“The secret of the whole matter is that a habit is not the mere tendency to repeat a certain act, nor is it established by the mere repetition of the act. Habit is a fixed tendency to react or respond in a certain way to a given stimulus; and the formation of habit always involves the two elements, the stimulus and the response or reaction. The indolent lad goes to school not in response to any stimulus in the school itself, but to the pressure of his father's will; when that stimulus is absent, the reaction as a matter of course does not occur.”