a very intense sound is one which en grosses our attention, which supplants all the others. But take away the shock, the well- marked vibration, which you sometimes feel in your head or even throughout your body: take away the clash which takes place between sounds heard simultaneously: what will be left except an indefinable quality of the sound which is heard? But this quality is immediately inter preted as quantity because you have obtained it yourself a thousand times, e.g. by striking some object and thus expending a definite quan tity of effort. You know, too, how far you would 44 TIME AND FREE WILL CHAP.
have to raise your voice to produce a similai sound, and the idea of this effort immediately comes into your mind when you transform the intensity of the sound into a magnitude. Wundt * has drawn attention to the quite special con nexions of vocal and auditory nervous filaments which are met with in the human brain. And has it not been said that to hear is to speak to one self? Some neuropaths cannot be present at a conversation without moving their lips; this is only an exaggeration of what takes place in the case of every one of us. How will the ex pressive or rather suggestive power of music be explained, if not by admitting that we repeat to ourselves the sounds heard, so as to carry ourselves back into the psychic state out of which they emerged, an original state, which nothing will express, but which something may suggest, viz., the very motion and attitude which the sound imparts to our body?
Thus, when we speak of the intensity of a sound of medium force as a magnitude, we allude principally to the greater or less effort "* which we should have ourselves to *! expend in order to summon, by our own effort, the same auditory sensation. Now, besides the intensity, we distinguish another characteristic property of the sound, its pitch.
1 Grundziige der Physiologischen Psychologic, 2nd ed., (1880), Vol. ii, p. 437.
CHAP, i SENSATION OF SOUND 45 Are the differences in pitch, such as our ear perceives, quantitative differences? I grant that a sharper sound calls up the picture of a higher position in space. But does it follow from this that the notes of the scale, as auditory sensations, differ otherwise than in quality? Forget what you have learnt from physics, exa mine carefully your idea of a higher or lower note, and see whether you do not think simply of the greater or less effort which the tensor muscle of your vocal chords has to make in order to produce the note? As the effort by which your voice passes from one note to another is discon tinuous, you picture to yourself these successive notes as points in space, to be reached by a series of sudden jumps, in each of which you cross an empty separating interval: this is why you establish intervals between the notes of the scale. Now, why is the line along which we dispose them vertical rather than horizontal, and why do we say that the sound ascends in some cases and descends in others? It must be remembered that the high notes seem to us to produce some sort of resonance in the head and the deep notes in the thorax: this perception, whether real or illusory, has undoubtedly had some effect in making us reckon the intervals vertically. But we must also notice that the greater the tension of the vocal chords in the chest voice, the greater is the surface of the body affected, if the singer is inexperienced; this is just the reason why the 4$ TIME AND FREE WILL CHAP, i effort is felt by him as more intense. And as he breathes out the air upwards, he will attribute the same direction to the sound produced by the current of air; hence the sympathy of a larger part of the body with the vocal muscles will be represented by a movement upwards. We shall thus say that the note is higher because the body makes an effort as though to reach an object which is more elevated in space. In this way it became customary to assign a certain height to each note of the scale, and as soon as the physicist was able to define it by the number of vibrations in a given time to which it corresponds, we no longer hesi tated to declare that our ear perceived differ ences of quantity directly. But the sound would remain a pure quality if we did not bring in the muscular effort which produces it or the vibra tions which explain it.
The experiments of Blix, Goldscheider and Donaldson l have shown that the points on the The sensations surface of the body which feel cold are oli(LheatThae°e not the same as those which feel heat. Lfle^tive^and Physiology is thus disposed to set up a £e?<Sn2 distinction of nature, and not merely of degree, between the sensations of heat and cold. But psychological observation goes further, for close attention can easily discover specific differences between the different sensa tions of heat, as also between the sensations of 1 " On the Temperature Sense " Mind, 1885.
