Chapter II: The Special Facts of Consciousness (1)

_A._ _THE ELEMENTS OF MENTAL LIFE_

§ 4. SENSATION

1. _The Newly Discovered Kinds of Sensations_

We shall discuss first the simplest facts of mental life, later their complications. It has often been objected that such a treatment is not in harmony with the fact that we are more familiar with the complications than with the simpler facts. But we are also more familiar with our body than we are with muscle cells, nerve cells, and blood corpuscles, and yet we do not object to beginning the study of biology by a study of the structural elements and their chief properties. No one understands this to mean that the cells of various kinds existed first separately and were then combined into the body which consists of them. No one should believe that the simple mental states existed separately and were then combined into those complications with which we have become familiar in everyday life. Simple mental states are abstractions. But we cannot hope to understand the complexity of mental life without using abstractions.

Through the sense organs our mind receives information about the external world. The traditional classification of the sensations divided them into five groups. But the distinction of five senses has been found to be insufficient. At least twice as many must be distinguished.

When psychologists tried to explain all human knowledge in terms of experience, they met with some difficulty in the description of our experience of solid bodies. Tactual sensation was found to be insufficient for this explanation, since it informs us only of the side-by-side position of things, that is, of only two dimensions. It was soon recognized that the movements of our limbs were important factors in this experience, and the question was asked: How do we perceive the spatial relations of our limbs and the resistances offered to changes in these spatial relations, that is, to movements? The first answer to this question was, that the muscles, being obviously a kind of sense organ which gives us the familiar sensations of fatigue and muscular pain, are also capable of sending in definite groups of afferent nervous processes according to their conditions of contraction and tension. This answer was quite true, as far as it went; and about 1870 the sensory neurons of muscles were actually discovered. The tendons connecting the muscles with the bones were also found to contain sensory neurons.

But this cannot be all, for we are able to judge the position of our limbs even when the muscles are completely relaxed and a limb is moved by another person. It is further a fact that a weight and the distance through which it is moved can be estimated with fair accuracy, whether the arm is sharply bent or straightened out, although the contraction and tension of the muscles is very different in these two cases. It is now known with some certainty how these estimations are made possible. The surfaces of the joints are furnished with nerves. Make a slow movement of the hand or a finger and attend to the sensation resulting from it. There is little doubt that the sensation is localized in the joint. This view is supported by the fact that electrical stimulation of a joint considerably decreases the accuracy of the estimation of weight and movement.

The three classes of sensations--muscular, tendinous, and articular--are customarily grouped together under one heading as _kinesthetic_ sensations, meaning literally sensations of movement. But, as we have noted, these sensations occur as the result not only of movements of our limbs, but also of pressure or pull when the limb is at rest. They always occur together with tactual sensations, but must nevertheless be strictly distinguished from them.

Soon after this distinction had been recognized, the tactual, or rather cutaneous, sense was found to consist of several senses. The impressions of touch, that is, of pressure on the skin, of temperature, and of pain had always been distinguished; but it had not been known that the areas of greatest sensitivity for touch are not identical with those for temperature, and that the sensitivity for pain may be greatly diminished without a corresponding change in the sensitivity for touch. It was only about 1880 that these observations were explained, when an anatomical separation of the neurons serving these different sensations was demonstrated. If we test the sensitivity of the skin by carefully stimulating single points, it is found that not every point of the skin is sensitive, but that the sensitive points are isolated by larger or smaller insensitive areas. It is further found that the points sensitive to warmth are different from those sensitive to cold or to pressure or to pain. This can easily be demonstrated for the cold points by touching the skin in a number of successive points with a steel pen or a lead pencil. Generally only the touch is perceived, but now and then an intense sensation of cold is felt on definite points, always recurring when these points are touched. It is somewhat more difficult to demonstrate the points sensitive to warmth. The sensation is in this case much less noticeable. The points sensitive to touch are on hairy parts of the skin always close to a hair; on other parts, for instance the palm of the hand and particularly the finger tips, they are located so close together that their separateness can be proved only by the use of very delicate instruments. The same is to be said of the pain points of the skin. We cannot, therefore, regard the skin as one organ of sense, but must regard it as containing four classes of organs serving the senses of warmth, cold, pressure, and pain.

We must be sure, of course, to distinguish between pain, as a sensation, and the feeling of unpleasantness which almost without exception accompanies pain. We must further distinguish the sensation of pain from intense cold, intense heat, strong pressure, dazzling light, all of which may produce pain as a secondary effect. But the sensation of pain is quite dissimilar from the sensations of cold, heat, pressure, and light, to which it is added in consequence of physiological conditions. The independence of the sensation of pain can easily be demonstrated by touching the cornea of the eye with a hair. Pain is then perceived without any touch or temperature sensation. The pricking sensation in our nose resulting from the breathing of chlorine or ammonia may also be mentioned as an illustration of the same point. Let us further understand that pain is not only a cutaneous sensation, but also a sensation localized in internal organs; for instance, headache, toothache, colic.

