It is less easy to define smell and the odorous object, than the subjects which have just been treated of, as the nature of odour is not so clear to us as is that of either sound or colour; and the reason of this is, that our sense of smell is inaccurate, is less delicate, in fact, than it is in many animals. Thus, man has but a coarse smell, and is never sensible of any thing odorous without associating therewith an impression of something painful or grateful; and this seems to indicate an organ imperfectly constituted. It is probable that colours are perceived by creatures which have hard eyes in this same manner, and that shades of colour invariably make upon them an impression of something to be afraid of or otherwise. The human race is circumstanced in a like manner with respect to odours; and there seems to be an analogy between taste and kinds of savours, and smell and kinds of odours, but as taste is a kind of touch, and touch of all man's senses the most perfect, his taste is more delicate than his smell. With respect to other senses, man is far behind many animals, but he is especially distinguished from them all by the accuracy of his Touch; and to this he is indebted for being of all the most intelligent. As proof of which, individuals of the human race are according to the constitution of this sense and nothing else, clever or dull for those with hard flesh are slow, and those, on the contrary, with soft flesh are quick of understanding.
As one savour is sweet and another bitter, so it is with odours; but some bodies impart an analogous savour and odour, impart, I mean, a sweet odour and a sweet savour, while other bodies give out their contraries. Some odours equally with savours are termed pungent, sour, and oily, but, as we have already explained, owing to their not being so distinguishable by us as savours, odours have derived their appellations from these, on account of the similarity of the objects from which they both proceed. Thus, the odour from saffron and honey is called sweet, that from thyme and other herbs of the kind pungent, and so for other bodies and odours.
There is a close analogy between the other senses and the hearing: for as it is sensible of the audible and the inaudible, so is vision of the visible and invisible, and smell of the odorous and the inodorous, and by inodorous is meant whatever is either altogether without odour, or has but a very faint odour; and a sense analogous to this is attached to the term insapid. The smell is perceptive through a medium, such as air or water, for aquatic animals seem to be sensible of odour; and so, likewise, are sanguineous and insanguineous creatures, as well as those which wing the air. Thus, some of these are to be seen proceeding from a distance towards food, of which they have been made sensible by the odour emanating from it. And hence the difficulty of determining why, if other creatures are sensible of odours in a like manner, man alone can smell neither when expiring nor when holding his breath but, only when inspiring; and this whether the odorous object be at a distance from or close to him, or placed immediately within the nostrils. It is common, it is true, to all the sentient organs to be insensible to impressions when objects are placed immediately upon them; but it is peculiar to man (as may be proved experimentally), to be unable to perceive odours without inspiring. So that as insanguineous creatures do not breathe, they ought to have some other sense besides those spoken of, but yet this cannot be, since they do perceive odour; for the perception of odour, whether agreeable or disagreeable, is smell; and as these appear to be destroyed by the same powerful odours as those which destroy man (odours, for instance, from pitch, sulphur, and other like substances), we must conclude that they have smell, although they do not breathe. The olfactory organ in man appears to differ from that in other animals as his eyes differ from those of creatures in which they are hard; for man's eyes are furnished with a rampart, and a kind of sheath in lids, without the elevation and drawing asunder of which he cannot see, while hard eyes, having no such provision, are instantly sensible of whatever may be present in the diaphanous medium. In accordance with this, the olfactory organ is, in such creatures, like the eye, uncovered; but, in such as breathe, it is furnished with a cover, which during inspiration is lifted up, as the veins and pores are then dilated. On which account, creatures which breathe cannot smell while in the water, as in order to smell they must inspire, and while in the water they cannot possibly inspire.
In fine, odour is derived from what is dry, as savour is from what is moist; and the olfactory organ, when in potentiality, is analogous to that from which odour is derived.