CHAP, i SENSATION OF HEAT 47 cold. A more intense heat is really another kind of heat. We call it more intense because we have experienced this same change a thousand times when we approached nearer and nearer a source of heat, or when a growing surface of our body was affected by it. Besides, the sensations of heat and cold very quickly become affective and incite us to more or less marked reactions by which we measure their external cause: hence, we are inclined to set up similar quantitative differences among the sensations which correspond to lower intensities of the cause. But I shall not insist any further; every one must question himself carefully on this point, after making a clean sweep of everything which his past experience has taught him about the cause of his sensations and coming face to face with the sensations themselves. The result of this examination is likely to be as follows: it will be perceived that the magnitude of a repre sentative sensation depends on the cause having been put into the effect, while the intensity of the affective element depends on the more or less important reactions which prolong the external stimulations and find their way into the sensation itself.
The same thing will be experienced in the case of pressure and even weight. When you say The sensa- that a pressure on your hand becomes tions of pressure and stronger and stronger, see whether you weight mea- ' sured by ex- do not mean that there first was a tent of organism affected, contact, then a pressure, afterwards a 4g TIME AND FREE WILL pain, and that this pain itself, after having gone through a series of qualitative changes, has spread further and further over the surrounding region. Look again and see whether you do not bring in the more and more intense, i.e. more and more extended, effort of resistance which you oppose to the external pressure. When the psychophysi- cist lifts a heavier weight, he experiences, he says, an increase of sensation. Examine whether this increase of sensation ought not rather to be called a sensation of increase. The whole question is centred in this, for in the first case the sensation would be a quantity like its external cause, whilst in the second it would be a quality which had become representative of the magnitude of its cause. The distinction between the heavy and the light may seem to be as old-fashioned and as childish as that between the hot and the cold. But the very childishness of this distinction makes it a psychological reality. And not only do the heavy and the light impress our consciousness as generically different, but the various degrees of lightness and heaviness are so many species of these two genera. It must be added that the difference of quality is here translated spontane ously into a difference of quantity, because of the more or less extended effort which our body makes in order to lift a given weight. Of this you will soon become aware if you are asked to lift a basket which, you are told, is full of scrap-iron, whilst in fact there is nothing in it. You will think you CHAP, i SENSATION OF WEIGHT 49 are losing your balance when you catch hold of it, as though distant muscles had interested them selves beforehand in the operation and experi enced a sudden disappointment. It is chiefly by the number and nature of these sympathetic efforts, which take place at different points of the organism, that you measure the sensation of weight at a given point; and this sensation would be nothing more than a quality if you did not thus introduce into it the idea of a magnitude. What strengthens the illusion on this point is that we have become accustomed to believe in the immedi ate perception of a homogeneous movement in a homogeneous space. When I lift a light weight with my arm, all the rest of my body remaining motionless, I experience a series of muscular sensa tions each of which has its " local sign," its pecu liar shade: it is this series which my conscious ness interprets as a continuous movement in space. If I afterwards lift a heavier weight to the same height with the same speed, I pass through a new series of muscular sensations, each of which differs from the corresponding term of the preceding series. Of this I could easily convince myself by examining them closely. But as I interpret this new series also as a continuous movement, and as this movement has the same direction, the same duration and the same velocity as the pre ceding, my consciousness feels itself bound to localize the difference between the second series of sensations and the first elsewhere than in the 5O TIME AND FREE WILL CHAM movement itself. It thus materializes this differ ence at the extremity of the arm which moves; it persuades itself that the sensation of movement has been identical in both cases, while the sensa tion of weight differed in magnitude. But move ment and weight are but distinctions of the reflec tive consciousness: what is present to conscious ness immediately is the sensation of, so to speak, a heavy movement, and this sensation itself can be resolved by analysis into a series of muscular sensations, each of which represents by its shade its place of origin and by its colour the magnitude of the weight lifted.
Shall we call the intensity of light a quantity, or shall we treat it as a quality? It has not perhaps been sufficiently noticed what a large The sensation J of light. Qua- number of different factors co-operate in litative.
