It is a task of considerable difficulty to point out accurately the precise limits of the functions of the senses, because, in every act of perception, their instrumentality is combined with that of the internal faculties of the mind; and it is not easy to discriminate to what extent the act depends upon the one, and to what extent upon the other. For the elucidation of this point, I submit the following considerations to the attention of the reader.

The senses themselves do not form ideas. For example, when an impression is made upon the hand, it is not the organs of touch which form the conception of the object making the impression; but the nerves of feeling in the hand receive the impression, communicate it to the brain, and a faculty of the mind perceives the object. Without the nerves of feeling, the internal faculty could not experience the perception; because the medium of communication betwixt it and the object would be wanting. But neither could the hand experience the perception without the instrumentality of the internal faculty, because the nerves of feeling do not perform the function of percep

tion.

Hence, previous to every perception, there must be an impression on the organs of sense; and the function of these organs appears to consist in receiving and transmitting this impression to the brain and internal faculties. The nature of the impression depends on the constitution of the senses, and on the relations established betwixt them and external objects; and, as it is absolutely impossible for the human will to change either the constitution of the senses, or the relations betwixt them and the external world, it is clearly absurd to speak of acquired impressions.

But, as the senses are constituted with a determinate relation to external objects, so the brain and internal faculties are constituted with a determinate relation to the organs of sense. In virtue of the first relation, a certain object makes a certain impression; and, in virtue of the second, a certain

impression gives rise to a certain perception; and both depend on nature, and not on the will, nor on exercise or habit.

But we must distinguish betwixt the perceptions we experience of external objects, and the inferences concerning their qualities, which we draw by reasoning from these perceptions. All those ideas which are pure perceptions are formed intuitively, on the presentation of objects fitted to excite them. Inferences from these, on the other hand, are the result of our reasoning powers. What are sometimes called" acquired perceptions," are merely habits of reasoning, from the impressions naturally made on the senses; and these habits are just as much a part of our nature as the original perceptions. It appears to me, that the visible and tangible appearances of bodies are simple perceptions, because, after the amplest experience of some of these being deceitful, we cannot, in the slightest degree, alter our perceptions of them. For example, a rod half immersed in water, appears crooked, in defiance of every endeavour to see it straight. When we stand three or four yards distant from a mirror, and perceive our image in it, we cannot, by any efforts, succeed in perceiving the image as if formed on the surface of the mirror, although we know perfectly that it is so. It appears always at the same distance behind the surface as we are before it. If a picture be painted according to the rules of perspective, and the laws of optics, so as to represent a vista in the country, or a long street in a city, we are altogether incapable, when in the proper position for viewing it, of perceiving the surface to be plain. The picture appears to us to represent objects at different distances, and the most determined resolution to see them all equally near, is of no avail, although we know that, in point of fact, they are so.

If, previous to experience, all objects seen by the eye appear only as of different colours and shades, and all equally near, although really at different distances; and if we learn by experience only, that this natural appearance is

deceitful, and that, in point of fact, one object is near and another distant; I cannot perceive a reason why we might not learn by experience also, to perceive pictures as plain surfaces, and images as if formed on the surfaces of mirrors; and, in short, to get quit altogether of the illusions of optics. If it be easy to acquire, by habit, the power of perceiving objects as at different distances, which naturally appear to the eye, as all equally near, it ought to be no difficult matter to learn by experience, to perceive a surface to be plain which really is so, after we are certain of the fact; and yet, I have never heard of an instance of a person who had made such an acquisition. Colour, Form, Magnitude, and Distance, appear to be objects of intuitive perception; and, accordingly, no experience, and no repetition of acts of volition, can alter such appearances, if the refraction of light, state of the eye, and the internal faculties, remain the same.

The following appears to me to be a correct mode of ascertaining the limits of the functions of the senses: Whatever perceptions or impressions received from external objects, can be renewed by an act of recollection, cannot depend exclusively upon the senses; senses; because the organs of sense are not subject to the will, and never produce the impressions which depend upon their constitution, except when excited by an external cause. On the other hand, whatever impressions we are unable to recal, must, for the same reason, depend on the senses alone.

These principles will be best elucidated by examples. For instance, when a bell has been rung in our presence, and the impressions have ceased, they cannot be recalled by an effort of the will; because their existence depended on the apparatus of the ear being in a certain state of excitation, which cannot be reproduced by an act of volition. Hence these impressions belong to the ear alone. But if an individual is endowed with the internal faculty of Tune, and if a piece of music be played over in his presence, then, after the sound of the instrument has ceased, although he cannot

recal that sound, he can with facility reproduce the internal impressions which the notes made upon his mind; in short, he can enjoy the tune internally anew, by an act of recollection. The power of experiencing the perception of melody, and of enjoying the impressions which it makes, appears, therefore, to depend on the internal faculty of Tune, while the sound alone depends upon the ear. Hence the perfection of the power of perceiving melody in any individual, is not in proportion to the perfection of the external ear alone, but in joint proportion to the perfection of that organ, and the internal faculty. Without the external ear the internal faculty could not receive the impressions; but the external ear could never of itself produce the perceptions of melody. Accordingly, we see every day that many individuals enjoy the sense of hearing unimpaired, who have no perception of melody. The same principles applied to the other senses will point out distinctly the precise limit of their functions. We may take an example from the sense of touch. If we embrace a square body with the hands, certain impressions are made on the nerves of touch, called Sensations, in consequence of which the mind forms an idea of the figure of the body. Now, we can recal the conception of the figure; but not the sensation which excited it. The conception, therefore, depends on an internal faculty; the sensation on the nerves of touch. The functions of the nerves of touch appear to produce the sensation; but the power of conceiving is not in invariable proportion to the power of feeling, but in proportion to the perfection of the internal faculty and the external senses jointly. The perception, however, depends as entirely on nature as the sensation; and the power of perceiving the form of the body is not acquired by experience.

Dr SPURZHEIM observes on this head, that, where the same ideas are acquired by the instrumentality of two or more senses, the ideas cannot possibly be formed by the senses, because Nature, so far as man has discovered, never endows different instruments with the same functions, in the

same individual. For example, we can acquire ideas of Form by the instrumentality of the sense of Sight, and likewise by means of Touch. Now, from this circumstance alone, it is evident that the conception of figure is formed, not by the eyes, or by the nerves of Feeling, because this would be an instance of two separate senses performing the same functions; but by an internal faculty, which perceives Figure, in consequence of impressions made on either of these two different senses. The impressions made upon the eye are totally different from those made upon the nerves of Touch, but the internal faculty is adapted by nature to both; and hence the same perceptions are experienced by means of the same faculty, although through the instrumentality of different media; but the same function is not performed by distinct senses.

These views of the functions of the senses are illustrated and confirmed by the phenomena which take place when the organs of Sense are diseased. For example, when the Ear becomes inflamed, it often happens that spontaneous sensations of sound are experienced; when too much blood flows into the Eye, impressions, like those of light, are felt; when the nerves of Taste become diseased, disagreeable savours are experienced; when the nerves of Touch are excited by internal causes, a tickling or disagreeable sensation is felt; when the muscular system is relaxed by nervous diseases, and flying spasms occur over the body, impressions occasionally arise from these spasmodic affections, so preeisely resembling those of touch, that the individual is at a loss to distinguish them.

Every one is acquainted with the ridiculous theories which have been framed by philosophers, to account for the phenomena of perception. ARISTOTLE taught, says Dr REID, "That, as our senses cannot receive external material objects themselves, they receive their species, that is, their images or forms without the matter, as wax receives the form of the seal, without any of the matter of it *"

Essay on Intellectual Powers, p. 25.