our conception of the nature of odours must be analogous to that of savours; inasmuch as the sapid dry effects in air and water alike, but in a different province of sense, precisely what the dry effects in the moist of water only. we customarily predicate translucency of both air and water in common; but it is not qua translucent that either is a vehicle of odour, but qua possessed of a power of washing or rinsing [and so imbibing] the sapid dryness.

for the object of smell exists not in air only: it also exists in water. this is proved by the case of fishes and testacea, which are seen to possess the faculty of smell, although water contains no air (for whenever air is generated within water it rises to the surface), and these creatures do not respire. hence, if one were to assume that air and water are both moist, it would follow that odour is the natural substance consisting of the sapid dry diffused in the moist, and whatever is of this kind would be an object of smell.

that the property of odorousness is based upon the sapid may be seen by comparing the things which possess with those which do not possess odour. the elements, viz. fire, air, earth, water, are inodorous, because both the dry and the moist among them are without sapidity, unless some added ingredient produces it. this explains why sea-water possesses odour, for [unlike ‘elemental’ water] it contains savour and dryness. salt, too, is more odorous than natron, as the oil which exudes from the former proves, for natron is allied to [’elemental’] earth more nearly than salt. again, a stone is inodorous, just because it is tasteless, while, on the contrary, wood is odorous, because it is sapid. the kinds of wood, too, which contain more [’elemental’] water are less odorous than others. moreover, to take the case of metals, gold is inodorous because it is without taste, but bronze and iron are odorous; and when the [sapid] moisture has been burnt out of them, their slag is, in all cases, less odorous the metals [than the metals themselves]. silver and tin are more odorous than the one class of metals, less so than the other, inasmuch as they are water [to a greater degree than the former, to a less degree than the latter].

some writers look upon fumid exhalation, which is a compound of earth and air, as the essence of odour. [indeed all are inclined to rush to this theory of odour.] heraclitus implied his adherence to it when he declared that if all existing things were turned into smoke, the nose would be the organ to discern them with. all writers incline to refer odour to this cause [sc. exhalation of some sort], but some regard it as aqueous, others as fumid, exhalation; while others, again, hold it to be either. aqueous exhalation is merely a form of moisture, but fumid exhalation is, as already remarked, composed of air and earth. the former when condensed turns into water; the latter, in a particular species of earth. now, it is unlikely that odour is either of these. for vaporous exhalation consists of mere water [which, being tasteless, is inodorous]; and fumid exhalation cannot occur in water at all, though, as has been before stated, aquatic creatures also have the sense of smell.

again, the exhalation theory of odour is analogous to the theory of emanations. if, therefore, the latter is untenable, so, too, is the former.

it is clearly conceivable that the moist, whether in air (for air, too, is essentially moist) or in water, should imbibe the influence of, and have effects wrought in it by, the sapid dryness. moreover, if the dry produces in moist media, i.e. water and air, an effect as of something washed out in them, it is manifest that odours must be something analogous to savours. nay, indeed, this analogy is, in some instances, a fact [registered in language]; for odours as well as savours are spoken of as pungent, sweet, harsh, astringent rich [=’savoury’]; and one might regard fetid smells as analogous to bitter tastes; which explains why the former are offensive to inhalation as the latter are to deglutition. it is clear, therefore, that odour is in both water and air what savour is in water alone. this explains why coldness and freezing render savours dull, and abolish odours altogether; for cooling and freezing tend to annul the kinetic heat which helps to fabricate sapidity.

there are two species of the odorous. for the statement of certain writers that the odorous is not divisible into species is false; it is so divisible. we must here define the sense in which these species are to be admitted or denied.

one class of odours, then, is that which runs parallel, as has been observed, to savours: to odours of this class their pleasantness or unpleasantness belongs incidentally. for owing to the fact that savours are qualities of nutrient matter, the odours connected with these [e.g. those of a certain food] are agreeable as long as animals have an appetite for the food, but they are not agreeable to them when sated and no longer in want of it; nor are they agreeable, either, to those animals that do not like the food itself which yields the odours. hence, as we observed, these odours are pleasant or unpleasant incidentally, and the same reasoning explains why it is that they are perceptible to all animals in common.

the other class of odours consists of those agreeable in their essential nature, e.g. those of flowers. for these do not in any degree stimulate animals to food, nor do they contribute in any way to appetite; their effect upon it, if any, is rather the opposite. for the verse of strattis ridiculing euripides

use not perfumery to flavour soup,

contains a truth.

those who nowadays introduce such flavours into beverages deforce our sense of pleasure by habituating us to them, until, from two distinct kinds of sensations combined, pleasure arises as it might from one simple kind.

of this species of odour man alone is sensible; the other, viz. that correlated with tastes, is, as has been said before, perceptible also to the lower animals. and odours of the latter sort, since their pleasureableness depends upon taste, are divided into as many species as there are different tastes; but we cannot go on to say this of the former kind of odour, since its nature is agreeable or disagreeable per se. the reason why the perception of such odours is peculiar to man is found in the characteristic state of man’s brain. for his brain is naturally cold, and the blood which it contains in its vessels is thin and pure but easily cooled (whence it happens that the exhalation arising from food, being cooled by the coldness of this region, produces unhealthy rheums); therefore it is that odours of such a species have been generated for human beings, as a safeguard to health. this is their sole function, and that they perform it is evident. for food, whether dry or moist, though sweet to taste, is often unwholesome; whereas the odour arising from what is fragrant, that odour which is pleasant in its own right, is, so to say, always beneficial to persons in any state of bodily health whatever.

