An Introduction to Comparative Psychology
Chapter 8: The Analysis of Impressions
C. Lloyd Morgan
WE have already seen that an impression is that which is brought to the focus of consciousness as the direct result of the excitation of afferent nerves ; and that an idea is that which is brought to the focus indirectly, that is to say, through the' intervention of an impression or of another idea. In the case of the. impression, the concomitant cerebral conditions are due to afferent impulses coming from outside the brain ; in the case of the idea, the concomitant cerebral conditions are determined from within by preceding brain-states. Impressions are therefore conveniently spoken of as presentative, as contrasted with those representative states we Call ideas. Neither presentative impressions nor representative ideas constitute by themselves, however, states of consciousness. They are the focal constituents, it is true, but they are set in a margin of subconscious elements, which not only form integral parts of the states of consciousness, but also serve to modify, and in part determine, the character of the impressions and ideas themselves. And these marginal constituents are also in part presentative and in part representative.
We must now direct our attention more closely to the impressions of which the ideas are representative, and learn what we can, through the application of analysis, concerning their nature and origin.
Let us begin by inquiring what is presented to consciousness when we receive through the channels of sight an impression of such an object as a violet. It is necessary
(126) here to distinguish between that which is directly presented to consciousness and that which rapidly follows on this presentation. We actually see the violet as coloured, of a certain figure, and definitely located in space; all this seems to be part of the direct impression. Its scent, its weight, how it feels in the hand, the taste of a petal in the mouth, its name, -- these or some of them, and perhaps more besides, may be rapidly suggested by the sight of the flower; but they seem to be representative and added to the direct impression. They are suggested through association, and are the results of experience. For the child who has made acquaintance only with wild dog-violets, the sight of the flower will not give rise to any representative scent element, such scent not having been associated with the flower through experience. It would seem, then, that the presentative impression, as such, does not contain more than elements of colour, of form, and of position in space. May it not, however, contain less ? Is all this indubitably presentative ? Is not, for example, the position in space something superadded to the direct impression ? For the adult consciousness it would appear not. We cannot get a definite visual impression of an object without this element. We cannot see a violet without seeing it somewhere. We do not first see it and then locate it in space. When we see it at all we see it out there. It would seem indeed that all our visual impressions carry with them this externality. Even when we lie on our backs and gaze up into the cloudless blue of a summer sky, we feel that there is in impression an inalienable element of depth.
Externality is also given us in auditory impressions, though out ability to localize the source of such impressions is here very markedly less than in the case of sight. When we hear a sound there appears to be presented, with the tone or noise, a certain outness ; and this does not seem to be merely suggested by the impression, but to be part and
(127) parcel of the impression itself. In olfactory impressions and impressions of radiant heat, on the other hand, the element of outness seems to be very vague, and to be rather suggested as the result of oft-repeated association than directly presented and involved in the impression as such. In tactile impressions there would seem to be an immediate reference outwards to the part of skin-surface affected, while in the case of gustatory impressions a general localization in the neighbourhood of the mouth is probably equally immediate.
Of all impressions of the special senses, visual impressions are the most clearly defined, and for human beings are the dominant impressions, at least so far as the information they convey is concerned. Touch is sight's constant ministrant, ever ready to verify the visual impressions. Hearing widens the field of consciousness, and for us, through oral communication, is a channel of universal suggestiveness. Smell, taste, and the temperature sense are in this connection of secondary importance. Let us then return to the visual impression and endeavour to analyse it further. Inseparable though the form and distance of the violet may be from its shading and colour, we nevertheless feel that these several elements are distinguishable on careful introspection. Nay, further, we shall learn that, inseparably combined as they are in the psychological impression, analysis decomposes them into elements which are physiologically separate and distinct. In this inquiry, indeed, it is necessary to dig down to the organic and physiological foundations in order that we may adequately grasp the nature of the psychological superstructure. We must even go further and see how these physiological foundations are determined by physical conditions external to the organism.
