Sec. 16. Sound shares with the lower senses the disadvantage of having no intrinsic spatial character; it, therefore, forms no part of the properly abstracted external world, and the pleasures of the ear cannot become, in the literal sense, qualities of _things._ But there is in sounds such an exquisite and continuous gradation in pitch, and such a measurable relation in length, that an object almost as complex and describable as the visible one can be built out of them. What gives spatial forms their value in description of the environment is the ease with which discriminations and comparisons can be made in spatial objects: they are measurable, while unspatial sensations commonly are not. But sounds are also measurable in their own category: they have comparable pitches and durations, and definite and recognizable combinations of those sensuous elements are as truly _objects_ as chairs and tables. Not that a musical composition exists in any mystical way, as a portion of the music of the spheres, which no one is hearing; but that, for a critical philosophy, visible objects are also nothing but possibilities of sensation. The real world is merely the shadow of that assurance of eventual experience which accompanies sanity. This objectivity can accrue to any mental figment that has enough cohesion, content, and individuality to be describable and recognizable, and these qualities belong no less to audible than to spatial ideas. There is, accordingly, some justification in Schopenhauer's speculative assertion that music repeats the entire world of sense, and is a parallel method of expression of the underlying substance, or will. The world of sound is certainly capable of infinite variety and, were our sense developed, of infinite extensions; and it has as much as the world of matter the power to interest us and to stir our emotions. It was therefore potentially as full of meaning. But it has proved the less serviceable and constant apparition; and, therefore, music, which builds with its materials, while the purest and most impressive of the arts, is the least human and instructive of them.