Lecture 20 — How smell occurs

Smelling also takes place through a medium, which is either air or water. For aquatic animals seem also to sense odors, both those with blood and those without, like animals that live in the air; for some of them traverse long distances for their food, being drawn to it by smell.

So there seems to be a difficulty whether all smell in the same way, for man smells by inhaling; when exhaling or holding his breath, he smells nothing, neither from a distance nor close at hand, not even if the object is placed inside the nose. (That a thing should be imperceptible when placed on the very organ of sensation is indeed common to all, but to be unable to sense without breathing is peculiar to man. This is evident to those who make the experiment.) Since, then, bloodless animals do not breathe, they would seem to have some other sense besides those which have been spoken of.

But this is impossible, if it is odor they sense. For the sense for odors, good odors or bad, is smell. Furthermore, they seem to be overcome by the same strong odors as man, such as sulfur and the like. Therefore, even if they do not breathe, they must smell.

Now, this sense seems to differ as between man and other animals, as human eyes differ from the hard eyes of some animals. The former have a covering or protection, the eyelids, and unless these are moved or withdrawn, one does not see. Hard-eyed animals have nothing like this; they see at once whatever happens to be present in the transparent medium. In the same way, then, the organ of smell is in some animals like an eye with no covering; in others, which inhale air, it has a covering that is withdrawn when they respire and so distend the veins and pores. And for this reason, animals that breathe do not smell in water. For they must respire if they are to be acted upon by odor, and they cannot do this in a liquid.

Odor is of dry things as savor is of liquid; and the sense organ of smell is such in potency.

491. Smelling also takes place (421b8). After concluding about the odorous as such, the Philosopher now deals with its impact on the sense;

and first, as to the medium;

second, at odor is of dry things (422a6; [500]), as to the organ of the sense of smell.

As to the former point, he does two things.

First, he indicates the medium of the organ of smell;

second, at so there seems to be a difficulty (412b13; [496]), he raises a question about the conclusion reached.

First, then, he observes that the organ of smell is affected by the odorous through a medium, that is, air or water. That air is a medium of smell is obvious, for we smell through the air. So we need only prove that water is such a medium. This he shows by the fact that aquatic animals, and not only the sanguineous but the non-sanguineous also, sense odors, like animals that live in the air. A sign of this is that some of them travel a long distance for their food, which could not happen unless they were attracted by smell—like vultures, which are said to come long journeys to carrion. But just how smell is diffused to such a distance in space is not certain.

492. An opinion which some have maintained should be noted here, that all sensation is consummated in a kind of touch: a contact, they said, must occur of sense and sense-object, but not in the same way in sight as in the other senses. For they said that certain visual lines come forth from sight, proceeding all the way to the thing seen, and that when these touch, the visible object is seen; whereas in the case of the other senses, a reverse process takes place, and the sense-object comes to the power. This, they said, is particularly obvious in tasting and touching, which sense by a sort of contact, and would seem to occur also in hearing, which implies a movement of air to the ear, and also in smelling; for they maintained that odors are conveyed to the sense of smell by a kind of fume that bodies give off by evaporation.

493. The cause of their drawing this distinction between two modes of sensation seems to have been that these early thinkers did not assert or notice anything of the spiritual modification of a medium but only of a natural modification. Now, a natural modification of the medium is evident in all the other senses, but not in sight: for, obviously, odors and sounds are carried or impeded by the wind, which is certainly not the case with color. Also, the species of contrary colors are carried through one and the same section of air to sight, as when one man sees white and another black at the same time and through the medium of the same air, which is not the case with odor, for contrary odors impede one another, even in the medium. Therefore, not perceiving the way the visible object modifies the medium, these philosophers maintained that sight was conveyed to the thing seen, whereas, observing in other sensations the changes that took place in the medium, they believed that in these cases, the sense-objects came to the sense.

494. But it is clear that this cannot occur in smelling. When vultures smell carrion at a distance of fifty miles or more, this cannot be due to any bodily evaporation from the carrion being diffused over so great a space. This is the more evident if we consider that a sense-object affects its medium for the same distance in all directions, if not impeded. There would not be enough of the object to occupy so much space, even if the whole corpse were to evaporate; for there is a fixed limit of expansion for all natural bodies—the rarity of fire—and in any case, the corpse can smell and be smelt in this way without being sensibly altered.

495. We are, therefore, compelled to say that whatever gaseous vapor may come from an odorous substance, it does not reach as far as the point where the odor is perceived but that beyond the point reached by this vapor the substance affects the medium spiritually. That such spiritual modification of the medium is effected by the object of sight more than by that of the other senses is due to the fact that, by their visible qualities, corruptible bodies share something with incorruptible bodies; hence, they have a more formal and noble being than do the other sense-objects, which are proper to corruptible bodies.

496. Then, at so there seems to be a difficulty (421b13), he states a problem suggested by what he has been saying, bringing forward objections:

first, from one side;

second, at but this is impossible (421b21; [497]), from the other;

until third, at now, this sense seems (421b26; [499]), he provides a solution.

First, then, he observes that there seems to be a difficulty whether all animals smell in the same way, as having the same sense of smell. It would seem not indeed, for man smells when he breathes in air, but when he is exhaling air or holding his breath, he smells nothing, neither from a distance nor close to the object, even if the latter is put into his nose. Now, that the object is imperceptible if it is placed right against the sense organ is of course common to all the senses in all animals; but that odors are imperceptible without breathing is peculiar to man, as is evident. And as bloodless animals do not breathe, it would seem to follow that they had some sense other than smell and the other senses which have been spoken of in man.

497. Then, at but this is impossible (421b21), he adduces two arguments on the other side.

First, the senses are distinguished according to their objects; and since smell is the sense of odors, whether good or bad, and these are sensed both by the breathing and by the non-breathing animals, it follows that the sense of smell in man and these animals is the same.

498. Second, the same things are destructive of the same senses: sight does not suffer from sounds nor hearing from colors. But the non-breathing animals seem to be overcome in their senses by the same strong and oppressive odors as are grievous to man, for example, from bitumen (a compound of the juice of herbs), sulfur and the like. Hence, other animals, even if they do not breathe, have a sense of smell like man.

499. Next, at now, this sense seems to differ (421b26), he proposes his solution: this diversity in the mode of breathing is due, he says, not to a difference in the senses, but to different constitutions of the organ, the organ of smell in man being different from that in other animals, as a man’s eyes differ from the hard eyes of certain beasts.

The human eye has a covering, that is, an envelope for protection, called eyelids, which must be withdrawn before a man can see. This is not the case with hard-eyed animals; they see at once whatever the things whose species come to be in the transparent medium. And so it is with some non-breathing animals; their olfactory organs have no covering, whereas that of animals that breathe is covered over until they distend the veins and pores in the act of respiration. Therefore, the latter animals that breathe do not smell in water, for they must be acted upon by breathing the odor, which cannot happen in water.

500. Then, at odor is (422a6), he indicates the organ of smell, saying that as the foundation of odor is dryness and that of taste is moisture, the organ of smell must be in potency to odor and dryness, as the organ of sight is to colors and the luminous.