I said just now that on careful introspection the shading and colour of the violet are distinguishable from the elements of form and position in space. Of these, however, shading
(128) and colour seem much more intimately united together than is either of them with distance or position in space, while form appears in some respects to be intermediate in character. In so far as the form involves only extension in two dimensions, on a plane at right angles to the line of vision, it appears to be closely associated with the shading and colour; but in so far as it involves solidity, which introduces the third dimension of space, it appears to be more closely associated with position in space. Leaving this solidity to be considered presently, together with distance and position, we will now direct our attention to the extension, shading, and colour of the violet or other such visible object in the external world. Physical science tells us that the visibility of the object is dependent on the surface reflection of rays of light, which are transmitted in right lines to the corneal window of the eye through the medium of the ether. A beam of sunlight consists of transverse vibrations of the etherial medium of almost inconceivable rapidity, of which all that lie between about 800 billions per second and 400 billions per second are capable of affecting the retina of the eye. There is a long series of vibrations of less rapidity than 400 billions per second which may be felt as radiant heat ; and there is a long series of vibrations of greater rapidity than 800 billions per second, of which we have indirect knowledge through their chemical effects on certain substances ; but neither series is capable of affecting the retina. By means of a spectroscope the vibrations which do affect the eye may be arranged in a continuous series, being spread out in the order of their frequency from 400 billions per second at one end to 800 billions per second at the other end, so that between the extremes there is a physical gradation through a great number of minute' stages, When a beam of sunlight comprising vibrations of all stages of frequency between and beyond these limits,, falls on such an object as a violet, the vibrations are
(129) partly absorbed and partly reflected. In the petal of the violet there is relative absorption of the waves of less frequency, and reflection of the waves of greater frequency. But all that reaches the retina from the violet, or other external object, is. a more or less selected sample of etherial vibrations lying between the limits of visibility. Such, in briefest outline, is the physical story of that which goes on external to the organism. Let us now turn to the physiological story of what takes place when the etherial vibrations reach the organism.
When the vibrations, transmitted in right lines, reach the eye, they pass through the convex transparent cornea, through the pupil surrounded by its iris curtain, through the crystalline lens and vitreous humour, and are thus made to impinge on the retina. All these transparent structures are merely accessory, and for the purpose of throwing a clear-cut, welldefined vibration-image on the essential receiving structure, the retina, which is in direct communication, through the optic nerve, with the brain. The retina itself, in its focal and most sensitive area, is composed of an exquisitely delicate mosaic of minute cones, of which there are some, 2,000 in the restricted area, less than 1/100th of an inch in diameter, of most acute focal sensibility. Outside this area there are, in the retinal mosaic, delicate rods as well as cones. Each of these minute cones and rods has probably its individual connection, through the fibres of the optic nerve, with a particular part of the brain ; and each is physiologically sensitive (using) this term in a non-psychical sense, as a photographer speaks of a sensitive plate) to light vibrations. Since, therefore, the retina is an extended surface, on which is thrown a vibration-image having extension, we have here a physiological basis for that extension in two dimensions with which we are psychologically acquainted. Only that part of the retinal mosaic on which there falls the image of the violet, or other external object, is stimulated by the
(130) etherial vibrations. So far there is little difficulty ; and if
the external world appeared of a uniform grey of merely varying intensity, we could sufficiently account for the shading of the extended surface by differential stimulation of the retinal cones or rods. But objects appear not only shaded in neutral grey, but tinted of many hues. Herschel thought that the workers on the mosaics of the Vatican could distinguish at least 30,000 shades of colour. Now there is no reason to doubt that each retinal cone is capable, when stimulated, of initiating the impulse which gives rise to any of the numerous colour sensations. There are not some cones set aside to respond to certain particular vibrations,others set aside to respond to other vibrations, and so on; each cone may respond to any vibration within the visible scale. But it is clear that it does not respond in only one way to vibrations of all frequencies. If it did this the world would appear neutral tinted, without variations of colour.