The most interesting discovery of a new sense organ concerns the labyrinth of the ear. It was made quite unexpectedly. The labyrinth consists of the inner ear proper, or the cochlea, the system of three semicircular canals, and between these two organs a pair of small sacs, each containing a little stone or otolith, built of microscopic lime crystals. All these organs, being all of the nature of cavities filled with fluid and communicating, were originally regarded as serving the sense of hearing, although no one was able to say how. It was observed, however, that stimulation or lesion of the semicircular canals and of the sacs did not affect hearing, but resulted in disturbances of the coördination of the muscular activities in locomotion and normal position. For more than fifty years these observations remained unexplained; and even then their explanation was but slowly accepted.

It is now recognized that the semicircular canals and the sacs are not organs of hearing, but organs informing the organism about the movements or position of the head, and indirectly of the body as a whole. The sensations coming from these organs are usually so closely bound up with kinesthetic and tactual sensations that we have not learned to become conscious of them as a separate kind. Nevertheless we may perceive them separately under favorable circumstances. If we close our eyes, turn quickly a few times on our heel, and suddenly stop, we are vividly conscious of being turned in the opposite direction. This is a perception mediated by the semicircular canals. The fluid ring in the horizontal canal gradually assumes the motion of the body, in consequence of its friction against the walls; and when the body suddenly stops moving, the fluid ring continues to move and to stimulate the sensory neurons for some time. If the body moves in a larger circle, for example on a merry-go-round or on a street car passing around a curve, the mind perceives an inclination of the body towards the convex side of the curve. If we go up in an elevator, we have the impression, just after the elevator has stopped, of moving a short distance down. These are sensations of the otolith organs.

The otoliths are slightly movable, one in the horizontal, the other in the vertical direction. If the body moves through a curve, the otolith which by centrifugal force is driven outwards stimulates the sensory neurons in the same manner in which it stimulates them when the body is inclined. The perception of the body’s position is therefore the same. If the body is quickly moved up or down, the vertical otolith at first lags behind, and at the stop, through its inertia, continues to move a little in the same direction. The result is a brief perception of the body moving in the opposite direction.

Artificial stimulation or lesion of the semicircular canals or otolith organs in animals tends to produce certain unexpected reflex movements of the body which the animal tries to counteract voluntarily, so that all kinds of unusual movements are observed. If these organs are destroyed, one source of information about the position and the movements of the body is lost. This loss is not very serious in man, in whom it occurs as a result of diseases of the ear; man can obtain his orientation from visual, kinesthetic, and pressure sensations in spite of this loss. It is far more serious in aquatic and flying animals. Pressure differences are of no account when the body has nothing but water or air on all sides. In a greater depth of water vision is practically impossible. Under these circumstances the semicircular canals and the otolith organs are highly important for an animal’s life. Unfortunately no definite names have thus far been adopted for these senses. They are frequently called the static sense or the sense of equilibrium. But these names are of doubtful value, since other senses too may inform us about our equilibrium.

The enumeration of our senses is not yet completed. What is hunger? What is thirst? What is nausea? These mental states are certainly similar, in some respects, to tones and odors. They are sensations. There is the difference, however, that we do not project them into external space, but think of them as characteristics of our own body’s condition. How is consciousness of these sensations brought about? No doubt, in a manner similar to that of the mediation of such sensations as odors and tones: through the stimulation of sensory neurons and the propagation of nervous processes toward the motor points of the body. The place of stimulation must be somewhere in our organs of nutrition, and thus these organs must be regarded also as a kind of sense organ. That the sensory function can be attributed to an organ in addition to another function has been proved by the example of the skin, muscles, and joints. The same may be said of other organs, for instance the lungs giving us the sensation of suffocation.

We possess, therefore, a large number of organs whose primary function is of an active kind, but which also give information as to the condition of those active functions. The sensations resulting from them are as independent of each other as tones are of color or taste. But they do not permit of as many subdivisions as the sensations of the so-called higher senses. For the emotional part of our mental life they are of the greatest significance. Since we do not project them into the external world, but think of them as significant of the functions of our internal organs, they are rightly called by the common name of _organic sensations_.

2. _The Other Sensations_

Besides the cutaneous sensations four classes were known to the older psychology: sensations of color, sound, odor, and taste. The relation of these sensations to the corresponding stimuli comprises a vast number of problems and theories, but we shall here state merely that which is of more general interest.