chants of daily life in giving us information about colour inter-, ° preted as the nature of the luminous source. We quantitative chaws in know from long experience that, when we intensity ol, j • rr luminous have a difficulty in distinguishing the outlines and details of objects, the light is at a distance or on the point of going out. Experience has taught us that the affective sensa tion or nascent dazzling that we experience in cer tain cases must be attributed to a higher intensity of the cause. Any increase or diminution in the number of luminous sources alters the way in which the sharp lines of bodies stand out and also the shadows which they project. Still more important are the changes of hue which coloured CHAP, i SENSATION OF LIGHT 51 surfaces, and even the pure colours of the spec trum, undergo under the influence of a brighter or dimmer light. As the luminous source is brought nearer, violet takes a bluish tinge, green tends to become a whitish yellow, and red a bril liant yellow. Inversely, when the light is moved away, ultramarine passes into violet and yellow into green; finally, red, green and violet tend to be come a whitish yellow. Physicists have remarked these changes of hue for some time;J but what is still more remarkable is that the majority of men do not perceive them, unless they pay attention to them or are warned of them. Having made up our mind, once for all, to interpret changes of quality as changes of quantity, we begin by assert ing that every object has its own peculiar colour, definite and invariable. And when the hue of objects tends to become yellow or blue, instead of saying that we see their colour change under the influence of an increase or diminution of light, we assert that the colour remains the same but that our sensation of luminous intensity increases or diminishes. We thus substitute once more, for the qualitative impression received by our con sciousness, the quantitative interpretation given by our understanding. Helmholtz has described a case of interpretation of the same kind, but still more complicated: " If we form white with two colours of the spectrum, and if we increase or 1 Rood, Modern Chromatics, (1879), pp. 181-187.
52 TIME AND FREE WILL CHAP, i diminish the intensities of the two coloured lights in the same ratio, so that the proportions of the combination remain the same, the resultant colour remains the same although the relative intensity of the sensations undergoes a marked change...This depends on the fact that the light of the sun, which we consider as the normal white light during the day, itself undergoes simi lar modifications of shade when the luminous inten sity varies." l But yet, if we often judge of variations in the luminous source by the relative changes of hue of the objects which surround us, this is no Does eiperi-,,, ment prove longer the case in simple instances where that we can...,., /.
measure di- a single object, e.g. a white surface, rectly our sen-..,,,.,„,, •ations oi passes successively through different de grees of luminosity. We are bound to insist particularly on this last point. For the physicist speaks of degrees of luminous intensity as of real quantities: and, in fact, he measures them by the photometer. The psychophysicist goes still further: he maintains that our eye itself estimates the intensities of light. Experi ments have been attempted, at first by Delbceuf,2 and afterwards by Lehmann and Neiglickj3 with 1 Handbuch der Physiologischen Optik, ist ed. (1867), pp. 318-319- 2 Elements de psychophysique. Paris, 1883.
8 See the account given of these experiments in the Revue philosophiqiie, 1887, Vol. i, p. 71, and Vol. ii, p. 180.
CHAP, i SENSATION OF LIGHT 53 the view of constructing a psychophysical formula from the direct measurement of our luminous sensations. Of these experiments we shall not dispute the result, nor shall we deny the value of photometric processes; but we must see how we have to interpret them.
Look closely at a sheet of paper lighted e.g. by four candles, and put out in succession one, two, Photometric three of them. You say that the surface ^ep5c(5ve remains white and that its brightness ?nflderafferh-ade8 diminishes. But you are aware that ?rttdstheSetas one candle has just been put out; or, if ~ y°u do n°t know it, you have often light, observed a similar change in the appear ance of a white surface when the illumination was diminished. Put aside what you remember of your past experiences and what you are accus tomed to say of the present ones; you will find that what you really perceive is not a diminished illumination of the white surface, it is a layer of shadow passing over this surface at the moment the candle is extinguished. This shadow is a reality to your consciousness, like the light itself. If you call the first surface in all its brilliancy white, you will have to give another name to what you now see, for it is a different thing: it is, if we may say so, a new shade of white. We have grown accustomed, through the combined influence of our past experience and of physical theories, to regard black as the absence, or at least as the minimum, of luminous sensation, and the succes- $4 TIME AND FREE WILL CHAP, i sive shades of grey as decreasing intensities of white light. But, in point of fact, black has just as much reality for our consciousness as white, and the decreasing intensities of white light illuminat ing a given surface would appear to an unpre judiced consciousness as so many different shades, not unlike the various colours of the spectrum. This is the reason why the change in the sensation is not continuous, as it is in the external cause, and why the light can increase or decrease for a certain period without producing any apparent change in the illumination of our white surface: the illumination will not appear to change until the increase or decrease of the external light is suffi cient to produce a new quality. The variations in brightness of a given colour — the affective sensa tions of which we have spoken above being left aside — would thus be nothing but qualitative changes, were it not our custom to transfer the cause to the effect and to replace our immediate impressions by what we learn from experience and science. The same thing might be said of degrees of saturation. Indeed, if the different intensities of a colour correspond to so many different shades existing between this colour and black, the degrees of saturation are like shades intermediate between this same colour and pure white. Every colour, we might say, can be regarded under two aspects, from the point of view of black and from the point of view of white. And black is then to intensity what white is to saturation.
