Lecture 13 — How non-breathing animals smell

It is clear that the ones that do not breathe have sensation of the odorous. For both fish and the whole class of insects keenly sense from a distance because of the nutritive species of odor, although they are very far from their proper food. So do bees in relation to honey; and the genus of small ants some call “hexapods”; and, among marine animals, the purple-fish. And many other such animals acutely sense their food because of odor.

But what they sense it with is not so clear. Therefore someone might raise the difficulty what it is they sense odor with, since smelling takes place in all animals that do breathe in one and the same way: for this is what seems to happen in all animals that do breathe, but none of these animals breathe, and yet they do sense. Unless there is some other sense besides the five? But this is impossible: for the odorous is the object of the sense of smell, and the odorous is what they sense.

But perhaps not in the same way. In the ones that breathe, the breath removes something that lies over as a covering, which is why they do not sense when they are not breathing. But in the ones that do not breathe this is absent. It is similar with the eyes. Some animals have eyelids, and when these are not opened they cannot see at all. But animals that have hard eyes do not have these, and so they do not need anything to open them; rather, they see immediately by the faculty in them.

Likewise, none of the other animals disdains what is of itself foul with respect to odor, unless something happens to be injurious, and they are injured by such things in the same way. Just as human beings suffer cold in the head from coal-smoke, and are often injured by it, and are injured by the power of sulfur, so other animals also avoid these because of the affection. But they do not care about the foulness itself in itself—although many growing things have foul odors—but only whether it affects the taste or the food. The senses exist in an odd number, and an odd number has a middle.

The sense for smelling seems to be a middle between the tactile senses, touch and taste, and those that sense through something else, sight and hearing. Thus the odorous is an affection of nourishment; the objects of the former senses are in the same genus. But it is also in the genus of the visible and the audible; thus things are smelled both in air and in water. So the odorous is something common to both; it is both in the genus of the tangible, and in that of the transparent and the audible. Therefore odorous enchymous dryness in moistness and fluidity is reasonably compared to a tincture and a wash. Let this much be said, then, about how we should speak of species of the odorous, and how we should not.

What some Pythagoreans say is not reasonable: they say that some animals are nourished by odors.

For, first, we see that food must be composite, for what is nourished is also non-simple. This is also why a superfluity of food is produced, whether inside, or, in the case of plants, outside. Moreover, water alone is not going to nourish by itself, for what is going to build up has to be something bodily. Moreover, it is much less reasonable that air should become bodily.

In addition to this, we see that all animals have a place that is able to receive food, from which, after the food enters, the body receives it. But the part for the odorous is in the head, and odor enters with the breath, and so goes to the place for breathing. It is clear, then, that the odorous as such does not contribute to nourishment.

But it is clear that it does contribute to health, both from observation and from what has been said. Thus the odorous is to health what flavor, in the nourishing part, is to nourishment.

Let this be the determination, then, with respect to each sensitive part.

191. It is clear that the ones that do not breathe (444b7), etc. After the Philosopher has shown that men and certain other animals smell by breathing, here he inquires into the way non-breathing animals smell. On this point, he does two things.

First, he shows what is clear in such animals;

second what is unclear, where he says but what they sense it with (444b15; [193]).

Accordingly, he first says that it is clear that animals that do not breathe sense the odorous, because, as we see, fish and the whole class of insects—that is, partitioned animals such as ants, bees, and the like—acutely sense their nourishment from a distance, when they are too distant from their proper food to be able to perceive it by their own sense of sight. Hence it is clear that they perceive it because of the nutritive species of odor—that is, inasmuch as they perceive the kind of odor that is analogous to flavor and indicates quality of nourishment.

192. He gives the example of bees, which are moved from a distance to seek honey; and that of small six-legged ants, which are also moved from a distance to seek their food; and that of certain other animals called purple-fish because of their color. Likewise many non-breathing animals are found that acutely sense their food from a distance because of odor.

193. Then, when he says but what they sense it with (444b15), he shows what the difficulty is with such animals. On this point, he does three things.

First, he raises the difficulty.

Second, he solves it, where he says but perhaps not in the same way (444b21; [195]).

Third, he clarifies the solution by means of a comparison, where he says likewise, none of the other animals (444b28; [197]).

Accordingly, he first says that although it is clear that the above-mentioned animals perceive odor, what they sense it with is not so clear. The reason for the difficulty is that all breathing animals sense odor in one and the same way, namely by breathing: for it is clear from experience that this seems to happen in all breathing animals. But in the case of the abovementioned animals, it is clear that they do not breathe, and that yet they do sense the odorous.