The taste--in the ordinary sense--of a substance is by no means made up exclusively of taste sensations in the special sense of this term. It is usually a complex of different sensations which almost invariably occur together. Only gradually do we learn to analyze this complex into its elements. Touch sensations of the tongue and palate often enter into the combination, for instance in a burning or astringent taste. Sensations of smell are of particular importance in this connection. The different kinds of meat, of wine, of bread, and of many other foods and beverages are distinguished almost exclusively by the smell. Aside from these accompanying sensations, there are only four tastes proper: sweet, sour, salt, bitter, in all their possible mixtures and relative degrees of intensity. In a manner comparable to the distribution of cutaneous sensations, the taste sensations have their end organs at definite points in the papillæ of the tongue and soft palate. The so-called taste buds contained in the walls of the papillæ seem to be sensitive according to the principle of the division of labor, some serving chiefly this, others chiefly that taste. It is possible that all the taste buds of the same papilla mediate the same taste sensation, so that each papilla might be said to be in the service of a particular taste.

The number of distinguishable odors is very large. Gaseous, fluid, and solid substances, minerals, plants, and animals have usually their characteristic, although often very faint, odors. As new substances are discovered or new mixtures of substances invented, the number of odors is increased. Unfortunately it has thus far been impossible to arrange this multitude of odors in a system according to a simple plan. Various groups of related odors have been formed by investigators (for example, the odor of flowers, fruit, musk, onion, decaying matter). But it is difficult to include all possible odors in such groups; and the relation between these groups is still unknown. One reason for this difficulty in understanding theoretically the sense of smell is the obvious fact that this sense has degenerated in man. The organ of smell, a spot in the upper part of each nasal cavity, is of small extent in man compared with that of animals. Even more superior are the animals to man with respect to the development of the olfactory nerve center. The degeneration is the result of a lack of use. Man, walking upright, has but rarely an opportunity of approaching objects with his nostrils closely enough to be able to smell them. The animal, searching for food on the ground, smells unceasingly.

The opposite is true for color sensations. They, too, are numerous, perhaps a million. But it is easy to group them into a system which permits us to understand their interrelations. The relations between the various colors are so simple that they can be symbolically represented by a geometrical figure, a double pyramid with a four-cornered base, like the one in figure 14. The vertical axis represents the visual sensations which are colorless, arrayed so that the brightest white is at one end, the darkest black at the other, the various grays between. The base of the pyramids, which is not perpendicular to the axis, but slanting, represents the series of colors of

the spectrum plus the non-spectral purples, between red and violet, all arranged in an orderly manner around the axis. The nearer we approach the axis, the less saturated, that is, the more whitish, or grayish, or blackish are the colors represented. The most saturated colors are therefore represented by the peripheral line of the base. The base is slanted because the most saturated colors are not all of the same brightness (meaning by this term exclusively lightness as opposed to darkness). The saturated yellow is much brighter than the saturated blue and must therefore be located here, symbolically, nearer the point of white than of black, while blue must be located nearer the point of black than of white. The figure shows clearly that it is impossible to deviate from the peculiar brightness of each saturated color without diminishing the saturation, for we cannot move up or down from any point of the peripheral line of the base and yet remain within the double pyramid, without approaching the axis. But if our starting point is a color of less than the maximum of saturation, we may change the brightness within certain limits without changing the saturation, for we may then, to a certain extent, move up and down parallel to the axis.

Some have represented the color system by a double cone, using as common base a circle. But a four-cornered base represents an additional fact of experience which is lost sight of in the circular plane. The four colors red, green, blue, and yellow possess this property: that any one of them is entirely dissimilar in color tone to any of the other three, while any given color other than these must resemble just two of these. No other four or any other number of colors can be found which fulfill exactly these conditions. In order to represent this fact symbolically, we ought to give the colors red, green, blue, and yellow distinguished places in the periphery of the basal plane, and this can be done most easily by choosing as a base a four-cornered plane.

By the aid of this color system it is easy to understand an abnormality of our color sense which occurs rather frequently, so-called color blindness. It is found almost exclusively among men, three per cent of them being affected, whereas it is very rare among women, although it is inherited through woman. Instead of three dimensions, two are sufficient for the representation of the color sensations of such individuals: a plane which is placed through the points white, black, blue, and yellow. The color sensations represented by those points of the pyramid which lie outside the plane just mentioned appear to the color-blind person yellowish if they are located on either side of the yellow triangle, so to speak; they appear bluish if they are located on either side of the blue triangle, and colorless if located exactly on either side of the axis. There are, however, a large number of minor differences not included or even expressed incorrectly in the above brief statement; the color-blind person, for instance, is more likely to see things yellowish than bluish. Since color-blind people may sometimes confuse such conspicuously different colors as red and green, they are often called red-green-blind. That they also confuse greenish blue with violet seems less remarkable to the normal person than the former fact. In testing a color-blind person one must not expect to find that he will confuse any red with any green. Brightness and saturation play here very important parts, and all kinds of individual differences have been observed. Nevertheless color-blind people fail to distinguish red and green much more frequently than people having a normal color sense, and should therefore be strictly excluded from any service in which the distinction of red and green is of importance, as in railway and marine signaling. For the normal person red and green are the ideal colors of signals, because yellow is not always sufficiently different from white, and a saturated blue is too dark.

