Looking then, in accordance with the principle stated above, for deeper conditions, we find rhythm explained in connection with such rhythmical events as the heart beat and pulse, the double rhythm of the breath; but these are, for the most part, unfelt; and moreover, they would hardly explain the predominance of rhythms quite other than the physiological ones. Another theory, closely allied, connects rhythm with the conditions of activity in general, but attaches itself rather to the effect of rhythm than to its cause. Thus we are reminded of the "heightened sense of expansion, or life, connected with the augmentation of muscular movements induced by the more extensive nervous discharges following rhythmic stimulation."<1> But why should it be just rhythmic stimulation that produces this effect? We are finally thrown back on physiology for the answer that in rhythmical stimulation there are involved recurrent activities of organs refreshed by immediately preceding periods of repose. Here again, however, we must ask, why on this hypothesis the periods themselves must be exactly equal. For within the periods the greatest variety obtains. One measure of a single note may be succeeded by another containing eight; within the periods, that is, the minor moments of activity and repose are quite unequal. <1> H.R. Marshall, _Pain, Pleasure, and Aesthetics._ Last of all, we must note the view of rhythm as a phenomenon of expectation (Wundt). But while we can undoubtedly describe rhythm in terms of expectation and its satisfaction, rhythm is rhythm just through its difference from other kinds of expectation. All these explanations seem either merely to describe the facts we seek to explain, or to fail to notice the peculiar intimate nature of the rhythmical experience. But if it could be shown not only that in all stimulation there must be involved an alternation of activity and repose, but also that an equality of such periods was highly favorable to the organism, we should have the conditions for a physiological theory of rhythm. Now the important psychological facts of so-called subjective rhythmizing seem to supply just this need. It has been shown<1> that we can neither receive objectively equal sense-stimuli, nor produce regular movements, without injecting into these a rhythmical element. A series of objectively equal sound-stimuli--the ticking of a clock, for instance--is heard in groups, within each of which one element is of greater intensity. A series of movements are never objectively equal, but grouped in the same way. Now this subjective rhythm, sensory and motor, is explained as follows from the general physiological basis of attention. <1> T.L. Bolton, _Amer. Jour. Of Psychol._, vol. vi. The classical historical study of theories of rhythm remains that of Meumann, _Phil. Studien_, vol. x.