The Essays of Adam Smith: ADAM SMITH ON THE EXTERNAL SENSES - Of the Sense of SEEING.

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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 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 

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