It is interesting to observe that colors are never simple or
complex in the sense in which a musical tone is simple and a chord
is a multitude of tones, or lemonade is a mixture of sour and
sweet. Any color sensation which is uniform over its area is as
simple as any other. The colors which, in our color pyramid, are
located between two of the four fundamental colors red, green,
blue, and yellow are “mixtures” only in the sense that the mixed
color _resembles_ two of those four, not that we are conscious of
two separate sensations in one act of perception.

Nevertheless we often have to speak of mixed colors and of
principal colors entering into mixtures. These phrases have many
different meanings. Most colors which we see in actual life are
mixtures in a physical sense, mixtures of ether waves, although our
sense organ does not inform us as to whether they are mixtures or
homogeneous light. White or gray or purple can never be anything
but mixtures in this physical sense. In actual life the only color
which is often simple, homogeneous light, is dark red, for
physical causes which do not concern us here. But this physical
complexity is irrelevant for the psychological question as to the
simplicity or complexity of color sensation.

Even more confusion has been carried into the psychology of color
by the fact that in dyeing and painting chemical substances are
sometimes applied as they occur in nature or come from the factory,
sometimes they are first mixed together and then applied. The
painter cannot afford to have an infinite number of color pigments
on the palette. He selects therefore a small number, at least
white, red, yellow, and blue. This is for many ends sufficient, and
he may therefore call these pigments his principal colors, and
wonder why one should call green a “fundamental” color, since he
can produce it by mixing blue and yellow. It is indeed no difficult
task to find people who, like Goethe, are convinced that they are
able to perceive in the green the yellow and the blue which the
painter used in order to give us the impression of green.

Still another difference occurs in the use of the terms simple and
mixed colors in physiology, with reference to the processes going
on in the eye and the part of the nervous system connected with the
eye. It is plain, therefore, that whenever we speak of colors we
must state in what sense we do this.

Auditory sensations are usually divided into two classes: tones and noises. They do not often appear separately. A violin tone, for example, is accompanied by some noise, and in the howling of the wind tones may be discerned. Both may be perceived in many different intensities, and both may be said to be low or high. Many thousands of tones may be distinguished from the lowest to the highest audible. Within one octave, in the middle region, more than a thousand can be distinguished. The fact that in music we use only twelve tones within each octave arises from special reasons: first, the difficulty of handling an instrument of too many tones; and especially the fact that with a particular tone only a limited number of others can be melodically or harmonically combined with a pleasing result.

Just as the colors, so the tones are a continuum, that is, one can pass from the lowest to the highest tones without at any moment making a noticeable change. We refer to this continuum by the word pitch. But tones also possess what is called quality; that is, they are either mellow or shrill. This mellowness is to some extent dependent on the pitch of each tone, for low tones are never very shrill and high tones never very mellow. But to some extent a tone may be made more or less shrill and yet retain exactly the same musical value, the same pitch. This is brought about by the overtones, of which a larger or smaller number is nearly always added to musical tones. Without being perceived as separate pitches the overtones influence our consciousness of the mellowness of a tone--the fewer overtones, the mellower; the more overtones, the shriller the tone. Each musical instrument has its characteristic quality of tone, and in some instruments, especially in organ pipes, the quality is skillfully controlled by the builder, who “voices” each pipe so that it produces the required number of overtones of the right intensities.

It was said above that the overtones, as a rule, are not perceived as separate pitches added to the pitch of the fundamental tone. It is not impossible, however, to perceive them thus. Those who experience difficulty in perceiving the overtones as separate pitches may use at first special instruments, resonators, which are held against the ear and greatly increase each the intensity of a special overtone. After some practice one becomes aware of the pitch of an overtone without the aid of a resonator.

Noises may be classified into momentary and lasting noises. Examples of the former are a click and the report of a gun; examples of the latter, the roaring of the sea or the hissing of a cat. Many noises, as thunder, rattle, clatter, and the noises of frying and boiling, are mixtures of momentary and lasting noises.

From all we have said it follows that the function of hearing is an analyzing function, enabling the mind to separate that which has lost its separate existence when it acts upon the tympanum. Two or three tones sounding together are usually perceived as two or three tones. In hearing music we can simultaneously listen to several voices. When two people talk together we may to some extent follow them separately. This is obviously an ability of great importance in animal life, since different objects, characterized by different tones or noises, rarely separate themselves spatially as the colors of different objects do, but act upon the sense organ as a single compound.

There are, however, certain exceptions to the analyzing power of the ear. If two tones differ but little in pitch, they are not perceived as two, but a mean tone is heard beating as frequently in a second as the difference of the vibration rates indicates. The ear thus creates something new, but of course something definitely depending on the external processes. If two tones not quite so close in pitch are sounded, one or even several new tones are created, combination tones or difference tones, the pitch of the new tone being determined by the difference of the rates of vibration. These difference tones do not seem to serve any purpose in animal life. They are merely secondary phenomena, of little practical consequence, but of much interest to the student of the function of the organ of hearing.