194. However, one might give a reason for this difference by saying that such animals sense the odorous by some other sense besides the five senses to which names have been given. This answer might seem probable for the following reason. To sense is to be affected by something, and so a different mode of sensing is as it were a different mode of being affected that indicates a difference of passive power. It is like the way in which a different mode of acting signifies a difference of active power; as we see, the stronger heat is, the more vehement is the action of heating. Likewise, then, on the side of what is passive, what is affected in a different mode seems to have a different passive power, and thus what senses in a different mode seems to have a different sense power.

But it is impossible for them to sense the odorous in a different mode, because where there is the same sensible object there is the same sense power, for powers are distinguished according to objects. But it is the same sensible object that both kinds of animal sense, namely the odorous. Hence it cannot be that the sense powers are different.

195. Then, when he says but perhaps not in the same way (444b21), he solves the difficulty presented as follows: the two kinds of animal perceive the same object of smell, and with the same sense power, but not in the same way. For it must be considered that the mode of sensing can be varied in two ways.

One is per se: this is according to the different relationships of sensible objects to sense powers, and such variation in mode of sensing makes for different senses. For example, one sense, such as touch, perceives an object united to it, but another, such as sight, perceives a remote object.

But there is another variation in mode of sensing that is accidental: this does not make for different sense powers; rather, it is understood with respect to removal of an impediment. It is this kind of variation in mode of sensing that is under discussion.

For in animals that breathe, something that lies over the organ of smell as a covering is removed by the breathing, and so when they are not breathing they are prevented from smelling because of this covering. But animals that do not breathe lack such a covering, and so they do not need to breathe in order to smell.

Likewise we see that some animals have eyelids, and if these are not opened such animals cannot see.

196. Nature gave such eyelids to animals that need a more acute sense of sight in order to preserve their eyes, which are soft. Hence animals that have hard eyes—animals that, as it were, do not need an acute sense of sight—do not have such eyelids, and so do not need any movement of opening the eyelids in order to see. Rather the eye has the faculty for seeing immediately, without the removal of anything.

197. Then, when he says likewise, none of the other animals (444b28), he clarifies the above-mentioned solution through another comparison involving the sense of smell, in which there is another variation among animals that does not make for difference of sense power.

None of the other animals besides the human being is pained by what has foul odor of itself—that is, not in relation to nourishment. He said this above. (1.5, 444a3–5; cf. ch. 12, n. 184.)

198. But there might be a difficulty on the point, because some animals seem to avoid foul odors of this kind; and so he has returned to the point to remove this difficulty.

He says that the other animals do not avoid odors that are of themselves foul except accidentally, that is, inasmuch as a foul odor of this kind happens to be injurious. For since odor is caused by heat, moisture, and dryness, as was said above, (1.5, 442b27–443a8; cf. ch. 8, n. 118; ch. 9, n. 141.) it sometimes happens that a foul odor comes from a great disorder in the qualities mentioned, and so, in receiving the odor, the medium is simultaneously altered to an extremely harmful condition that destroys the bodies of the other animals as well as that of the human being. The other animals sense this alteration by touch, and so flee such foul things.

199. He gives the example of human beings suffering cold in the head from coal-smoke, because of its disorder, sometimes to the point of destruction; and it is the same with sulfur. Hence animals avoid these destructive things because of the affection of destruction they perceive. But they do not care about the foulness itself of the odor considered in itself—although many things that grow from the earth have foul odors—but only according as the foulness of the odor represents something concerning taste, or concerning the suitability of their proper nourishment.

200. Then when he says the senses exist in an odd number (445a4), he compares the sense of smell to other senses.

First, he determines the truth.

Second, he eliminates an error, where he says what some Pythagoreans say (445a16; [202]).

On the first point it must be considered that, following the custom of the Pythagoreans, the Philosopher here uses a property of number in order to draw a comparison among the senses.

An odd number cannot be divided into two halves like an even one: rather there remains something undivided in the middle between two equal parts, as in a group of five there remains an intermediate unit between two groups of two. Now the senses are set up in an odd number, that is, a group of five. Two of them are tactile, because they do not perceive their objects, which are united to them, through external media, and these are touch and taste. Two of them, namely sight and hearing, perceive their remote objects through something else, that is, external media. But smell is in the middle between the two groups of two, and so it also has something in common with both groups.

It has something in common with touch and taste, which are the senses of nourishment, as is said in On the Soul 2, (Aristotle, On the Soul 2.3, 414b6–14; 3.12, 434b18–19; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 5, n. 290.) inasmuch as the odorous is an affection of nourishment, according as odor is analogous to flavor, and thus objects of touch and taste are in the same genus with objects of taste.

