Closely connected with these facts, perhaps only a somewhat different aspect of them, is the phenomenon of motor mechanization. Any movement repeated tends to become a circular reaction, as it is called; that is, the end of one repetition serves as a cue for the beginning of the next. Now, in regularly recurring stimuli, giving rise, as will be later shown, to motor reactions, which are differentiated through the natural periodicity of the attention (physiologically the tendency to motor discharge), we have the best condition for this mechanization. In other words, a rhythmical grouping once set up naturally tends to persist. The organism prepares itself for shocks at definite times, and shocks coming at those times are pleasant because they fulfill a need. Moreover, every further stimulus reinforces the original activity; so that rhythmical grouping tends not only to persist, but to grow more distinct,--as, indeed, all the facts of introspection show. All this, however, is true of the repetition of objectively equal stimuli. It shows how an impulse to rhythm would arise and persist subjectively, but does not of itself explain the pleasure in the experience of objective rhythm. It may be said in general, however, that changes which would occur naturally in an objectively undifferentiated content give direct pleasure when they are artificially introduced,--when, that is, the natural disposition is satisfied. This we have seen to be true in the of color contrast; and it is perhaps even more valid in the realm of motor activity. Whatever in sense stimulation gives the condition for, helps, furthers, enhances the natural function, is felt both as pleasing and as furthering the particular activity in question. Now, the objective stress in rhythm is but emphasis on a stress that would be in any case to some degree subjectively supplied. Rhythm in music, abstracting from all other pleasure-giving factors, is then pleasurable because it is in every sense a favorable stimulation.