The Essays of Adam Smith · Adam Smith
EXTERNAL SENSES;
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.
The Sun, the great source of both Heat and Light, is at an immense {449} distance from us. His rays, however (traversing, with inconceivable rapidity, the immensity of the intervening regions), as they convey the Sensation of Light to our eyes, so they convey that of Heat to all the sensible parts of our body. They even convey the power of exciting that Sensation to all the other bodies that surround us. They warm the earth and air, we say; that is, they convey to the earth and the air the power of exciting that Sensation in our bodies. A common fire produces, in the same manner, all the same effects; though the sphere of its action is confined within much narrower limits.
The odoriferous body, which is generally too at some distance from us, is supposed to act upon our organs by means of certain small particles of matter, called Effluvia, which being sent forth in all possible directions, and drawn into our nostrils by the inspiration of breathing, produce there the Sensation of Smell. The minuteness of those small particles of matter, however, must surpass all human comprehension. Inclose in a gold box, for a few hours, a small quantity of musk. Take out the musk, and clean the box with soap and water as carefully as it is possible. Nothing can be supposed to remain in the box, but such effluvia as, having penetrated into its interior pores, may have escaped the effects of this cleansing. The box, however, will retain the smell of musk for many, I do not know for how many years; and these effluvia, how minute soever we may suppose them, must have had the powers of subdividing themselves, and of emitting other effluvia of the same kind, continually, and without any interruption, during so long a period. The nicest balance, however, which human art has ever been able to invent, will not show the smallest increase of weight in the gold box immediately after it has been thus carefully cleaned.
The Sensation of Sound is frequently felt at a much greater distance from the sounding, than that of Smell ever is from the odoriferous body. The vibrations of the sounding body, however, are supposed to produce certain correspondent vibrations and pulses in the surrounding atmosphere, which being propagated in all directions, reach our organ of Hearing, and produce there the Sensation of Sound. There are not many philosophical doctrines, perhaps, established upon a more probable foundation, than that of the propagation of Sound by means of the pulses or vibrations of the air. The experiment of the bell, which, in an exhausted receiver, produces no sensible Sound, would alone render this doctrine somewhat more than probable. But this great probability is still further confirmed by the computations of Sir Isaac Newton, who has shown that, what is called the velocity of Sound, or the time which passes between the commencement of the action of the sounding body, and that of the Sensation in our ear, is perfectly suitable to the velocity with which the pulses and vibrations of an elastic fluid of the same density with the air, are naturally propagated. Dr. {450} Benjamin Franklin has made objections to this doctrine, but, I think, without success. Such are the intermediate causes by which philosophers have endeavoured to connect the Sensation in our organs, with the distant bodies which excite them. How those intermediate causes, by the different motions and vibrations which they may be supposed to excite on our organs, produce there those different Sensations, none of which bear the smallest resemblance to vibration or motion of any kind, no philosopher has yet attempted to explain to us.
_Of the Sense of_ SEEING. DR. BERKLEY, in his New Theory of Vision, one of the finest examples of philosophical analysis that is to be found, either in our own, or in any other language, has explained, so very distinctly, the nature of the objects of Sight: their dissimilitude to, as well as their correspondence and connection with those of Touch, that I have scarcely any thing to add to what he has already done. It is only in order to render some things, which I shall have occasion to say hereafter, intelligible to such readers as may not have had an opportunity of studying his book, that I have presumed to treat of the same subject, after so great a master. Whatever I shall say upon it, if not directly borrowed from Dr. Berkley, has at least been suggested by what he has already said. That the objects of Sight are not perceived as resisting or pressing upon the organ which perceives them, is sufficiently obvious. They cannot therefore suggest, at least in the same manner as the objects of Touch, their externality and independency of existence.
We are apt, however, to imagine that we see objects at a distance from us, and that consequently the externality of their existence is immediately perceived by our sight. But if we consider that the distance of any object from the eye, is a line turned endways to it; and that this line must consequently appear to it, but as one point; we shall be sensible that distance from the eye cannot be the immediate object of Sight, but that all visible objects must naturally be perceived as close upon the organ, or more properly, perhaps, like all other Sensations, as in the organ which perceives them. That the objects of Sight are all painted in the bottom of the eye, upon a membrane called the _retina_, pretty much in the same manner as the like objects are painted in a Camera Obscura, is well known to whoever has the slightest tincture of the science of Optics: and the principle of perception, it is probable, originally perceives them, as existing in that part of the organ, and nowhere but in that part of the organ. No optician, accordingly, no person who has ever bestowed any moderate degree of attention upon the nature of Vision, has ever pretended that distance from the eye was the immediate object of Sight. How it is that, by {451} means of our Sight we learn to judge of such distances Opticians have endeavoured to explain in several different ways. I shall not, however, at present, stop to examine their systems.
