Let us apply our formula to one more simple case. While reading in the late afternoon, I find the daylight growing dim, rise and turn on the electric light. The stimulus that sets this series of acts going is the dim light; the first, inner response is a _need_ for light. This need tends, by force of habit, to make me turn the button, but it does not make me execute this movement in the air. I only make this movement when the button is in reaching distance. My first {82} reaction, rising from my chair, is preparatory and brings the button close enough to act as a stimulus for the hand reaction. The button within reach is not by itself sufficient to arouse the turning reaction, nor is the need for light alone sufficient. The two conditions must be present together, and the preparatory reaction is such that, given the need, the other condition will be met and the reaction then aroused. What a Tendency Is, in Terms of Nerve Action Very little need be added to our neural conception of a reaction in order to get a satisfactory conception of a tendency to reaction. Principally, we must add this fact, that a nerve center aroused to activity does not always discharge instantly and completely into the muscles, or into some other center, and come to rest itself. It does so, usually, in the case of a reflex, and in other momentary reactions; as when A makes you think of B, and B at once of C, and so on, each thought occupying you but a moment. But a tendency means the arousing of a nerve center under conditions which do not allow that center to discharge at once. The center remains in a condition of tension; energy is dammed up there, unable to find an outlet. We have already seen what the conditions are that cause this damming up of energy. The center that is aroused tends to arouse in turn some lower motor center, but by itself does not have complete control over that lower center, since the lower center also requires a certain external stimulus in order to arouse it to the discharging point. Until the proper external stimulus arrives to complete the arousal of the lower center, the higher center cannot discharge its energy.