LECTIO TWENTY

             § 491. After concluding about the odorous as such, the Philosopher now deals with its impact on the sense; and first, as to the medium; secondly, at 'Odour is of dry things', 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; next, at 'So there seems to be a difficulty', 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, i.e. 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, perceive odours, 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 from sight to the object seen certain 'visual lines' proceed; and that when these touch the object it is seen; whereas in the case of the other senses a reverse process takes place, and the sense-object comes to the faculty. This (they said) is particularly obvious in tasting and touching, which perceive 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 odours 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 had no theory or perception of a spiritual modification of a medium, but only of a material one. Now a material modification of the medium is evident in all mediated sensations other than sight: for, obviously, odours and sounds are carried or impeded by the wind; which is certainly not the case with colour. Also, in one and the same section of air contrary colours can appear, 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 odour, for contrary odours impede one another, even in the medium. Therefore, knowing nothing about the way the visible object affects 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 rarification for all natural bodies,--the rarity of fire; and in any case, and especially, the corpse can smell and be smelt in this way without being sensibly altered.

             § 495. We are therefore compelled to say that whatever gaseous vapour may come from an odorous substance, it does not reach as far as the point where the odour is perceived; but that beyond the point reached by this vapour 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 participate in the mode of being of incorruptible bodies; hence these qualities exist in a more formal and noble manner than do the other sense-objects which are proper to bodies precisely as corruptible.

             § 496. Then at 'So there seems to be a difficulty', he states a problem suggested by what he has been saying; bringing forward objections, first from one side and then, at 'But this is impossible', from the other; until, at 'Now this sense seems', he provides a solution.

             First, then, he observes that it is doubtful whether all animals smell in the same way, as having identically the same sense of smell (aquatic animals, for instance, smell through water). It would seem not indeed. Man smells when he breathes in air, but when he is exhaling air or holding his breath he cannot smell at all either at a distance or 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 odours are imperceptible without respiration is peculiar to man, as experience shows. And as non-sanguineous animals do not breathe, it would seem to follow that they had some sense other than smell and the other senses attributed to man.

             § 497. Then, at 'But this is impossible, if it is odour that they perceive', he adduces two arguments on the other side. (1) The senses are distinguished according to their objects; and since smell is the sense of odours, whether good or bad, and these are perceived 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. (2) The same things are destructive of the same senses: sight does not suffer from sounds nor hearing from colours. But the non-breathing animals seem to suffer in their senses from the same strong and oppressive odours as are grievous to man, e.g. from bitumen (a compound of the juice of herbs), sulphur 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', 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 protective covering or envelope called the eyelid, which must be withdrawn before a man can see. This is not the case with hard-eyed animals; they see at once whatever appears in the transparent medium. And so it is with non-breathing animals; their olfactory organ is uncovered; whereas that of breathing animals is covered over until the pores are dilated in the act of respiration. The latter therefore cannot smell through water; for they cannot breathe in it.

             § 500. Then at 'Odour is etc.', he indicates the organ of smell, saying that as the basis of odour is dryness (as that of taste is moisture) the organ of smell must be in potency to odour and dryness (as that of sight is to colours and light).

422a 8-422b 17