In connection with the exercise of our senses there are not a few points on which popular ideas stand in need of correction. When we speak of ‘seeing’ or ‘hearing,’ the exercise of these or any other of our senses indeed, is usually referred to the organ concerned—eye, ear, nose, or tongue, as the case may be. A little consideration, however, will shew us that we make a very grievous mistake in referring the act of sensation or perception to the organ itself. Let us consider for a moment what happens when we acquire ideas regarding the form of an object through the sense of touch. We may in the first place ‘will’ to touch the object in question; the act of ‘volition’ as it is termed, originating in the brain, being transformed into nerve-force, and being further directed along the particular nerves which supply the muscles of one finger or along those which supply all the fingers. The muscles are thus stimulated to action, and through their agency the fingers are brought into contact with the desired object. Leaving the sense of sight out of consideration for a moment, we know that we can through the sense of touch gain ideas regarding the form, size, hardness, and other qualities of the object. Our nervous system is thus bringing us into relation with the outer world and specially with that portion of it represented by the object we have touched. But how have we gained our knowledge? The reply to this question leads us at once to perceive that the tips of the fingers do not represent the seat of knowledge. And a further consideration makes it equally clear that the brain must be credited not only with the task of perceiving, but also with that of appreciating what has been perceived. Hence we are forced to conclude that just as the first nervous impulse shot through the nerves to the fingers, so a second impulse has passed from the fingers to the brain. Our sense of touch has given origin to a subtle force which has passed upwards to the brain, and has there become transformed, through a mechanism—of the working of which we know as yet absolutely nothing—into perception and thought. Similarly with the work of the eye, of the ear, and of other senses.
When we talk of seeing or hearing, we are in reality speaking of the act of the brain, not of the eye or ear, which are merely the ‘gateways’ through which the brain obtains its knowledge. And that the brain is the true seat of the senses, may be proved to us from the side of pathology—the science which makes us acquainted with the causes and nature of disease. Cases are well known in which injury of the brain as the seat of sense has given origin to depraved sensations. Post-mortem examinations of persons who were continually conscious of a disagreeable odour have proved that these persons had laboured under brain-disease; whilst one case is on record in which, after a fall from a horse, and for several years before his death, a person believed that he smelt a bad odour. So also the sense of sight may be altered from internal causes, and on this ground may be explained the real nature of many cases of so-called ghost-seeing and spectral illusions. One well-known case, in illustration of this latter point, was that of Nicolai, a Berlin bookseller, who, neglecting to be bled in accordance with his usual custom, began to see strange persons in his room, and faithfully described the appearance of the figures. The figures disappeared when he had been bled once more. Thus in all such cases we must believe that those parts of the eye or ear which would have been concerned in seeing the supposititious objects or hearing the supposititious sounds—had either existed—were irritated from the brain and produced the delusive sensations. Thus the common phrase that ‘seeing is believing’ is in one sense literally true; for the act of sight apparently exercised in the person who suffers from optical illusions is in reality performed by the brain and is thus an act of belief, even if it be one of unconscious kind. The entire subject of physiological errors teems with valuable applications, but with none more practical or worthy of remark than that which would insist on the advantages, in the ruling wisely of our lives, to be derived from even an elementary acquaintance—such as should be included in the curriculum of every school—with the science of life.