We have seen that the number of classes of sensations is fairly large; but to state this number exactly is impossible. According as we count the muscles, the joints, the lungs, the digestive organs as several sense organs or as a single group, the number of classes of sensations is larger or smaller. However, it matters little whether we count them or not. We know that provision is made for everything needed. Information about the most distant things is obtained through the eye; information about the things in contact with the body or the body itself comes through the cutaneous and organic sense organs. Most varied is the information about things at a moderate distance, obtained through eyes, ears, and nose combined.

Many of the higher animals surpass man in one or the other respect through their sensory equipment. Many of the birds (for example, the carrier pigeons) have a sharper eye; dogs and other animals, a keener sense of smell. The sense of hearing in man seems to be equal to that of the higher animals, and the cutaneous sense perhaps superior. In one respect man is better equipped than his mode of living justifies, that is, in possessing the semicircular canals and the otolith organs, for which he has scarcely any use. In another respect he, as well as the animals, is very poorly equipped, that is, for the direct perception of the electromagnetic-optic phenomena of physics, only a small range of which can be perceived as a particular kind of sensations, namely, as colors.

3. _Temporal and Spatial Attributes_

The study of the simple in mental life, as previously mentioned, is always a study of abstractions. The actual experience even of the briefest moment never consists of a single sensation. And actual sensations are always characterized by more than the properties which we have thus far discussed. Colors always occupy space of a certain size and shape; tones come from a certain direction; both colors and tones are either continuous or intermittent, they are perceived simultaneously or in succession. We naturally inquire into the laws of these spatial and temporal relations. Unfortunately psychologists have not yet agreed on a definite answer to the question concerning space and time. The question is beset with difficulties, partly real, partly imaginary.

Is it possible to perceive temporal relations as sensory qualities as we perceive colors, tones, tastes, and smells as sensory qualities? We certainly lack a sense organ of time. But aside from this, it seems impossible to perceive duration at its beginning, when the end is not yet known; impossible to perceive it at the end, when its beginning no longer exists and can only be recalled in memory. It seems equally impossible to get direct knowledge of a spatial relation. Imagine one particular point _a_ of the skin or the retina of the eye. If this is stimulated, our mind receives a definite impression of touch or color, but no indication of or reference to any other point, since no other point is stimulated. Let the same be true for the point _b_. How, then, if _a_ and _b_ are stimulated simultaneously, can the mind receive an impression of distance between the two points, since there is no such consciousness in the perception of either of them? If the mere fact of an objective distance between the stimulated neurons were a sufficient explanation, then tones too should be localized differently.

Those who took these objections seriously tried to think of some means by which the objective, but not directly impressive, spatial relations could become known to the mind. It was suggested that the almost unceasing movements of the eyes and fingers, the chief organs of space perception, might have significance in this connection; that perhaps the kinesthetic sensations of eye and finger movement, being added to the visual or tactual impressions, made up the consciousness of spatial relationship.

All attempts, however, to prove the correctness of this and similar theories by applying them to the details of special experience, have failed. While there is no doubt that movements of our eyes and fingers are of great importance for the development and extension of the spatial consciousness in the individual as well as in the race, they are not the source from which springs the individual’s ability to perceive spatial relationship. The fundamental part of our ability of _spatial_ perception is inborn, just as our ability to perceive light or blueness or cold is inborn. From this inborn capacity for spatial perception the individual’s delicate and elaborate sense of space is derived.

The most convincing proof that there is an innate capacity for spatial perception, is the spatial consciousness of persons born blind, to whom an operation has given eyesight. The crystalline lenses of these persons have been as little transparent as ground glass, so that they have been unable to recognize any outlines of things. Nevertheless, they make spatial distinctions immediately after the operation for removal of the lens. Of course they cannot, without further experience, tell that a round thing is the ball with which they have been familiar through the sense of touch, or a long and narrow thing a walking stick. But they immediately perceive the round thing as something different from the long and narrow thing, without any tendency to confuse them. Spatial extent is therefore an attribute of visual and tactual sensation as brightness or darkness is an attribute of visual sensation, and mellowness or shrillness an attribute of tone; with this difference only, that spatial extent is not restricted to one sense, but is common to visual and cutaneous sensations. That this is founded on some kind of similarity of these senses cannot be doubted. But this similarity is to be looked for in structural peculiarities of the nerve centers, not in accessory mental states serving as special agents of spatial consciousness.

