This reference to the future may be in the nature of an end or goal that controls a series of activities or it may be of a momentary and casual kind. In any case the character of the stimulus changes with the progress of the act. The book on the table must become successively book-to-be-reached-for, book-to-be-picked-up, and book-to-be-opened, unless the process is to drop back to the type of reflex. This development of the stimulus gives genuine continuity, since every moment in the process comes as a fulfilment of its predecessor and as a transition-point to its successor. In a purely mechanical act response follows stimulus like the successive strokes of a clock. It is a touch-and-go affair; the stimulus presses the button and then subsides, while the neural organization does the rest. In conscious behavior, on the other hand, stimulus and response keep step with each other. A mere succession of stimuli would reduce conscious behavior to a series of explosive jerks, on the principle of the gasoline engine. To be conscious at all is to duplicate in principle the agility of the tight-rope performer, who continuously establishes new co-ordinations according to the exigencies of the moment and with constant reference to the controlling consideration of keeping right side up. The sensory stimulus provides continuously for its own rehabilitation or appropriate transformation, and in a similar way the neural organization is never a finished thing, but is in constant process of readjustment to meet the demands of an adaptation that still lies in the future.