The methods by which the larger musical wholes are built up illustrate principles of aesthetic structure with which we are already familiar. There is the harmonious unification of parts through the simple repetition of motives, their inversion or imitation in higher or lower keys, either successively or simultaneously; the execution of the same theme in another time or tempo; and through the interweaving of themes. There is the balance of contrasted or competing themes; the subordination of the lesser to the more striking and insistent motives; the preparation for, emergence and triumph of, a final passage that resolves all dissonances and adjusts all conflicts. Because of music's abstractness, the connection between the parts of a musical composition may be loose or subtle, taxing the synthetic powers even of the educated listener; yet some contrast or analogy of feeling must always unite them. The structure of the whole may be either static or dramatic; in the former case the dramatic element is confined to the themes, the purpose of the whole being merely to work out all their significant variations,--to embroider and repeat them in new keys and rhythms and tempos, and to contrast them with other themes. Repetition is the great creative principle of musical development, the composer seeking to say over again in ever new forms what he has said before. And this, again because of the abstractness of music, is a significant process; to repeat the concrete is tiresome and trivial, but an abstract form is always enriched by appearing in a new shape. The explanation of musical expression thus far given, although it suffices to account for the basis of all musical feeling, is, I think, inadequate to its full volume and intensity. There is a concreteness of emotional content in some musical compositions--an arousal of terror and longing and despair and joy--infinitely richer than any abstract forms of feeling.