As time goes on, however, art passes through an evolution, becoming gradually more potent in its treatment of reality. Its progress takes the form of a curious zigzag, the resultant of two alternating tendencies; what happens is something like this. For a while it develops its power of synthesis (a power dependent both upon technical skill in handling material and on organizing force of thought) until it is able to present a few simple factors of effect in clear, salient unity. This is what is called a period of classicism. Then, dissatisfied with its attainment, desiring a richer reflection of the great whirl of experience, it reaches out after novel effects; its vision is for a while more extended than clear, and, presenting many effects which it cannot yet unify, it becomes brilliant, suggestive, fragmentary, turgid, inchoate. There has been a sacrifice of the old simple clarity for a richer chaos, or, in the trite terminology, a romantic movement. Now, however, technical skill and synthetic power of thought again advance, and a new and complexer order supervenes on the temporary confusion. Unity of effect is regained, art is classic once more (but with increased wealth of meaning), and the time is ripe for another burst of romanticism. By this alternation of impulses art grows, and when either tendency is defective we have a diseased art. If there be no romantic movement, if art remains contented with its acquired scope, there is stagnation, pedantry, academicism; if there be no classical period of assimilation, we have vagueness and turgidity, qualities even more fatal, since, as we have seen, the justification of art is its power to clarify. The general formula for wholesome artistic advance might, then, run thus: «Increase in the variety of the selected elements, without loss of the ideal unity imposed upon them.» And the ideal goal of art is a representation of the whole of life, stamped with complete unity. Turning now to music, we must point out that, although it has in a general way undergone a development like that of the other arts, made up of alternating classic and romantic movements, it has had from the first certain advantages over them in the struggle for richness and clarity, advantages proceeding from its fundamental nature. For tones are unique in our mental experience as being at once more directly expressive of the emotional essence of life than any other art-material, and more susceptible of orderly structure.