Nor, on the other hand, can we suppose that it contains some thousands of substances of differentiated sensibility, each I of which is stimulated by vibrations of a definitely assigned frequency or range of frequency., It seems that the phenomena can be best explained if we assume that there are in each cone a few primary modes of response, by the subtle combination of which all the diverse colour-effects of experience may be produced. It is found, for example, that if we take vibrations of a certain frequency in the red end of the spectrum, others of a certain frequency in the green, and others of a certain frequency in the blue, we can, by suitable combinations and by varying the relative intensities of these three, match very fairly any colour or shade which we meet with in normal experience. If we similarly take four primaries, red, yellow, green, and blue, we can, it is said, make a somewhat better match. Into the physiological theories of colour-vision, however, we cannot enter. By the development of what organic products (or in what other
(131) way) these different modes of response are rendered possible; whether there be three or more fundamental modes of response in the retinal cone ; whether the sensation of white is physiologically simple or compound; whether I black is merely negation of response or a separate mode of response,-- these are questions which must be left for physiology to answer. It must here suffice to note that they are questions under discussion, and ones to which a definite reply is by no means easy.
For psychology, vision begins with the visual sensations; and for us the primary question is, What is a visual sensation ? A sensation may be defined as an undecomposable psychological element directly due to an afferent impulse or combination of afferent impulses. It is thus an ultimate result of psychological analysis as such. I would specially draw attention to the fact that the sensation is the result of analysis, and to the sense in which this statement holds good. The data of experience are states of consciousness. With them we commence our psychological study. We wish to know all that we can about them, and we therefore submit them to analysis. The first result of our analysis of a state of consciousness is to distinguish a focal impression or idea from a marginal setting of subconscious elements. We then proceed to analyse the focal impression, say of a violet, and we find that the impression is somewhat complex involving colour, shading, form, and position. We therefore analyse further, and taking the element of colour we reach a sensation of violet, which appears to be psychologically undecomposable. Here is an end of our psychological analysis in this direction. But we, may carry back the analysis further physiologically, and may show that the sensation of violet is due to the combination of physiological impulses, namely (to take one interpretation), to strong impulses due to physical vibrations of a certain rapid frequently with feeble impulses due to vibrations of other
(132) frequencies, medium and relatively slow. The steps of the analysis, therefore, are -- state of consciousness with which we start: impression: sensation: physiological impulse. The impression and sensation are psychological: the impulse physiological. Hence the sensation is, the ultimate result of psychological analysis.
But now suppose that instead of starting from the data of experience, states of consciousness, we endeavour to explain how these states of consciousness have been built up, then we proceed to utilize the results of our analysis and make the elementary sensations our point of departure: and this method is so common in psychology, that the fact that the sensations are the results of our analysis of states of consciousness is apt to be lost sight of.
To return now to the visual sensations which psychological analysis discloses as elementary constituents of impressions. There can be no question that, for psychology, black and white are primary sensations. But what about grey? Can we analyse a uniform grey into black and white, as we can analyse a musical chord into its constituent tones? So, too, with such colours as orange or purple. When we look at a homogeneous patch of orange, uniformly illuminated, can we analyse it into red and yellow elements ? or does it produce an effect which, as a sensation, is incapable of analysis however well we may know that it is the result of the combination of diverse physiological impulses? Opposite answers are given by different observers. For myself, I am quite incapable of analysing orange when it is presented to me as part of a definite impression. I happen to know that pink, to take another example, is the result of, so to speak, -watering down carmine with white. But when 1 look at a uniform pink surface I am incapable of analysing the sensation. When I look at a purple surface I can see that the colour has certain affinities to blue and certain affinities to red. But I cannot pretend to analyse it as I can analyse a
(133) musical chord. For myself, therefore, and those whose experience is similar to mine, it would seem that grey, orange, purple, pink, and so forth, are primary sensations which result from the combination of physiological impulses below the threshold of consciousness. For us their "dissociation point," to borrow an analogy from chemical science, lies in the infra-conscious physiological region. This does not, however, preclude the view that for others differently constituted the combination occurs above the threshold of consciousness, and that they can analyse in a region into which the conditions of my conscious nature preclude MY following them. For them pink is not a sensation, but the result of the combination of sensations of carmine and of white.
