De Anima

Of smell and the object of smell it is less easy to speak definitely than of the senses above-mentioned: for the nature of odour is by no means so clear as is the nature of sound or of colour. The reason is that this sense in us is not exact, but inferior to that of many animals. In fact, man has a poor olfactory sense and perceives none of the objects of smell unless they be painful or pleasant, which implies that the organ is wanting in accuracy. It is reasonable to suppose that animals with hard eyes perceive colour in the same vague way and do not distinguish the varieties of colour except in so far as they do, or do not, inspire fear. And this is the way in which mankind perceive odours. For it would seem that, while there is an analogy to taste and the varieties of flavour answer to the varieties of smell, our sense of taste is more exact because it is a modification of touch and the sense of touch is the most exact of man's senses. In the other senses man is inferior to many of the animals, but in delicacy of touch he is far superior to the rest. And to this he owes his superior intelligence. This may be seen from the fact that it is this organ of sense and nothing else which makes all the difference in the human race between the natural endowments of man and man. For hard-skinned men are dull of intellect, while those who are soft-skinned are gifted.

As with flavours, so with odours: some are sweet, some bitter. (But in some objects smell and flavour correspond; for example, they have sweet odour and sweet flavour: in other of odour ~~ things the opposite is the case.) Similarly, too, an odour may be pungent, irritant, acid or oily. But because, as we said above, odours are not as clearly defined as the corresponding flavours, it is from these latter that the odours have taken their names, in virtue of the resemblance in the things. Thus the odour of saffron and honey is sweet, while the odour of thyme and the like is pungent; and so in all the other cases. Again, smell corresponds to hearing and to each of the other senses in that, as hearing is of the audible and inaudible, and

sight of the visible and invisible, so smell is of the odorous and inodorous. By inodorous may be meant either that which is wholly incapable of having odour or that which has a slight or faint odour. The term tasteless involves a similar ambiguity.

Further, smell also operates through a medium, namely, air or water. For water animals, too, whether they are, or are not, possessed of blood, seem to perceive odour as much as the creatures in the air: since some of them also come from a great distance to seek their food, guided by the scent.

Hence there is an obvious difficulty, if the process of smell is the same everywhere, and yet man smells when inhaling but does not smell when instead of inhaling he is exhaling or holding his breath, no matter whether the object be distant or near, or even if it be placed on the inside of the nostril. The inability to perceive what is placed immediately on the sense-organ man shares with all animals: what is peculiar to him is that he cannot smell without inhaling. This is made plain by experiment. Consequently bloodless animals, since they do not breathe, might be thought to have a distinct sense other than those commonly recognised. But, we reply, that is impossible, since it is odour which they perceive. For perception of odour, be it fragrant or noisome, constitutes smelling. Moreover, it is found that these bloodless animals are destroyed by the same powerful odours as man, such as asphalt, brimstone and the like. It follows then that they do smell, but not by inhaling breath.

It would seem, again, that in man the organ of this sense differs from that of the other animals, as his eyes differ from those of hard-eyed animals. Man's eyes have, in the eyelids, a sort of screen or sheath and without moving or opening them he cannot see: while the hard-eyed animals have nothing of the kind, but at once see whatever is taking place in the transparent medium. So, too, it seems, the organ of smell in some animals is unenclosed, just as is the eye, but in those which take in the air it has a curtain, which is removed in the process of inhaling, by dilatation of the veins and passages. And this is the

reason why animals which breathe cannot smell in the water. For it is necessary for them to take in breath before smelling and this they cannot do in the water. Odour is included under that which is dry, as flavour under that which is moist, and the organ of smell is potentially dry also.