Very much the same is the case with time. Let us admit that the temporal consciousness of our ordinary life is largely mediated by accessory sensations and images. Minutes, hours, days, weeks, are not experienced directly as properties of sense perception, but are extensions of simpler experiences. But such extensions would be impossible if duration and succession were not, somewhere in our mental life, direct experiences. They are direct experiences in some very brief temporal perceptions occupying, say, only a fraction of a second. The flash of a lighthouse signal, the quick succession of sounds when a person knocks at a door, are perceived as having temporal attributes without any mediation by conscious states acting as agents. The _temporal_ attributes are elements of perception no less direct than the intensity of the light or of the sound. The same holds for all other sensations. Time is an attribute common to all. But here, as in space, we cannot tell exactly in what respect all senses are similar so far as the nervous processes are concerned. It seems that these processes or their after effects continue a certain time after the stimulation has ceased.

Another attribute common to all sense impressions is the belonging-together of sensations, the _unity in variety_, so to speak. The most striking example is the relationship of tones in harmony and melody. Tones of certain comparatively simple ratios of vibration belong together in a higher degree than others. We cannot explain this by reference to conscious agents mediating the effect. It is a fundamental attribute of each tonal combination, the conscious effect of our inherited nature. It is a property of sense, not of thought.

In other cases our consciousness of relationship is indirect, mediated by other conscious agents; for instance, when I group together voluntarily four or five adjoining holes of a sieve and perceive them as a unit. This grouping together would be impossible if the mind did not possess the native ability to perceive a number of sensational elements as a unit without altogether losing the consciousness of variety. It is a mere consequence of our inborn nature when we perceive as such units, for example, an animal romping among unchanging surroundings, a picket fence divided into groups by the fence posts, a familiar compound perfume, a dish made up of several familiar food substances. The same holds for successive elements. We could never perceive tones or noises in various rhythm forms if our mind did not possess the native ability to perceive a number of successive elements of sensation under certain conditions as a sensory unit.

Our numerical concepts are obviously only abstract symbols for units containing each a certain variety of elements.

4. _Sensation and Stimulus_

It is most interesting to observe the astonishing _absolute sensitiveness_ of some of our senses, that is, their ability to respond to exceedingly small stimuli. It has been a difficult task to design physical instruments as sensitive to sound as the ear. It has not been possible, thus far, to surpass the ear. The sensitiveness of the eye to the faintest light is estimated to be a hundred times that of the most sensitive photographic plates. Remember what a long exposure is necessary to photograph things in a rather dark room; but the eye takes a snap shot, so to speak, of a star of the fifth magnitude, or of a landscape in diffused moonlight. Man’s organ of smell is far inferior to that of many animals. Nevertheless a trace of tobacco smoke or musk in the air whose presence no chemist could detect is easily perceived through the nose. A gram is about one twenty-eighth of an ounce; a milligram is one thousandth of a gram. One millionth of a milligram of an odorous substance is sufficient to affect the organ of smell. Taste also is sensitive, particularly when supported, as in tasting wine or tea, by smell. The cutaneous and kinesthetic senses, on the other hand, are not very sensitive. A weak pressure, a small weight, a slight tremor of our limbs, a spatial extent, can be detected much more readily by delicate instruments than by our fingers or our kinesthetic organs.

Very important is the range of perceptibility. Our measuring laboratory instruments are, as a rule, adapted only to a small range. To weigh a heavy thing, like a stack of hay, we have to use a balance differing from that used by the prescription druggist. The watchmaker’s tools are much like those of the machinist, but neither could use the other’s tools. Nature cannot well provide separate sets of tools for delicate and gross work. With our hand we estimate the weight of ounces, pounds, and hundredweights. The same ear which perceives a falling leaf can be exposed to the thunder of cannon without ceasing to respond in its normal way. The eye which perceives a small fraction of the light of a firefly, can look at the sun somewhat covered by mist, radiating light many million times as intense. No laboratory instrument has an equal range of applicability.

This wide range of usefulness is made possible partly by purely mechanical provisions, partly by a special law of nervous activity usually called Weber’s law. The iris of the eye with pupil in the center is a readily changeable diaphragm. The stronger the external light, the smaller the pupil, and the reverse; so that the eye is capable of functioning at a stronger and also at a fainter illumination than it could function if the width of the pupil were of a medium, unchangeable diameter. The nose can smell faint odors better if larger quantities of the odorous substances are by sniffing brought into contact with the organ. Too strong odors are kept away by blowing out the air.

More important, however, than such mechanical devices is the effect of Weber’s law. If a stimulus is increased, the nervous excitation is also increased,--not absolutely, but only relatively to the stimulus before the increase. Suppose an oil lamp of ten candle power needs an addition of a two candle power light to make me observe that the illumination has changed. Nevertheless I shall not be able to observe a change of illumination if to an incandescent gas light of sixty candles two candles are added. The addition must be in proportion to the stimulus. Since sixty is six times ten and twelve is six times two, twelve candles must be added to make me observe the difference in illumination. To an arc light of two thousand candles four hundred have to be added to obtain the same result. If a postal clerk is able to recognize that a letter which he weighs on his hand and which is one twentieth heavier than an ounce, requires more than the one postage stamp attached to it, he will probably be found capable of observing in the same manner that a package of newspapers prepaid for one pound does not have the correct number of stamps if it is actually one twentieth heavier than a pound.