To sum up then concerning the physical, physiological, and psychological aspects of this matter, we have first in the physical field of external nature a long series of etherial vibrations capable of stimulating the retina with a wide range of graduated frequency; we have, secondly in the physiological field of the retina, a few differentiated modes of response to groups of these etherial vibrations; and we have, lastly, in the psychological field of consciousness, a great number of different sensations, for some of us incapable of analysis in this field, due to varied combinations of the few physiological responses. These visual sensations are, however, merely elements in the visual impressions which carry with them other elements, such as solid figure and position in space, to which we shall have to return presently.
Auditory impressions, like visual impressions, carry with them, as we have seen, an element of outness or externality. Apart from this they consist of auditory sensations. Physical science tells us that sounds take their origin from some external vibrating body, and are conveyed to the ear as waves of elastic compression of the air. The vibrations thus transmitted are longitudinal or to and fro, not
(134) transverse as in the case of light. Within the external opening of the ear, these waves encounter the drum membrane which they set in vibration. Into the nature of the accessory parts, and of the essential structure of the auditory organ, we cannot here enter. Suffice it to say that by the accessory parts, which though organic are to a large extent mechanical in their function, the vibrations are transmitted to a fluid contained in the essential organ, which, itself bathed in fluid, lies in a cavity in one of the bones of the skull. In a certain part of this organ, termed the cochlea, there is a spiral membrane with delicate radial fibres, and this membrane is so supported and arranged as to be tense radially, but loose longitudinally, or in the direction of the spiral. The fibres of the spiral membrane are not themselves directly connected with the fibres of the auditory nerve, but they are connected indirectly through their supporting structures. If any particular radial fibres are caused to vibrate, their vibration stimulates certain hair-cells indirectly associated with them; and thus it is possible that every radial fibre sends its special and peculiar excitation along its associated auditory fibre to a particular part of the brain.
When we sing a note into an open piano, the loud pedal being held down, the sound waves caused by the voice, sweep over the strings, and tend to set them in vibration. But only those wires whose tones are in unison with ;he note we are singing are thrown into vigorous vibration. The other wires remain comparatively unaffected. And when our voice ceases, the note is continued through the vibration of those piano-wires which answer to it in period. If a noise be made near the piano, such as the striking of a book with a stick, a confused murmur will be heard among the strings. For the noise is a chaotic mixture of vibrations, and from this each string mechanically selects the vibrationi whose period answers to its own. Now it seems probable,
(135) though there are still unexplained details, that the spiral membrane is in some manner selective like the wires of our recipient piano. When a note falls upon the ear, vibrations of the fluid are made to sweep along the membrane; that part of the membrane which is attuned to the vibrations is affected, and through it the associated hair-cells in connection with auditory fibres are stimulated. When a noise falls upon the ear a considerable area of the membrane is set in vibration, and a somewhat chaotic set of impulses travel together along the fibres of the auditory nerve.
The range of vibrations capable of stimulating the auditory membrane varies a good deal in different individuals. About thirty vibrations per second is the lower limit; the upper limit is about 20,000 vibrations per second, but some individuals have their range extended to 40,000. It should be clearly noted that although physicially there is a close analogy between the series of light-vibrations of increasing frequency, and the series of sound-vibrations of increasing frequency, yet physiologically there is this great difference. Any light-vibrations belonging to the visible series can and may stimulate each retinal cone, and the differentiation for colour is somehow effected within the cone itself or in some structures connected therewith on the way to or within the brain. Sound-vibrations on the other hand affect, according to their frequency of vibration, different auditory hair-cells. Hence the combination of visual impulses to form a physiologically compound colour such as purple is of a different order from the combination of auditory impulses to form a chord. Psychologically I am unable to analyse purple; the combination of impulses is for me infra-conscious. But I can quite easily analyse a chord into its constituent notes ; the dissociation point of a musical chord is within the conscious region. I must, therefore, speak of a combination of impulses (physiological) to form the sensation purple; but I can speak of a combina-
(136) -tion of auditory sensations (psychological) to produce a musical chord. Noises, however, from their chaotic nature are only to a slight extent, and with difficulty, analysable.
