e organs, but which had no connections with the muscular system, and
on the other hand, a brain which had motor nerves and muscular
adjustment would be entirely useless if it had not sensory nerves and
sense organs connected with it. In the one case the world would be
experienced, but no response would be possible; in the other case, the
means for response would be given, but no adjustment could set in
because no experience of the surroundings would be possible. Adjustment
every moment demands the relation of the brain in both directions.
Through the sensory nerves the brain receives; through the motor nerves
the brain directs, and this whole arc from the sense organs through the
sensory nerves, through the brain, through the motor nerves and finally
to the muscles, is one unified apparatus of which no part can be thought
away. The brain in itself would be just as useless for the organism as
the heart would be without the arteries and veins.

We must keep this intimate and necessary relation between the sensory
and motor parts constantly in view, and must understand that there
cannot be any sensory process which does not go over into motor
response. Then only the ways are open to develop physiological views
which give a physical basis to the processes of attention and vividness
and inhibition, just as well as to the processes of memory and
association. Such motor theories take many forms. Perhaps we shall most
quickly bring the most essential factors together, if we say that full
vividness belongs only to those sensations for which the channels of
motor discharge are open, while those are inhibited for which the
channels of discharge are closed; and any channel of discharge is
closed, if action is proceeding in the opposite channel. If I open my
hand, the motor paths which lead to closing my fist are blocked; and if
I close my fist, the channels which lead to the opening of the hand are
closed. Now if only those ideas are vivid which find the channels open,
it is clear that all the ideas which would lead to the opposite action
have no chance for development; they remain inhibited, and just this
relation between the vividness of certain ideas and inhibition for those
ideas which lead to the opposite action is the characteristic of the
process of attention.

From such a point of view, the total mental life can be brought into the
psychophysical scheme. We now have not two variable factors, but three,
namely, the qualitie