The above cases show clearly that physiological processes, and not deception of judgment, are responsible for contrast of color. To say this, however, is not to maintain that our perception of a color is never in any degree modified by our judgment of what the particular colored thing before us may be. We have unquestionable illusions of color due to wrong inferences as to what object is before us. Thus Von Kries[30] speaks of wandering through evergreen forests covered with snow, and thinking that through the interstices of the boughs he saw the deep blue of pine-clad mountains, covered with snow and lighted by brilliant sunshine; whereas what he really saw was the white snow on trees near by, lying in shadow].[31]
Such a mistake as this is undoubtedly of psychological origin. It is a wrong classification of the appearances, due to the arousal of intricate processes of association amongst which is the suggestion of a different hue from that really before the eyes. In the ensuing chapters such illusions as this will be treated of in considerable detail. But it is a mistake to interpret the simpler cases of contrast in the light of such illusions as these. These illusions can be rectified in an instant, and we then wonder how they could have been. They come from insufficient attention, or from the fact that the impression which we get is a sign of more than one possible object, and can be interpreted in either way. In none of these points do they resemble simple color-contrast, which unquestionably is a phenomenon of sensation immediately aroused.
I have dwelt upon the facts of color-contrast at such great length because they form so good a text to comment on in my struggle against the view that sensations are immutable psychic things which coexist with higher mental functions. Both sensationalists and intellectualists agree that such sensations exist. They fuse, say the pure sensationalists, and make the higher mental function; they are combined by activity of the Thinking Principle, say the intellectualists. I myself have contended that they do not exist in or alongside of the higher mental function when that exists. The things which arouse them exist; and the higher mental function also knows these same things. But just as its knowledge of the things supersedes and displaces their knowledge, so it supersedes and displaces them, when it comes, being as much as they are a direct resultant of whatever momentary brain-conditions may obtain. The psychological theory of contrast, on the other hand, holds the sensations still to exist in themselves unchanged before the mind, whilst the 'relating activity' of the latter deals with them freely and settles to its own satisfaction what each shall be, in view of what the others also are. Wundt says expressly that the Law of Relativity is "not a law of sensation but a law of Apperception;" and the word Apperception connotes with him a higher intellectual spontaneity.[32] This way of taking things belongs with the philosophy that looks at the data of sense as something earth-born and servile, and the 'relating of them together' as something spiritual and free. Lo! the spirit can even change the intrinsic quality of the sensible facts themselves if by so doing it can relate them better to each other! But (apart from the difficulty of seeing how changing the sensations should relate them better) is it not manifest that the relations are part of the 'content' of consciousness, part of the 'object,' just as much as the sensations are? Why ascribe the former exclusively to the knower and the latter to the known? The knower is in every case a unique pulse of thought corresponding to a unique reaction of the brain upon its conditions. All that the facts of contrast show us is that the same real thing may give us quite different sensations when the conditions alter, and that we must therefore be careful which one to select as the thing's truest representative.
There are many other facts beside the phenomena of contrast which prove that when two objects act together on us the sensation which either would give alone becomes a different sensation. A certain amount of skin dipped in hot water gives the perception of a certain heat. More skin immersed makes the heat much more intense, although of course the water's heat is the same. A certain extent as well as intensity, in the quantity of the stimulus is requisite for any quality to be felt. Fick and Wunderli could not distinguish heat from touch when both were applied through a hole in a card, and so confined to a small part of the skin. Similarly there is a chromatic minimum of size in objects. The image they cast on the retina must needs have a certain extent, or it will give no sensation of color at all. Inversely, more intensity in the outward impression may make the subjective object more extensive. This happens, as will be shown in [Chapter XIX], when the illumination is increased: The whole room expands and dwindles according as we raise or lower the gas-jet. It is not easy to explain any of these results as illusions of judgment due to the inference of a wrong objective cause for the sensation which we get. No more is this easy in the case of Weber's observation that a thaler laid on the skin of the forehead feels heavier when cold than when warm; or of Szabadföldi's observation that small wooden disks when heated to 122° Fahrenheit often feel heavier than those which are larger but not thus warmed;[33] or of Hall's observation that a heavy point moving over the skin seems to go faster than a lighter one moving at the same rate of speed.[34]
Bleuler and Lehmann some years ago called attention to a strange idiosyncrasy found in some persons, and consisting in the fact that impressions on the eye, skin, etc., were accompanied by distinct sensations of sound.[35] Colored hearing is the name sometimes given to the phenomenon, which has now been repeatedly described. Quite lately the Viennese aurist Urbantschitsch has proved that these cases are only extreme examples of a very general law, and that all our sense-organs influence each other's sensations.[36] The hue of patches of color so distant as not to be recognized was immediately, in U.'s patients, perceived when a tuning-fork was sounded close to the ear. Sometimes, on the contrary, the field was darkened by the sound. The acuity of vision was increased, so that letters too far off to be read could be read when the tuning-fork was heard. Urbantschitsch, varying his experiments, found that their results were mutual, and that sounds which were on the limits of audibility became audible when lights of various colors were exhibited to the eye. Smell, taste, touch, sense of temperature, etc., were all found to fluctuate when lights were seen and sounds were heard. Individuals varied much in the degree and kind of effect produced, but almost every one experimented on seems to have been in some way affected. The phenomena remind one somewhat of the 'dynamogenic' effects of sensations upon the strength of muscular contraction observed by M. Féré, and later to be described. The most familiar examples of them seem to be the increase of pain by noise or light, and the increase of nausea by all concomitant sensations. Persons suffering in any way instinctively seek stillness and darkness.
Probably every one will agree that the best way of formulating all such facts is physiological: it must be that the cerebral process of the first sensation is reinforced or otherwise altered by the other current which comes in. No one, surely, will prefer a psychological explanation here. Well, it seems to me that all cases of mental reaction to a plurality of stimuli must be like these cases, and that the physiological formulation is everywhere the simplest and the best. When simultaneous red and green light make us see yellow, when three notes of the scale make us hear a chord, it is not because the sensations of red and of green and of each of the three notes enter the mind as such, and there 'combine' or 'are combined by its relating activity' into the yellow and the chord, it is because the larger sum of light-waves and of air-waves arouses new cortical processes, to which the yellow and the chord directly correspond. Even when the sensible qualities of things enter into the objects of our highest thinking, it is surely the same. Their several sensations do not continue to exist there tucked away. They are replaced by the higher thought which, although a different psychic unit from them, knows the same sensible qualities which they know.
The principles laid down in Chapter VI seem then to be corroborated in this new connection. You cannot build up one thought or one sensation out of many; and only direct experiment can inform us of what we shall perceive when we get many stimuli at once.