says that the olfactory part—that is, the organ that has power to smell—must be in potentiality what actual smelling is in actuality. He clarifies this as follows: a sensible object makes a sense power act—that is, be in actuality, or again, operate. But the sensitive part must be in potentiality the sensible object: otherwise it would not be affected by it. Hence it remains that the sensitive part is in potentiality what sensing is in actuality.

Now it is clear that odor is smoky evaporation. Not that “smoky evaporation” is the very essence of odor, for this was disproved in On the Soul 2 on the grounds that odor spreads farther than does smoky evaporation. (Gauthier, p. 30, indicates that Aquinas is referring to his own commentary on On the Soul 2.9, 421b8–13. Gauthier also notes the contradiction between Aristotle’s claim in 438b24 that odor is a smoky evaporation and his argument in 443a21–b2 that it is not; see Ross, Parva naturalia (Oxford: Oxford University Press, 1955), p. 194.) Rather this is said because smoky evaporation is a cause of odor being perceived. But smoky evaporation comes from fire, or whatever is hot. Therefore the sense of smell is brought to actuality by heat, which is in fire as in its principle. This is why flowers have stronger odor in hot times and places.

69. Then, when he says for this reason (438b25), he concludes from the foregoing that the organ of smell should be in a place near the brain.

For the organ of smell is, in potentiality, odor in actuality; odor in actuality exists by heat or fire; therefore the organ of smell must be hot in potentiality. But the potentially hot is the matter of what is cold, because the matter of contraries is the same, and it cannot be in potentiality to one of them without being in actuality under the other, either perfectly or, when it is under the form of an intermediate, imperfectly. So the substance of the organ of smell must be something that is cold and moist in actuality, and this is especially the case around the brain. Hence the organ of smell is around the brain.

70. Then, when he says and the generation of the eye (438b27), he shows a point of agreement between the organ of smell and the organ of sight.

He says that the generation of the eye also has the same mode, inasmuch as it stems from the brain. It does so because the brain is the coldest and moistest of all parts of the body, and so it has the nature of water, which is naturally cold and moist. Thus the brain fits both with the organ of smell, which should be hot in potentiality, and with the organ of sight, which should be made of water.

71. But then it seems that Plato fittingly assigned sight to fire, as Aristotle here assigns smell to fire.

To this it must be said that the organ of smell is made of water inasmuch as water is potentially hot, heat being a feature of fire. But the organ of sight is made of water inasmuch as water is transparent, and consequently luminous in potentiality.

72. But because fire is also luminous in actuality, as well as hot, someone might then say that sight is fittingly assigned to fire.

To this it must be said that, just as Aristotle assigned smell to fire, so nothing prevents sight from being assigned to fire—not according to the proper qualities of fire, which are heat and dryness, but according as fire is luminous in actuality.

The other philosophers seem to have focused on this, basing their argument on the shining that appears when the eye is moved. Aristotle disproved their opinion in this regard not because they held that sight in actuality is fire, which in a way would be true, inasmuch as sight in actuality does not occur without light, as the act of smelling in actuality does not occur without heat; rather, he did so because they held that the organ of sight is luminous in actuality, holding as they did that sight occurs not by a taking in, but by an extromission.

73. Then, when he says but the tactile part (438b30), he makes a determination about the organs of the necessary senses.

First, he shows the element to which they are to be assigned.

Second, he shows the place where they are situated, where he says and so the sensitive part (439a1; [74]).

Accordingly, he first says that the organ of touch is assigned to earth, and similarly the organ of taste, which is a kind of touch, as was said in On the Soul 2. (Aristotle, On the Soul 2.9, 421a19; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 18, nn. 865–68, 873; lect. 17, n. 861.)

Now this is not to be taken to mean that the organ of touch or taste is simply made of earth, for we do not perceive by means of hair, which contains even more earth. It is rather that, as is said in On the Soul 3, earth is mixed into the organs of these senses to the greatest degree. (Aristotle, On the Soul 3.1, 425a7.)

With respect to the organ of touch, the reason, as is said in On the Soul 2, is this. (Aristotle, On the Soul 2.11, 423b26–424a10; 3.13, 435a21–22; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 23, nn. 546–48.) In order for the organ of touch to be in potentiality to contrary tangible qualities, it has to have a composition that is intermediate. Therefore, it has to have the greatest amount of earth, which has less active power than do the other elements.

With respect to the organ of taste, the reason is clear. The organ of smell has to contain water in order to be in potentiality the hot thing without which there is no smelling in actuality. Similarly, the organ of taste has to contain earth in order to be in potentiality the moist thing without which there is no tasting in actuality.

74. Then, when he says and so the sensitive part (439a1), he shows where the organ of taste and touch is based.

He says that it is near the heart, and he gives the reason, namely that the heart is opposite the brain with respect to position and quality. As the brain is the coolest part of the body, the heart is the warmest of all the parts of the body. For this reason they are placed in opposition to one another in order for the heart’s heat to be moderated by the brain’s coolness.

This is why those who have heads that are small in proportion to their other parts are impetuous, as if the heat from the heart were not being pushed back down enough by the brain. Conversely, those who have unusually big heads are very slow and dull, as if the heat from the heart were being impeded by the size of the brain.

Since the organ of touch is made of earth, it must be in the body’s warmest place as in its principle, so that by the heart’s heat the coolness of the earth can be brought to a moderate temperature.

75. This does not prevent an animal from perceiving by means of touch through the whole of its body, because as other senses perceive through an external medium, touch and taste perceive through an internal medium, namely flesh. And just as the principle of sight is not on the surface of the eye, but within, the principle of touch is also within, near the heart, a sign of which is that the most painful wounds are those around the heart.

76. But it cannot be said that there are two sensitive principles in an animal, one near the brain, where the visual, olfactory, and auditory principle is established, and one near the heart, where the tactile and gustatory principle is established. The sensitive principle is primarily in the heart, which is also where the source of heat is located in the body of an animal, because nothing is sensitive without heat, as is said in On the Soul. (Aristotle, On the Soul 3.1, 425a6; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 18, nn. 474–75.) The sensitive power flows from the heart to the brain, and from there it proceeds to the organs of three senses, sight, hearing, and smell. But touch and taste are relayed to the heart itself through a medium united to the body, as was said.

Finally, by way of an epilogue, he says let it be determined in this way about the sensitive parts of the body (439a4)—that is, as was done above.