Closely associated organical ly with the auditory organnay forming part, and almost unquestionably the original part of the structure long known as the auditory sac or membranous labyrinth of the ear-lies the seat of certain very different sensations; namely, those by which we are made sensible of changes in the direction or in the amount of the movements of the body as a whole, and of the head. The exact nature of the physiological impulses which conspire to give rise to these sensations is not thoroughly understood. No doubt the impulses orginate in movements in the fluids which fill, and surround the membranous labyrinth with its semicircular canals; but exactly how these movements stimulate the end-organs of the afferent nerve-fibres is not known with certainty. Even the sensations themselves, though they are now recognisable without difficulty, are so little obvious in ordinary experience, that, like the motor sensations to be considered presently, they play little or no part in the older psychology. We may be said, indeed, to owe their recognition to the alliance between physiology and psychology, and to the introduction of methods of experimental investigation.
If a man sit blindfolded on a smoothly running turntable he feels a curious sensation of being turned when the table is rotated;- but if the rotation be continued at a uniform rate the sensation dies away, but is again felt whenever the rate of motion is accelerated. If, on the other hand, the rate of rotation be diminished he has a sensation of being turned round the other way. If he sit on a swing, blindfolded, and the swing be gently moved he is at I once aware of the fact. If he be on a smoothly running car, he feels each change of rate of motion. And if it run for some time at a uniform rate and then slacken speed, he feels as
(137) if it were beginning to move in the opposite direction. Such are some of the experimental evidences of the real existence of these sensations. And I call them sensations because they do not appear to be psychologically analysable.
It will be unnecessary for us to analyse, the impressions of smell, taste, touch, and temperature, or consider olfactory, gustatory, tactile, and temperature sensations. The foregoing analysis will suffice to show the psychological and physiological nature of sensations and impulses due to the stimulation of the end-organs of the so-called special' senses. We may now note that in the impulses so arising there are differentiations of two kinds. There is a differentiation of similar stimuli according to the position of the stimulated end-organ on an extended surface, as in the case of touch. There is also a differentiation of, dissimilar stimuli acting on the same end-organ as in the case of colour vision, probably of smell, and perhaps of taste. And these differentiations may be combined as in the case of vision, where each cone of the area of maximum sensibility is not only differentiated in position, b ut exhibits differential response according to the wave-frequency of the light by which it is stimulated; or again, as in the case of hearing, where the differentiation according to position is associated, through the intervention of the accessory structure of the spiral (basilar) membrane, with sound stimuli of a given wave-frequency. We have also distinguished between the afferent. impulses, which are physiological in their nature, and the sensations to which they may give rise, and have seen that these sensations, and not the mew impulses, are the psychical elements which constitute those impressions that we experience. And now we proceed to note, as in accordance with all that we have already learnt, that at any moment there are raining in on the brain through the afferent nerves multitudinous impulses,
(138) some of which are definitely of conscious sensation-value, many of which are merely subconscious, and yet more of which are infra-conscious, merely physiological, and not entering the psychical wave at all, As I write there is, besides the focal visual impression, much that is merely subconscious in vision, and perhaps also visual impulses that are infra-conscious. But apart from these occurrences in the visual field there are very numerous tactile impulses. My clothes press gently on many parts of my body, my attitude involves stronger pressures, my fingers hold the pen. There are, too, numerous warmth-impulses from, the fire; and many auditory impulses fall upon the ear, the ticking of my clock the pleasant murmur and crackle of the burning log of wood, sounds of birds without, and of my pen running over the paper. There are also olfactory impulses from my cigarette and so forth. All these impulses pour in by the afferent nerves, and there arises from them a definite series of focal impressions and ideas suggested thereby, a more or less indefinite margin or setting to the same, and a physiological background which though infra-counscious is constantly so to speak hanging on the confines of the psychical wave.
