The pleasantness of sounds has a simple physical basis. All sensations are pleasant only between certain limits of intensity; but the ear can discriminate easily between noises, that in themselves are uninteresting, if not annoying, and notes, which have an unmistakable charm. A sound is a note if the pulsations of the air by which it is produced recur at regular intervals. If there is no regular recurrence of waves, it is a noise. The rapidity of these regular beats determines the pitch of tones. That quality or _timbre_ by which one sound is distinguished from another of the same pitch and intensity is due to the different complications of waves in the air; the ability to discriminate the various waves in the vibrating air is, therefore, the condition of our finding music in it; for every wave has its period, and what we call a noise is a complication of notes too complex for our organs or our attention to decipher. We find here, at the very threshold of our subject, a clear instance of a conflict of principles which appears everywhere in aesthetics, and is the source and explanation of many conflicts of taste. Since a note is heard when a set of regular vibrations can be discriminated in the chaos of sound, it appears that the perception and value of this artistic element depends on abstraction, on the omission from the field of attention, of all the elements which do not conform to a simple law. This may be called the principle of purity. But if it were, the only principle at work, there would be no music more beautiful than the tone of a tuning-fork. Such sounds, although delightful perhaps to a child, are soon tedious. The principle of purity must make some compromise with another principle, which we may call that of interest. The object must have enough variety and expression to hold our attention for a while, and to stir our nature widely. As we are more acutely sensitive to results or to processes, we find the most agreeable effect nearer to one or to the other of these extremes of a tedious beauty or of an unbeautiful expressiveness. But these principles, as is clear, are not coordinate. The child who enjoys his rattle or his trumpet has aesthetic enjoyment, of however rude a kind; but the master of technique who should give a performance wholly without sensuous charm would be a gymnast and not a musician, and the author whose novels and poems should be merely expressive, and interesting only by their meaning and moral, would be a writer of history or philosophy, but not an artist. The principle of purity is therefore essential to aesthetic effect, but the principle of interest is subsidiary, and if appealed to alone would fail to produce beauty.