ght 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.

When there is an "organic state" present, such as hunger or thirst,
this may act as a persistent stimulus to the sensory nerves and
through them to the higher center in {83} question; and then we can
readily understand how it is that the center remains active until the
organic state is relieved. But where there is no such persistent
organic stimulus, as there can scarcely be in the case of the
bloo