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62 that during the waking state any particular specification of the said activity (whether spontaneous or due to stimulation), which is capable of occasioning a particular specification of consciousness, must exceed in its turn a certain further degree of intensity for the consciousness actually to arise. . ..

"This state of things (in itself a mere fact needing no picture) may be made clearer by an image or scheme, and also more concisely spoken of. Imagine the whole psycho-physical activity of man to be a wave, and the degree of this activity to be symbolized by the height of the wave above a horizontal basal line or surface, to which every psycho-physically active point contributes an ordinate. . . . The whole form and evolution of the consciousness will then depend on the rising and falling of this wave; the intensity of the consciousness at any time on the wave's height at that time; and the height must always somewhere exceed a certain limit, which we will call a threshold, if waking consciousness is to exist at all.

"Let us call this wave the total wave, and the threshold in question the principal threshold."

[Since our various states of consciousness recur, some in long, some in short periods], "we may represent such a long period as that of the slowly fluctuating condition of our general wakefulness and the general direction of our attention as a wave