The Essays of Adam Smith

by Adam Smith

447 Thirdly, Those Sensations are incapable of motion. The bodies which excite them may be moved to a greater or to a smaller distance. The Sensations become fainter in the one case, and stronger in the other. Those bodies may change their direction with regard to the organ of Sensation. If the change be considerable, the Sensations undergo some sensible variation in consequence of it. But still we never ascribe motion to the Sensations. Even when the person who feels any of those Sensations, and consequently the organ by which he feels them, changes his situation, we never, even in this case, say, that the Sensation moves, or is moved. It seems to exist always, where alone it is capable of existing, in the organ which feels it. We never even ascribe to those Sensations the attribute of rest; because we never say that any thing is at rest, unless we suppose it capable of motion. We never say that any thing does not change its situation with regard to other things, unless we can suppose it to be capable of changing that situation.

Fourthly, Those Sensations, as they have no extension, so they can have no divisibility. We cannot even conceive that a degree of Heat or Cold, that a Smell, a Taste, or a Sound, should be divided (in the same manner as the solid and extended substance may be divided) into two halves, or into four quarters, or into any number of parts.

But though all these Sensations are equally incapable of division; there are three of them, Taste, Smell, and Sound; which seem capable of a certain composition and decomposition. A skilful cook will, by his taste, perhaps, sometimes distinguish the different ingredients, which enter into the composition of a new sauce, and of which the simple tastes make up the compound one of the sauce. A skilful perfumer may, perhaps, sometimes be able to do the same thing with regard to a new scent. In a concert of vocal and instrumental music, an acute and experienced Ear readily distinguishes all the different sounds which strike upon it at the same time, and which may, therefore, be considered as making up one compound sound.

Is it by nature, or by experience, that we learn to distinguish between simple and compound Sensations of this kind? I am disposed to believe that it is altogether by experience; and that naturally all Tastes, Smells, and Sounds, which affect the organ of Sensation at the same time, are felt as simple and uncompounded Sensations. It is altogether by experience, I think, that we learn to observe the different affinities and resemblances which the compound Sensation bears to the different simple ones, which compose it, and to judge that the different causes, which excite those different simple Sensations, enter into the composition of that cause which excites the compounded one.

It is sufficiently evident that this composition and decomposition is altogether different from that union and separation of parts, which constitutes the divisibility of solid extension.

448 The Sensations of Heat and Cold seem incapable even of this species of composition and decomposition. The Sensations of Heat and Cold may be stronger at one time and weaker at another. They may differ in degree, but they cannot differ in kind. The Sensations of Taste, Smell, and Sound, frequently differ, not only in degree, but in kind. They are not only stronger and weaker, but some Tastes are sweet and some bitter; some Smells are agreeable, and some offensive; some Sounds are acute, and some grave; and each of these different kinds or qualities, too, is capable of an immense variety of modifications. It is the combination of such simple Sensations, as differ not only in degree but in kind, which constitutes the compounded Sensation.

These four classes of Sensations, therefore, having none of the qualities which are essential to, and inseparable from, the solid, external, and independent substances which excite them, cannot be qualities or modifications of those substances. In reality we do not naturally consider them as such; though in the way in which we express ourselves on the subject, there is frequently a good deal of ambiguity and confusion. When the different meanings of words, however, are fairly distinguished, these Sensations are, even by the most ignorant and illiterate, understood to be, not the qualities, but merely the effects of the solid, external, and independent substances upon the sensible and living organ, or upon the principle of perception which feels in that organ.

Philosophers, however, have not in general supposed that those exciting bodies produce those Sensations immediately, but by the intervention of one, two, or more intermediate causes.

In the Sensation of Taste, for example, though the exciting body presses upon the organ of Sensation, this pressure is not supposed to be the immediate cause of the Sensation of Taste. Certain juices of the exciting body are supposed to enter the pores of the palate, and to excite, in the irritable and sensible fibres of that organ, certain motions or vibrations, which produce there the Sensation of Taste. But how those juices should excite such motions, or how such motions should produce, either in the organ, or in the principle of perception which feels in the organ, the Sensation of Taste; or a Sensation, which not only does not bear the smallest resemblance to any motion, but which itself seems incapable of all motion, no philosopher has yet attempted, nor probably ever will attempt, to explain to us.

The Sensations of Heat and Cold, of Smell and Sound, are frequently excited by bodies at a distance, sometimes at a great distance, from the organ which feels them. But it is a very ancient and well-established axiom in metaphysics, that nothing can act where it is not; and this axiom, it must, I think, be acknowledged, is at least perfectly agreeable to our natural and usual habits of thinking.