So far I have said nothing of sensations of pain, though these are often unpleasantly insistent elements in consciousness. The word pain is here used in its narrower acceptation, not as antithetical to pleasure, but as descriptive of a particular kind of sensation. If the skin be removed from any part of the body, and the underlying dermis be 'exposed, a stimulation, however light, does not give rise to a sensation of touch, or to one of heat or cold, but to one of pain. If we increase the pressure on a small area of the skin beyond a certain amount we cause a sensation of pain; and if we either chill or heat a portion of the skin-surface beyond certain limits pain arises. It seems proved 'that the nerve-endings which are stimulated to pain are different
(139) from, and probably lie deeper than, those which are concerned in sensations of touch or of temperature ; the nerves with which they are connected pursue a somewhat different course in the spinal cord, and end in different centres in the brain. In certain diseases, and under the influence of certain drugs, sensations of pain and sensations of touch and temperature are differently affected. It is worth noting that the sensations of pain, in the narrower sense, are not always painful or disagreeable in the wider sense. Moderate pain, as when we lightly press a bruise, or begin to make our hands tingle before the fire, is not by any means unpleasant.
These sensations of pain are developed not so much in the skin as beneath the skin ; nor are they restricted to the neighbourhood of the skin. From tendons, bones, joints, and the internal viscera, we may receive sensations of pain, and the only definite impressions we receive from within our skins are those originating in sensations of pain. The healthy man, sound in every part, pays very little heed to what is going on inside him, apart from the calls of hunger and thirst and other natural promptings. There is probably nevertheless a very considerable body of impulses pouring in on the brain from all the inner parts. This is often summed up under the head of "general sensibility." It is this which, as we have before seen, plays no unimportant part in determining those marginal factors of our psychic life which we term mood and temperament, and which were indicated as part of the organic basis of personality. It seems very probable then that pain is felt when these sensations of general sensibility are abnormally heightened. Incipient heightening or the beginnings of pain are not unpleasant, but they soon reach a stage when they are more or less distressing. Fatigue is probably a modification of general sensibility, and in its incipient stages, when we are, as we say, Just healthily tired, is not unpleasant. Hunger is
(140) associated especially with the condition of the mucous membrane of the stomach, and may range from a pleasantly healthy appetite to the gnawings of famine. So too with thirst, which appears to be due to a deficiency of water in the mucous membrane' of the soft palate. Fatigue, hunger, and thirst, with the pain felt in the viscera, the eyes, ears, head, and so forth, and in or beneath the skin, are more or less diffused presentative states, which carry with them a varying degree of localization.
The agreeableness or disagreeableness of impressions or states of consciousness-that emotional colouring or tone which makes them pleasant or distasteful-should probably be regarded rather as psychical qualities of sensations than as separate and distinct elements. It will be sufficient to make but a passing reference to them here; their differential influence on the control centres will be briefly considered hereafter. The pleasurable tone sometimes tinges the sensation or impression ; sometimes arises out of the relations of the focal and marginal elements of a state of consciousness; and sometimes is due to associated ideas, being of course in this case representative. A pure saturated colour, a full rich musical tone, a diffused sense of moderate warmth, certain scents and flavours ; -these are pleasurable in themselves. Other scents, other flavours, shrill high-pitched notes, pain in any of its intenser forms, are in themselves distasteful or even distressing. In the case of a musical chord or a sequence of such chords, in the case of a fine painting, the pleasure arises from the harmonious relations of the parts. And nothing is better calculated to enforce the complexity of the psychical wave in the instant of consciousness than a consideration of what -elements are present, focal and marginal, when our soul is stirred by some magnificent pageant, by a rich and full orchestral climax, or by the culminating scene in a great play or novel. It may perhaps be said that in such cases we have the
(141) accumulated effect of states of consciousness which have succeeded one another during a considerable space of time. But if it be true that we are limited to the present instant of consciousness, it is clear that at such moments of climax there are gathered up into the wave of consciousness a great number of elements, the harmonious correlation of which gives us a pleasure of peculiar richness and fulness. Some of these are representative; and pleasurable or painful tone is often associated rather with the suggested ideas than with the suggestive impression itself. There is nothing intrinsically pleasurable in the cawing of rooks, nor in the shrill cry of the swifts as they wheel in their flight through the summer air; but these tire sounds which for many of us are full of pleasant associations. The scent of violets in the early morning, the song of the lark in the summer sky, the sight of the young grass when the bay has been recently cut and carried, the warmth and brilliance of the sun after dark and cloudy days,-- all these are pleasant; each has its special quality of emotional tone; and all 'are due to complex correlated influences, presentative and representative, but mainly representative. So, too, with disagreeable or distressing states. The painful horror of the sight of blood, the loathing some feel for a spider or a toad, the indefinable fear of a snake,-these are due to the emotional tone of associations which are often complex. Lastly, the exhilaration of perfect freshness and health, and the depression of weariness and dyspepsia, are due to the emotional tone in association with the net result of multitudinous impulses from the nerves of general sensibility.
