In all these cases we meet with the same relations between sensations and ideas, as we saw in the metronome beats described above. Green or red, white or black, &c., are called visual sensations; a green surface or a black body is called a visual idea. The relation is exactly the same as between the single beat and the row of beats. Only in this case the combination of several sensations to an idea of a surface or of a body forces itself upon us much more directly, and it requires a very careful abstraction from this combination into an ideational complex, in order to retain the conception of a sensation. But we can vary our ideas of surfaces and bodies at will, while the colour remains the same. So at last we are forced to look upon this element, that remains the same in spite of all changes in the combinations, as a simple sensation. In the same way we consider a simple tone as a sensation of hearing, and a clang or chord, composed of several tones, as an auditory idea, and so on. If the tones follow each other in a melodious and rhythmical combination, then ideas of increasing complexity arise, and in the same manner several relatively simple visual ideas may be bound together into more extensive simultaneous or successive unities. The senses of sight and of hearing in especial form in this way a great variety of sensations and ideas, and they do this in two ways--firstly, through the qualities of their simple sensations, and secondly, through the complications of ideas, into which these sensations may be combined. The simple scale of tones, from the deepest to the highest tone that can be heard, consists of an infinite gradation of tonal qualities, out of which our musical scale chooses only certain tones, which lie at relatively large distances from each other. Musical clangs are combinations of a number of such simple tonal sensations, and the so-called compound clangs increase this complicated constitution of the clangs by emphasising to a greater degree certain partial tones. The simple light-sensations form a more concise manifoldness, but one that stretches into different directions. Red, for example, on the one hand goes over by constant gradations into orange and then into yellow, and on the other hand we have just as many constant gradations from each of these colour-shades through the lighter colour-tones into white, or through the darker ones into black, and so on. The ideas of this sense are absolutely inexhaustible. If we think of the manifold forms of surfaces and bodies, and of the differences in distance and direction, in which we perceive objects, it is obvious that it is absolutely impossible to find any limit here. Thus the richness in sensations and ideas, which each of the senses conveys, stands in close relation to the spatial distance of the objects which they introduce into consciousness. The narrowest region is that of the touch and organic sense, where the impressions all refer to our own body. Then