« VorigeDoorgaan »
The Platonists differed from ARISTOTLE in maintaining, "That there exist eternal and immutable ideas, which were prior to the objects of sense, and about which all science was employed." They agreed with him, however, as to the manner in which these ideas are perceived. Two thou sand years after PLATO, Mr LOCKE represents our manner of perceiving external objects, by comparing the understanding "to a closet, wholly shut from light, with only some little opening left, to let in external visible resemblances or ideas of things without." The notion of all these philosophers was, that, from the existence of these images or ideas, the mind inferred, by a process of reasoning, the existence of the external objects themselves.
Dr REID refuted, by a very simple process, these doctrines. He pointed out merely the fact, that the mind is so formed, that certain impressions, produced by external objects, on our organs of sense, are followed by certain sensations; and that these sensations are followed by percep→ tions of the existence and qualities of the bodies by which the impressions are made; and that all the steps of this process are equally involuntary and incomprehensible.
It will at once be perceived, that the doctrine here laid down regarding the functions of the senses, corresponds precisely with the philosophy of Dr REID.
The organs of each sense are double; and yet the consciousness of all impressions experienced by the mind is single. Various theories have been propounded to account for this fact; but none of them are satisfactory. Dr GALL ventured to give an explanation different from all these. "He distinguishes two states of activity in the organs of the senses, calling one active, the other passive. The functions are passive, if performed independently of the will; the eye, for instance, necessarily perceives the light which falls upon it, and the ear, the vibrations propagated to it. Now, we perceive passively with both organs, says he; we see with both eyes, hear with both ears, but the active state is confined to one organ, and commonly to the strongest. We see
with both eyes at the same time, but we look with one only; we hear with both ears, we listen only with one; we feel with both hands, we touch with but one, &c.
"There is no doubt that we look with one eye only. In placing a pencil or any other thin body between us and a light, keeping both eyes open, and throwing the axis of vision, the stick, and the light, into a right line, did we look with both eyes, the pencil should occupy the diagonal, and its shadow fall on the nose. But this always falls on one eye, on that which the person, who makes the experiment, ordinarily uses in looking with attention. If the pencil be kept in the same position, and the eye not employed in looking be shut, the relative direction of the objects will seem to remain the same; but if he shut the eye with which he looked, it will be altered, and the pencil will appear removed far from its former place. Again, let any one look at a point but a little way distant, both eyes will seem directed towards it; let him then shut his eyes alternately. If he close the one with which he did not look, the other remains motionless; but if he shut that with which he looked, the other turns immediately a little inwards, in order to fix the point. Moreover, the eyes of many animals are placed laterally, and cannot both be directed at once to the same object. Finally, the gestures of man and animals prove that they look with one eye, and listen with one ear; for they direct one eye or one ear towards the object to be seen or heard *.
"Notwithstanding what has been said, Dr GALL's explanation, seems to me," says Dr SPURZHEIM, "little satisfactory. Indeed, it is very remarkable, that passively, we perceive, at the same time, the impressions of both organs of any sense, not only if one, but also if different, objects impress the two. Even different impressions of different objects may be perceived by both organs of two senses at once. We may, for instance, with both eyes see different objects at the moment that with both ears we hear different
* Dr SPURZHEIM's Phrenology, p. 221.
sounds. As soon as we are attentive, however, as soon as we look or listen, we perceive but one impression. It is impossible, therefore, to attend to two different discourses at once. The leader of an orchestra hears passively all the instruments, but he cannot be attentive except to one. The rapidity of mental action deceives several, and makes them think it possible to attend to different objects at the same moment. It follows that there is a difference between the active and passive state of the senses; but whether this difference suffices to explain the single consciousness of every sense is another question; I think it does not.
"First, this explanation would only apply to functions in their active, not at all in their passive state; and the cause of single consciousness must be the same in both. Further, the active state is not produced by the external senses themselves, any more than voluntary motion by the mere muscles. Some internal power renders the senses active; they themselves are always passive, and merely pro.. pagate external impressions; they appear active only, when something internal employs them to receive and to transmit impressions to the brain. It is therefore probable, that the internal cause which excites only a single organ of the external senses to activity, is also the cause of the single consciousness of different impressions. Dr GALL's explanation of single consciousness is consequently not only grounded upon an inaccurate notion, but would be far from satisfactory, were the supposition even true *."
The mind has no consciousness either of the existence of the organs of sense, or of the functions performed by them. When the table is struck, and we attend to the subject of our own consciousness, we perceive the impression of a sound; but by this attention we do not discover that the impression has been experienced by the instrumentality of any organ whatever. Hence the perceptions of the mind are always directed to the objects which make the impressions, and not to the instruments by means of which the Lib. cit. p. 223.
impressions are experienced. The instruments perform their functions under Nature's care; and, as has been already observed, are not subject to the will. We should have been distracted, not benefited, by a consciousness of their internal action, when they perform their functions. It is when they become diseased that we become conscious of their action, and then the consciousness is painful. Every one must be sensible of this fact, whose eyes or ears have been diseased.
Dr SPURZHEIM observes, that "the brain seems to be necessary to every kind of perception, even to that of the immediate functions of the external senses; but it is not yet ascertained, though it is probable, that one fundamental power, inherent in a particular part of the brain, knows and conceives as sensations, all the varied impressions made on the external senses. Some phrenologists think that each external sense has a peculiar portion of brain for this end, and that the combined action of its nerve and of this cerebral part, is necessary to the accomplishment of its functions. That the nerve of taste and a portion of brain, for instance, are necessary to perceive savours; the olfactory nerve and a cerebral part, to distinguish odours, &c. I do not believe that consciousness happens without brain, but I see no reason to surmise that the immediate functions of each external sense require a particular portion of the brain, in order to be recognised as determinate sensations." (Dr SPURZHEIM'S Phrenology, p. 257.)
After these general considerations, which apply to all the external senses, a few words may be added on the specific functions of each sense in particular.
FEELING OR TOUCH.
DR SPURZHEIM inferred, from pathological facts, that the nerves of motion must be distinct from the nerves of feeling; and subsequent experiments have proved his inference
to be well founded. This subject has been treated of on page 59. The sense of feeling is continued, not only over the whole external surface of the body, but even over the intestinal canal. It gives rise to the sensations of pain and pleasure; of the variations of temperature; and of dryness and moisture. These cannot be recalled by the will; and I therefore consider them as depending on the sense alone. The impressions made upon this sense serve as the means of exciting in the mind perceptions of figure, of roughness and smoothness, and numerous other classes of ideas; but the power of experiencing these perceptions, is in proportion to the perfection of certain internal faculties, and of the sense of touch jointly, and not in proportion to the perfection of this sense alone.
THE functions of this sense are, to produce sensations of taste alone; and these cannot be recalled by the will. We may judge of the qualities of external bodies, by means of the impressions made on this sense; but to form ideas of such qualities is the province of the internal faculties.
By means of smell, the external world acts upon man and animals from a 'distance. Odorous particles are conveyed from bodies, and inform sentient beings of the existence of the substances from which they emanate. The functions of smell are confined to the producing of agreeable or disagreeable sensations, when the organ is so affected. These cannot be reproduced by an effort of the will. Various ideas are formed of the qualities of external bodies, by the impressions which they make upon this sense; but these ideas are formed by the internal faculties of the mind.