There is yet one more group of sensation-elements that remains for consideration,- the group which constitutes what is often spoken of as the muscular sense. The name is not quite satisfactory - and it will be better to speak not of muscular but of motor sensations, including under this head sensations of position, such position being regarded as
(142) the resting stage of movement. We need not here discuss the physiological question as to what share of these motorsensations is due to afferent impulses from the muscles themselves ; what share is due to movements and strains in the skin or other surrounding parts; and what share is due to impulses coming from the articular surfaces of joints, or from parts which move over each other, as the eye moves in its socket. Psychological introspection would seem to assign to the last class of impulses-- that of the movements of parts in their sockets-the leading position. But this does not exclude the other factors.
There can be little question that motor-sensations are capable of very exact definition, and that they are of great importance in building up the fabric of our presentative impressions, as they are also in affording data-to be considered hereafter-for the control of action in the field of practical skill. If with shut eyes and outstretched hand we trace a circle in the air, we shall perhaps be surprised at the accuracy with which we feel the movements and positions of the limb; or if under similar circumstances we draw a small circle on paper, or write our name slowly blindfolded, the delicacy of our motor feelings may, if our attention has not before been drawn to the matter, come upon us almost as a revelation. For it is characteristic of these motor-sensations, that they lurk somewhat persistently in the marginal region of consciousness, and require the application of our attention in an unwonted direction to he rendered focal. And this would seem to be due to the fact that the movement or position is in daily life merely a means to attain a given end, and that the end and not the means is what we habitually have in view. It is therefore worth while to carry out some self-observation directed to the ascertainment of the extent and limits of our motor-sensations. We are looking round us all day long with our eyes, and yet we may scarcely have realised how distinctly we can feel the, move-
(143) -ments of the eyes in their sockets. We are constantly adjusting our eyes for near and far vision, and yet may not have paid any attention to the motor-sensations which accompany these adjustments. Nevertheless, when our attention is drawn to the fact, we feel them distinctly.
We have seen that along the afferent nerves of the special senses, and again through the nerves of general sensibility, impulses are constantly pouring in upon the brain, some of which, presumably those which are dominant, enter into the focal impressions of consciousness ; others, as subconscious elements, fill in the margin of consciousness ; while yet others remain infra-conscious, -- no doubt of great organic importance, but psychologically extra-marginal. So, too, there is pouring in upon the brain a multitude of motorimpulses, some of which, as we shall see in the next chapter, enter into and modify the focal impressions, many of which are - at most marginal, while a residue, large and by no means unimportant, remains infra-conscious, and ministers to the physiological integrity of the bodily life.
It only remains to add that, while we speak of " a sensation " or "sensations " as undecomposable for consciousness, we often employ the word " Sensation " (without the article and not usable in the plural) as a grouping term for the whole of that area of mental life which is comprised under the head of sense-experience. Sensation in this usage is comparable in classification with Perception and Conception