Another way of speaking of the law is this: If we imagine a definite stimulus successively increased by such amounts that the change of the sensation is each time just as noticeable as it was the last time, the added amounts of the stimulus are a _geometrical progression_. Let us express the fact that the change of the sensation can always be noticed _with the same ease_, by saying that the additions to the sensation are an arithmetical progression. We can then state Weber’s law in these simple words: If the sensation is to increase in arithmetical progression, the stimulus must increase in geometrical progression. This statement is mathematically identical with the most widely adopted statement of the law, namely, that _the sensation is proportional to the logarithm of the stimulus_.

The practical result of the law in our mental life is this: The mind is informed of a further increase in the intensity of the stimulus (however great this intensity may have become before this last increase) without having to respond to the absolute intensity of the stimulus with a correspondingly enormous activity of the animal organism. Thus the mind is enabled, figuratively speaking, to weigh a stack of hay or a druggist’s herb on the same balance, to apply the same tool to a watch or to a railroad locomotive, or at least to perform its work with a much smaller number of tools than would otherwise be required. In the eye, for instance, we have, as we see below, only two different kinds of receiving instruments for faint and for strong light.

It must be mentioned, however, that Weber’s law does not hold good
over an unlimited range of intensities of stimulation. If the sun
were twice as bright, it would not appear brighter to the eye. For
such extreme intensities the law is no longer valid. Neither is it
valid for exceedingly low intensities; it makes no difference to
the eye whether the wall of a dark room is illuminated from a
distance of three or four yards by the glow of one cigarette or a
dozen. The logarithmic equation applies only to a certain--quite
large--range of medium intensities. For this range our
sensitiveness to change is not only constant, but also greatest.
Changes in illumination within this range can be perceived as soon
as the stimulus increases or decreases by about one hundred and
fiftieth.

Weber’s law has still another practical significance. A thing which
we recognize by the aid of the differences in illumination of its
parts (as, for example, a stone relief) or by its differences in
loudness (as a rhythm beaten on a drum) always retains, not the
same absolute differences, but the same quotients or proportions of
the different light or tone values, however our distance from the
thing varies. Weber’s law, then, enables us to perceive the
identity of the thing although the absolute light or tone values
have undergone change. If our nervous activities were not regulated
in accordance with Weber’s law, the relief and the rhythm might
become unrecognizable at a greater distance, and the relief also at
dusk.

A further important relation between our mental life and the external world consists in our much greater sensitiveness to the moving and changing than to the stable and permanent. A pencil point moved over the skin under slight pressure gives us a perception of the length and direction of the line traversed more accurate than the impression received from the edge of a screwdriver pressed on the skin. On the peripheral parts of the retina the sizes and distances of things are not easily perceived; but no difficulty is experienced in noticing a waving handkerchief or a starting animal. Only the small central part of the retina is adapted to the perception of the motionless.

The same statement holds for qualitative changes. The eye is not only more sensitive to that which qualitatively changes than to that which remains unchanged; it even loses its ability to perceive things if for a considerable time no qualitative changes occur. We have seen that our eye can take snap shots under conditions which would make this impossible for the photographic camera. But for time exposures, like those used in photographing faint stars, continued for hours, our eye is not suited. The eye, in such a case, would soon cease to distinguish anything. The eye completely fixed upon one set of objects soon sees their lighter parts darker, their darker parts lighter, their colored parts less colored--more grayish--that is, it sees everything gray on gray. This is technically called adaptation of the eye. Moving the eye suddenly, we become aware of this adaptation in peculiar after-images.

Similar adaptations occur in other sense organs. Constant pressure on the skin, unchanging temperature of not extreme degree, permanent odors, cease to be perceived. But what is new, what differs from the condition which was in existence just before, is perceived at once; and because of the sense organ’s adaptation for something else, as a rule it is seen with particular intensity. This is obviously the most favorable equipment for a struggle for life. Nothing is more dangerous in battle than surprise.

Our present knowledge of the mechanical, chemical, and physiological laws governing the peculiar dependence of the different kinds of sensations on special properties of the sense organs--that which is customarily called a theory of vision, a theory of audition, and so on, is rather unsatisfactory. Some thirty years ago much seemed to be perfectly explained which has since become mysterious again. This much has been learned, that the laws in question are far more complex than they were believed to be.