CHAP, i SENSATION OF LIGHT 55 The meaning of the photometric experiments will now be understood. A candle placed at a in photome- certain distance from a sheet of paper - illuminates it in a certain way: you SSlc°?ot double the distance and find that four buTaph?8Sicai candles are required to produce the same sensation. From this you conclude that if you had doubled the distance without increas ing the intensity of the luminous source, the result ant illumination would have been only one-fourth as bright. But it is quite obvious that you are here dealing with the physical and not the psy chological effect. For it cannot be said that you have compared two sensations with one another: you have made use of a single sensation in order to compare two different luminous sources with each other, the second four times as strong as the first but twice as far off. In a word, the physicist never brings in sensations which are twice or three times as great as others, but only identical sensa tions, destined to serve as intermediaries between two physical quantities which can then be equated with one another. The sensation of light here plays the part of the auxiliary unknown quantity which the mathematician introduces into his calcu lations, and which is not intended to appear in the final result.
But the object of the psychophysicist is entirely different: it is the sensation of light itself which he studies, and claims to measure. Some times he will proceed to integrate infinitely small 56 TIME AND FREE WILL CHAP. I differences, after the method of Fechner; some times he will compare one sensation The psycho- tJ r physicist directly with another. Ine latter claims to com J ^ t,.
pare and method due to Plateau and Delbceur, measure sensa-., tions. Dei- differs far less than has hitherto been boauf's ex- 1., periments. believed from Fechner s: but, as it bears more especially on the luminous sensations, we shall deal with it first. Delbceuf places an observer in front of three concentric rings which vary in brightness. By an ingenious arrangement he can cause each of these rings to pass through all the shades intermediate between white and black. Let us suppose that two hues of grey are simul taneously produced on two of the rings and kept unchanged; let us call them A and B. Delbceuf alters the brightness, C, of the third ring, and asks the observer to tell him whether, at a certain moment, the grey, B, appears to him equally dis tant from the other two. A moment comes, in fact, when the observer states that the contrast A B is equal to the contrast B C, so that, according to Delbceuf, a scale of luminous intensities could be constructed on which we might pass from each sensation to the following one by equal sensible contrasts: our sensations would thus be measured by one another. I shall not follow Delbceuf into the conclusions which he has drawn from these remarkable experiments: the essential ques tion, the only question, as it seems to me, is whether a contrast A B, formed of the elements A and B, is really equal to a contrast B C, which is differently CHAP, i SENSATION OF LIGHT 57 composed. As soon as it is proved that two sen sations can be equal without being identical, psy- chophysics will be established. But it is this equality which seems to me open to question: it is easy to explain, in fact, how a sensation of luminous intensity can be said to be at an equal distance from two others.
Let us assume for a moment that from our birth onwards the growing intensity of a luminous source in what case na<i always called up in our conscious- cofourQcemight ness> one after the other, the different as 'dVEnces colours of the spectrum. There is no oi magnitude, ^oubt that these colours would then appear to us as so many notes of a gamut, as higher or lower degrees in a scale, in a word, as magnitudes. Moreover it would be easy for us to assign each of them its place in the series. For although the extensive cause varies continuously, the changes in the sensation of colour are discon tinuous, passing from one shade to another shade. However numerous, then, may be the shades inter mediate between the two colours, A and B, it will always be possible to count them in thought, at least roughly, and ascertain whether this num ber is almost equal to that of the shades which separate B from another colour C. In the latter case it will be said that B is equally distant from A and C, that the contrast is the same on one side as on the other. But this will always be merely a convenient interpretation: for although the number of intermediate shades may be equal eg TIME AND FREE WILL CHAP, i