for this reason, too, the perception of odour [in general] effected through respiration, not in all animals, but in man and certain other sanguineous animals, e.g. quadrupeds, and all that participate freely in the natural substance air; because when odours, on account of the lightness of the heat in them, mount to the brain, the health of this region is thereby promoted. for odour, as a power, is naturally heat-giving. thus nature has employed respiration for two purposes: primarily for the relief thereby brought to the thorax, secondarily for the inhalation of odour. for while an animal is inhaling — odour moves in through its nostrils, as it were ‘from a side-entrance.’

but the perception of the second class of odours above described [does not belong to all animal, but] is confined to human beings, because man’s brain is, in proportion to his whole bulk, larger and moister than the brain of any other animal. this is the reason of the further fact that man alone, so to speak, among animals perceives and takes pleasure in the odours of flowers and such things. for the heat and stimulation set up by these odours are commensurate with the excess of moisture and coldness in his cerebral region. on all the other animals which have lungs, nature has bestowed their due perception of one of the two kinds of odour [i.e. that connected with nutrition] through the act of respiration, guarding against the needless creation of two organs of sense; for in the fact that they respire the other animals have already sufficient provision for their perception of the one species of odour only, as human beings have for their perception of both.

but that creatures which do not respire have the olfactory sense is evident. for fishes, and all insects as a class, have, thanks to the species of odour correlated with nutrition, a keen olfactory sense of their proper food from a distance, even when they are very far away from it; such is the case with bees, and also with the class of small ants, which some denominate knipes. among marine animals, too, the murex and many other similar animals have an acute perception of their food by its odour.

it is not equally certain what the organ is whereby they so perceive. this question, of the organ whereby they perceive odour, may well cause a difficulty, if we assume that smelling takes place in animals only while respiring (for that this is the fact is manifest in all the animals which do respire), whereas none of those just mentioned respires, and yet they have the sense of smell — unless, indeed, they have some other sense not included in the ordinary five. this supposition is, however, impossible. for any sense which perceives odour is a sense of smell, and this they do perceive, though probably not in the same way as creatures which respire, but when the latter are respiring the current of breath removes something that is laid like a lid upon the organ proper (which explains why they do not perceive odours when not respiring); while in creatures which do not respire this is always off: just as some animals have eyelids on their eyes, and when these are not raised they cannot see, whereas hard-eyed animals have no lids, and consequently do not need, besides eyes, an agency to raise the lids, but see straightway [without intermission] from the actual moment at which it is first possible for them to do so [i.e. from the moment when an object first comes within their field of vision].

consistently with what has been said above, not one of the lower animals shows repugnance to the odour of things which are essentially ill-smelling, unless one of the latter is positively pernicious. they are destroyed, however, by these things, just as human beings are; i.e. as human beings get headaches from, and are often asphyxiated by, the fumes of charcoal, so the lower animals perish from the strong fumes of brimstone and bituminous substances; and it is owing to experience of such effects that they shun these. for the disagreeable odour in itself they care nothing whatever (though the odours of many plants are essentially disagreeable), unless, indeed, it has some effect upon the taste of their food.

the senses making up an odd number, and an odd number having always a middle unit, the sense of smell occupies in itself as it were a middle position between the tactual senses, i.e. touch and taste, and those which perceive through a medium, i.e. sight and hearing. hence the object of smell, too, is an affection of nutrient substances (which fall within the class of tangibles), and is also an affection of the audible and the visible; whence it is that creatures have the sense of smell both in air and water. accordingly, the object of smell is something common to both of these provinces, i.e. it appertains both to the tangible on the one hand, and on the other to the audible and translucent. hence the propriety of the figure by which it has been described by us as an immersion or washing of dryness in the moist and fluid. such then must be our account of the sense in which one is or is not entitled to speak of the odorous as having species.

the theory held by certain of the pythagoreans, that some animals are nourished by odours alone, is unsound. for, in the first place, we see that food must be composite, since the bodies nourished by it are not simple. this explains why waste matter is secreted from food, either within the organisms, or, as in plants, outside them. but since even water by itself alone, that is, when unmixed, will not suffice for food — for anything which is to form a consistency must be corporeal-, it is still much less conceivable that air should be so corporealized [and thus fitted to be food]. but, besides this, we see that all animals have a receptacle for food, from which, when it has entered, the body absorbs it. now, the organ which perceives odour is in the head, and odour enters with the inhalation of the breath; so that it goes to the respiratory region. it is plain, therefore, that odour, qua odour, does not contribute to nutrition; that, however, it is serviceable to health is equally plain, as well by immediate perception as from the arguments above employed; so that odour is in relation to general health what savour is in the province of nutrition and in relation to the bodies nourished.

this then must conclude our discussion of the several organs of sense-perception.