The objects of Touch are solidity, and those modifications of solidity which we consider as essential to it, and inseparable from it; solid extension, figure, divisibility, and mobility. The objects of Sight are colour, and those modifications of colour which, in the same manner, we consider as essential to it, and inseparable from it; coloured extension, figure, divisibility, and mobility. When we open our eyes, the sensible coloured objects, which present themselves to us, must all have a certain extension, or must occupy a certain portion of the visible surface which appears before us. They must too have all a certain figure, or must be bounded by certain visible lines, which mark upon that surface the extent of their respective dimensions. Every sensible portion of this visible or coloured extension must be conceived as divisible, or as separable into two, three, or more parts. Every portion too of this visible or coloured surface must be conceived as moveable, or as capable of changing its situation, and of assuming a different arrangement with regard to the other portions of the same surface.
Colour, the visible, bears no resemblance to solidity, the tangible object. A man born blind, or who has lost his sight so early as to have no remembrance of visible objects, can form no idea or conception of colour. Touch alone can never help him to it. I have heard, indeed, of some persons who had lost their sight after the age of manhood, and who had learned to distinguish by the touch alone, the different colours of cloths or silks, the goods which it happened to be their business to deal in. The powers by which different bodies excite in the organs of Sight the Sensations of different colours, probably depend upon some difference in the nature, configuration, and arrangement of the parts which compose their respective surfaces. This difference may, to a very nice and delicate touch, make some difference in the feeling, sufficient to enable a person, much interested in the case, to make this distinction in some degree, though probably in a very imperfect and inaccurate one. A man born blind might possibly be taught to make the same distinctions. But though he might thus be able to name the different colours, which those different surfaces reflected, though he might thus have some imperfect notion of the remote causes of the Sensations, he could have no better idea of the Sensations themselves, than that other blind man, mentioned by Mr. Locke, had, who said that he imagined the Colour of Scarlet resembled the Sound of a Trumpet. A man born deaf may, in the same manner, be taught to speak articulately. He is taught how to shape and dispose of his organs, so as to pronounce each letter, syllable, and word. But still, though he may have some imperfect idea of the remote causes of {452} the Sounds which he himself utters, of the remote causes of the Sensations which he himself excites in other people; he can have none of those Sounds or Sensations themselves.
If it were possible, in the same manner, that a man could be born without the Sense of Touching, that of Seeing could never alone suggest to him the idea of Solidity, or enable him to form any notion of the external and resisting substance. It is probable, however, not only that no man, but that no animal was ever born without the Sense of Touching, which seems essential to, and inseparable from, the nature of animal life and existence. It is unnecessary, therefore, to throw away any reasoning, or to hazard any conjectures, about what might be the effects of what I look upon as altogether an impossible supposition. The eye when pressed upon by any external and solid substance, feels, no doubt, that pressure and resistance, and suggests to us (in the same manner as every other feeling part of the body) the external and independent existence of that solid substance. But in this case, the eye acts, not as the organ of Sight, but as an organ of Touch; for the eye possesses the Sense of Touching in common with almost all the other parts of the body.
The extension, figure, divisibility, and mobility of Colour, the sole object of Sight, though, on account of their correspondence and connection with the extension, figure, divisibility, and mobility of Solidity, they are called by the same name, yet seem to bear no sort of resemblance to their namesakes. As Colour and Solidity bear no sort of resemblance to one another, so neither can their respective modifications. Dr. Berkley very justly observes, that though we can conceive either a coloured or a solid line to be prolonged indefinitely, yet we cannot conceive the one to be added to the other. We cannot, even in imagination, conceive an object of Touch to be prolonged into an object of Sight, or an object of Sight into an object of Touch. The objects of Sight and those of Touch constitute two worlds, which, though they have a most important correspondence and connection with one another, bear no sort of resemblance to one another. The tangible world, as well as all the different parts which compose it, has three dimensions, Length, Breadth, and Depth. The visible world, as well as all the different parts which compose it, has only two, Length and Breadth. It presents to us only a plain or surface, which, by certain shades and combinations of Colour, suggests and represents to us (in the same manner as a picture does) certain tangible objects which have no Colour, and which therefore can bear no resemblance to those shades and combinations of Colour. Those shades and combinations suggest those different tangible objects as at different distances, according to certain rules of Perspective, which it is, perhaps, not very easy to say how it is that we learn, whether by some particular instinct, or by some application of either reason or experience, which {453} has become so perfectly habitual to us, that we are scarcely sensible when we make use of it.