The genus of the visible and the audible is the same as that of the odorous inasmuch as both are apprehended through external media; hence animals smell through air and water, just as they also see and hear through them.

Thus it is clear that the odorous is something common to both genera of sensible objects.

201. For it is in the genus of the tangible according as it is an affection of nourishment and thus comes together in the same genus with the tangible and tastable qualities. Likewise, it is in the genus of the transparent and the audible—that is, it is perceived through the transparent medium through which a thing is seen as well as heard, namely air and water, although it is perceived through these not inasmuch they are transparent, but inasmuch as they are receptive of enchymous dryness, as was said above. (1.5, 442b29–443a2; cf. ch. 11, n. 163.) And so some reasonably compare it to two things: the being of odorous enchymous dryness in moistness of water and fluidity—that is, fluidity of air—is, because of its ready diffusion, like a tincture—which refers to alteration of a medium by color—and like a wash—which refers to flavors—for odor has something in common with both.

After this, as an epilogue, he concludes that it has been said how we should distinguish species of the odorous, and how we should not, namely inasmuch as odors are taken in themselves.

202. Then when he says what some Pythagoreans say (445a16), he eliminates an error. On this point, he does three things.

First, he relates the erroneous opinion.

Second, he disproves it, where he says for, first, we see that food (445a17; [203]).

Third, he responds to a tacit objection, where he says but it is clear (445a29; [206]).

Accordingly, he first says that what some Pythagoreans said—that some animals are nourished by odors—is not reasonable. According to them, smell would not be intermediate among the senses, as was said (445a4–8), but would have to be completely included with the senses of nourishment. They were moved to say this because they saw that human beings and other animals are fortified by odors.

203. Then, when he says for, first, we see that food (445a17), he disproves the above-mentioned opinion by two arguments.

The first is that food must be composed of more than one element. For simple elements do not nourish, because animals that are nourished by them are composed of the four elements, and a thing is nourished by the same things by which it exists, as was said in On Generation 2. (Aristotle, On Generation and Corruption 2.8, 335a10–11; cf. Aquinas, Commentary on Aristotle’s On Generation and Corruption, bk. 2, lect. 7.) He concludes that a sign of this is that a superfluity is produced from food: whether within, as is evident in animals, in the bodies of which are places assigned for the gathering of superfluities; or outside, in the case of plants, the superfluities of which are immediately expelled outward, as is clear in the case of gums of trees and other such things. If an animal or plant were nourished by one simple element, no superfluity would occur, since in a simple element there is uniformity of parts. And no one element is suitable for nutrition, because of its simplicity.

204. Although, water in addition has a special impediment because of which it cannot nourish alone, without the admixture of something earthen, which is why farmers add manure to nourish plants with water mixed with something: nourishment builds up and generates something in the substance of what is nourished, and so it has to be something bodily and solid, which is not true of water. Hence water alone cannot nourish, and much less can air. And hence it remains that neither can odor nourish: for it is clear that odor, since it is a quality, cannot of itself build up a substance by nourishing it, except perhaps by reason of what is receptive of it, namely air or water. Even if odor were evaporation, or exhalation of smoke, as the ancients said, the argument would still stand, because both of these pertain to the nature of air, as was said above. (Cf. ch. 11, nn. 169–70.)

205. Then, when he says in addition to this, we see (445a23), he presents the second argument. He says that all animals have a place in which food is first received—that is, the stomach—from which it is drawn off into individual parts of the body. If we consider the odorous itself, it is clear (since most animals smell by breathing) that it is perceived by an organ near the brain, as was said above. (Cf. ch. 4, n. 69; ch. 8, n. 119.) But the air inbreathed, with which odor is drawn in, goes to the place for breathing, that is, the lung. Now it is clear that neither the brain nor the lung is the place in animals that first receives food. Hence it is clear that odor does not nourish. But it does fortify because of alteration by heat, moistness, and dryness, and because of pleasure; just as bad odor is destructive, as was said above. (Cf. ch. 12, n. 181.)

206. Then, when he says but it is clear (445a29), he responds to a tacit objection. For one might object: if odor does not nourish, it is utterly useless.

He answers that, although it does not nourish, nevertheless it does contribute to health, as is clear from observation, and from what has been said above. (Cf. ch. 12, nn. 185–89.) Hence he concludes that, as flavor is ordered to nutrition, so odor is ordered to health.

207. Finally, by way of epilogue, he concludes that sensible objects have been discussed with respect to each (445b1) organ of sense.