Only one statement about eyesight can here be made without fear of contradiction, that is, that the eye is a double instrument, one part of the organ serving in daylight, the other at dusk and in twilight. But this explains only a part of the total function of the eye. The retina of the eye consists of a great number of elements called rods and cones, forming a kind of mosaic. Twilight vision is served by the rods, which contain a sensitive substance called the visual purple. Most of the rods are in the peripheral parts of the retina, becoming less numerous toward the center. In the central area there are no rods at all. The only service of the rods is the mediation of a weak bluish-white sensation of various intensities, as in a moonlit landscape. Ordinary day vision is served by the cones, which are the only elements present in the center and become rare towards the periphery. All the variety of our color perception depends on the cones. In very faint illumination the colors of things cannot be perceived, although the things may still be distinguished from other objects. The rods alone are functioning then; the cones have “struck work.” Neither can the shape of things be perceived in dim light with normal definiteness, because the area of most distinct vision, the central area, contains only cones; reading, for instance, is impossible at twilight. The astronomer, in order to observe a very faint star, must intentionally look at a point beside the star, because of the lack of rods in the central area.

While the human eye normally possesses both rods and cones, certain species of animals have only one or the other kind of visual elements. Chickens and snakes possess only cones. This is the reason why chickens go to roost so promptly when the sun sets. Night animals, on the other hand, have mostly rods and few cones. This explains why bats come out only after sunset. In very rare cases human beings seem to possess only the rods, in cases of total color-blindness. The whole world appears colorless to them, only in shades of gray. They dislike greatly to be in brilliantly lighted places. They lack the keenness of normal eyesight because of the deficient function of the central area of the retina, which is normally best equipped.

A mechanical theory of hearing was worked out by Helmholtz nearly fifty years ago. This theory was at first generally accepted, but has in recent years lost much of its plausibility. The inner ear is a tube coiled up in the shape of a snailshell in order to find a better place in the lower part of the skull. Its coiling, of course, has little if any mechanical significance. The tube is divided into two parallel tubes by a kind of ribbon, the organ of Corti, containing the endings of the auditory neurons and also a comparatively tough membrane. Helmholtz made the hypothesis that the cross fibers of this membrane were under constant tension like the strings of a piano. The comparison with a piano was also suggested by the fact that the membrane in question tapers like the sounding board of a grand piano. As the piano resounds any tone or vowel, so this system of strings would resound any complex sound; that is, each of the tones contained in the complex would be responded to by those fibers whose tension, length, and weight determine a corresponding frequency of vibration. The analyzing power of the ear is well explained by this hypothesis, but there are considerable difficulties left. For instance, the fibers of the membrane, even the longest, are rather short for the low tones to which they are assumed to be tuned. And for the assumption of a constant tension of these fibers there is no analogon in the whole realm of biology, since living tissues always, sooner or later, adapt themselves and thus lose their tension.

Another theory avoids these difficulties by merely assuming that the ribbon-like partition of the tube, when pushed by the fluid, moves out of its normal position only to a slight extent and then resists, and that therefore the displacement of the partition must proceed along the tube. If successive waves of greater and lesser amplitude, as we find them in every compound sound, act upon the tympanum and indirectly upon the fluid in the tube, the displacement of the partition must proceed along the tube now farther, now less far, now again to another distance, and so on. Accordingly, one section of the partition is displaced more frequently, another section less frequently, others with still different frequencies in the same unit of time. This theory then makes the hypothesis that the frequency with which each section of the partition is jerked back and forth determines the pitch of a tone heard, and explains thus the analyzing power of the ear. What is chiefly needed in order to decide in favor of either of these or any other theory is a large increase in our knowledge through anatomical, physiological, and psychological investigation.

QUESTIONS

46. What are the newly discovered kinds of sensations?

47. How were they discovered?

48. What are the cutaneous senses?

49. What is the objection to speaking of the cutaneous sense as
one?

50. What is pain?

51. Of what importance are the labyrinth senses (other than
hearing) to man and various animals?

52. What is meant by organic sensations?

53. What are the four tastes?

54. How does the sense of smell in man compare with that of
animals?

55. Why is the color pyramid superior to the color cone?

56. What are the chief symptoms of defective color vision?

57. What is not meant, and what is meant, by color mixtures?

58. Why does music use only twelve tones?

59. What is meant by the qualities of the tones of various
instruments?

60. Are there any limits to the analyzing power of the ear?

61. What is the exact number of classes of sensations?

62. How does the sensory equipment of man compare with that of the
animals?

63. What do we learn from experiments on blind-born persons who
have been operated on?

64. In what experiences is time an attribute of sense perception?

65. Is tone relationship a property of sense or of thought?

66. Can you illustrate the absolute sensitivity of our sense
organs?

67. How does the range of applicability of our sense organs compare
with that of tools and instruments?

68. Can you illustrate Weber’s law?

69. What are the practical advantages obtained through Weber’s law?

70. Illustrate sensitiveness to change and movement.

71. How is the chief difference in the behavior of chickens and
bats to be explained?

§ 5. IMAGINATION

Mind is influenced not only by that which is present, but also by the past and--one may say--the future, and by that which exists at another place. Consciousness of this kind is called imagery. I imagine a lion and recognize that he looks different from a horse. I recall the room in a hotel where I have recently spent a night and see that it differs from my study.