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.