CHAP. XII.

SENSATION.

Vibrations-Perceptions-Sense of Feeling-Taste-Smell—Hearing— Sight.

THE bodily organs of sensation are the external organs, the nerves, and the brain. The external organs of sense are usually classed under five heads, the sight, hearing, feeling, smell, and taste. The sense of feeling might with convenience be divided into two or three; because the classes of sensation, which are referred to this sense, differ considerably in themselves, and in the external causes producing them.

Vibrations.-How it is that the changes produced in the external organs of sensation affect the mind, is a point respecting which we have, at present, no means of knowledge. Different hypotheses have been invented to account for it; but even if correct, they explain little more than the mode in which the external impressions are conveyed to the brain: how the changes in the brain affect the percipient principle, is utterly unknown, and, not improbably, will always continue so.

Dr. Hartley was of opinion, that the impressions made upon the external organs of sense produce vibrations in the minute parts of the nerves, which are propagated to the brain, and there excite sensations in the mind. This hypothesis was suggested by Sir Isaac Newton; but it was greatly extended by Hartley, who thought that it accounts for all the leading phe

nomena of sensation and association. It is much to be regretted, that this great philosopher burdened the important doctrine of association with the hypothesis of vibrations; they are really independent of each other: and those do Hartley and his philosophical system great injustice, who represent the matter in any other light. We do not believe that any hypothesis as to the corporeal occasions of thought, can, in the present state of our knowledge, really explain the operations of thought: but if any of our readers wish to know more of Hartley's hypothesis, we may refer them to Mr. Belsham's Elements of the Philosophy of the Mind, p. 38—55. Dr. Reid's Essays on the Intellectual Powers, Essay 11. chap. 3. they will find the arguments of that philosopher against it. And we think it right to add, that Haller (the great physiologist,) maintained, that it is totally incompatible with the nature of the nerves and medullary substance.

In

Of Perception.-To perceive the influence of each sense in forming our notions and feelings, it is necessary carefully to distinguish between the mere sensation, and those ideas which are, by association, so connected with it, as to appear at first view a part of the sensation.

By the laws of association, many simple ideas of sensation, received through the medium of different senses, become connected, and at last blended together, so as to form one very complex, though apparently uncompounded idea. The complex idea, or its component simple ideas, may have been the direct objects of the operations of the understanding; and thus the complex idea may have been farther modified. Now this complex idea is often recalled to the mind by a corresponding sensation; and, by association, it becomes so connected with that sensation, that the complex idea itself is often mistaken for a part of the sensation. For instance, the sensation produced by the impression made by a globe on the sense of sight, is, as can be proved, nothing more than that produced by a circle, with certain variations of light and shade; yet, immediately on the sensation being perceived, the ideas of its

solidity, of its hardness, its magnitude, and of its being something external to one's self, immediately rise up in the mind in one blended form: by their complete coalescence they appear to be one; and by their immediate and constant connexion with the sensation, they appear to the mind as a part of the sensation. Indeed, there are comparatively few people who ever think that the sensation derived from the sight, is nothing more than what is derived from a minute picture delineated on the back part of the eye called the retina. Things appear to us, at one glance of the sight, to be solid or flat, to be near or distant, to be large or small, to be conjoined with other things or separate from them, to be parts of our own frame or external to it, &c.; and all this we appear to learn by the sight alone: but the fact is, that all these ideas are derived from another sense at various times, and altogether blending together, and arising the moment the visible impression is communicated, they appear part of the visible impression. "The visible appearance of objects," as Berkeley observes, "is a kind of language serving to inform us of their distance, magnitude, and figure:" no sooner are these signs presented to the mind, than, with the rapidity of lightning, the ideas associated with them succeed, and appear to have been communicated by the sight, and to be in reality a part of the sensation.

When the sensation produced by an external object, is so closely united with ideas which have been previously derived from sensation, and modified by the operations of the understanding, that the whole shall appear to be one feeling, apparently derived immediately from the external impression, the whole together, the sensation and the complex idea,—is called a perception. And by the faculty of Perception we understand that compound power, (or rather combination of powers,) by which perceptions are formed of external objects. The real nature of perception is greatly misunderstood; and indeed it cannot be understood without an acquaintance with the influence of association in forming complex ideas, and in

enabling sensations at once to bring them into the view of the mind.

Admitting the powers of sensation to be in an equally sound and vigorous state, yet it is obvious that the perceptions may vary very greatly in different individuals, and in the same individual, at different periods. Suppose a watch to be subjected to the observation of three persons, whose organs of sense are alike healthy and vigorous: the one, a very ignorant person, totally unacquainted with its purposes and movements; the second, a well informed person, not however possessed of any acquaintance with the particular mechanism; the third, an artist, minutely and completely acquainted with it: the sensation may be precisely the same in all instances; the picture upon the retina may convey to the mind an equally impressive sensation of the object; but how different the perception! The first sees a number of minute objects, which attract his attention perhaps by their beauty and regularity; but nothing more: he has no idea of their subserviency to each other, or of their general use: there is little more in his case than sensation; indeed we may say, nothing more than sensation, besides those associated perceptions which so soon become connected with every impression from external objects, and to which we have already referred. The second, from his general knowledge of mechanism, has some ideas excited by the sensation, of use and connexion; but he cannot discern the specific kind of connexion, nor how each part tends to answer the end of the whole. If he set about to study the mechanism, he subjects each part to minute examination in its structure and connexions; and by degrees may acquire an acquaintance with the whole, which, on a subsequent inspection, would give him an immediate distinct perception of the parts and purposes. What he thus acquires by laborious and patient examination, the third saw at once. His perceptions have long been cultivated by daily attention to the movements and their dependencies, by studying their defects and excellences, by the actual formation of their various parts, and

the structure of the whole: and a great number of ideas produced by such observations and operations become so intimately united with the sensation, that at last this at once. excites them, and thus he sees, or, (more correctly,) perceives what lies totally out of the reach of the observation of others.

Sensations derived from the senses of Feeling, Taste, Smell, and Hearing-All the external organs of sensation communicate to the mind materials for its notions and feelings; and it is probable that elementary pleasures or pains from every organ enter, more or less, into the composition of our most refined mental feelings.

The sense of feeling differs from the other senses, in belonging to every part of the body, internal or external, to which nerves are distributed: the term touch is most correctly limited to the sensibility which is diffused over the surface of the body, and exists in the most exquisite degree, at the extremities of the hands and lips.

This sense is of the utmost importance to man, considered as an intellectual being. It furnishes us with all our elementary notions respecting the real qualities of substances; and it is the sole medium by which we gain a knowledge of external objects as such. It is by the touch, (and originally by the touch alone,) aided by the power of bodily motion, that we distinguish our own bodies from other substances that surround us, and from the impression which every one has, that the objects, which affect the sight, the hearing, &c., are external.

The sense of feeling conveys to us the sensations of heat, hardness, solidity, roughness, dryness, motion, distance, figure, &c.; and all those corporeal feelings which arise from a healthy or diseased state of the nerves, and of the part of the body to which they belong.

The pains of this sense are more numerous and vivid than those derived from any other sense; and, therefore, have the greatest share in the composition of our mental pains. Its