The distinctness of this Perspective, the precision and accuracy with which, by means of it, we are capable of judging concerning the distance of different tangible objects, is greater or less, exactly in proportion as this distinctness, as this precision and accuracy, are of more or less importance to us. We can judge of the distance of near objects, of the chairs and tables for example, in the chamber where we are sitting, with the most perfect precision and accuracy; and if in broad daylight we ever stumble over any of them, it must be, not from any error in the Sight, but from some defect in the attention. The precision and accuracy of our judgment concerning such near objects are of the utmost importance to us, and constitute the great advantage which a man who sees has over one who is unfortunately blind. As the distance increases, the distinctness of this Perspective, the precision and accuracy of our judgment gradually diminish. Of the tangible objects which are even at the moderate distance of one, two, or three miles from the eye, we are frequently at a loss to determine which is nearest, and which remotest. It is seldom of much importance to us to judge with precision concerning the situation of the tangible objects which are even at this moderate distance. As the distance increases, our judgments become more and more uncertain; and at a very great distance, such as that of the fixed stars, it becomes altogether uncertain. The most precise knowledge of the relative situation of such objects could be of no other use to the enquirer than to satisfy the most unnecessary curiosity.
The distances at which different men can by Sight distinguish, with some degree of precision, the situation of the tangible objects which the visible ones represent, is very different; and this difference, though it, no doubt, may sometimes depend upon some difference in the original configuration of their eyes, yet seems frequently to arise altogether from the different customs and habits which their respective occupations have led them to contract. Men of letters, who live much in their closets, and have seldom occasion to look at very distant objects, are seldom far-sighted. Mariners, on the contrary, almost always are; those especially who have made many distant voyages, in which they have been the greater part of their time out of sight of land, and have in daylight been constantly looking out towards the horizon for the appearance of some ship, or of some distant shore. It often astonishes a landsman to observe with what precision a sailor can distinguish in the offing, not only the appearance of a ship which is altogether invisible to the landsman, but the number of her masts, the direction of her course, and the rate of her sailing. If she is a ship of his acquaintance, he frequently can tell her name, before the landsman has been able to discover even the appearance of a ship.
Every visible object which covers from the eye any other visible object, must appear at least as large as that other visible object. It must occupy at least an equal portion of that visible plain or surface which is at that time presented to the eye. Opticians accordingly tell us, that all the visible objects which are seen under equal angles must to the eye appear equally large. But the visible object, which covers from the eye any other visible object, must necessarily be seen under angles at least equally large as those under which that other object is seen. When I hold up my finger, however, before my eye, it appears to cover the greater part of the visible chamber in which I am sitting. It should therefore appear as large as the greater part of that visible chamber. But because I know that the tangible finger bears but a very small proportion to the greater part of the tangible chamber, I am apt to fancy that the visible finger bears but a like proportion to the greater part of the visible chamber. My judgment corrects my eyesight, and, in my fancy, reduces the visible object, which represents the little tangible one, below its real visible dimensions; and, on the {455} contrary, it augments the visible object which represents the great tangible one a good deal beyond those dimensions. My attention being generally altogether occupied about the tangible and represented, and not at all about the visible and representing objects, my careless fancy bestows upon the latter a proportion which does not in the least belong to them, but which belongs altogether to the former.
It is because the visible object which covers any other visible object must always appear at least as large as that other object, that opticians tell us that the sphere of our vision appears to the eye always equally large; and that when we hold our hand before our eye in such a manner that we see nothing but the inside of the hand, we still see precisely the same number of visible points, the sphere of our vision is still as completely filled, the retina of the eye is as entirely covered with the object which is thus presented to it, as when we survey the most extensive horizon. A young gentleman who was born with a cataract upon each of his eyes, was, in one thousand seven hundred and twenty-eight, couched by Mr. Cheselden, and by that means for the first time made to see distinctly. 'At first,' says the operator, 'he could bear but very little sight, and the things he saw he thought extremely large; but upon seeing things larger, those first seen he conceived less, never being able to imagine any lines beyond the bounds he saw; the room he was in, he said, he knew to be but part of the house, yet he could not conceive that the whole house would look bigger.' It was unavoidable that he should at first conceive, that no visible object could be greater, could present to his eye a greater number of visible points, or could more completely fill the comprehension of that organ, than the narrowest sphere of his vision. And when that sphere came to be enlarged, he still could not conceive that the visible objects which it presented could be larger than those which he had first seen. He must probably by this time have been in some degree habituated to the connection between visible and tangible objects, and enabled to conceive that visible object to be small which represented a small tangible object; and that to be great, which represented a great one. The great objects did not appear to his sight greater than the small ones had done before; but the small ones, which, having filled the whole sphere of his vision, had before appeared as large as possible, being now known to represent much smaller tangible objects, seemed in his conception to grow smaller. He had begun now to employ his attention more about the tangible and represented, than about the visible and representing objects; and he was beginning to ascribe to the latter the proportions and dimensions which properly belonged altogether to the former.
Ideas discussed in this chapter
Text from Project Gutenberg, public domain in the United States.