of thought
may be reached.

Yet even the highest development of the association theories did not
seem to do justice to the whole richness of the inner life. We may well
understand through those association processes that a rich supply of
memory pictures is at our disposal, that ideas stream plentifully to our
minds and enter into new and ever new combinations. But that alone is
not an account of our inner experience. If there is anything essential
for inner life, it is the attention which gives emphasis to certain
states and neglects others. And that means that certain mental contents
are growing not only in strength but in vividness and clearness, and
that others are losing their vividness, are inhibited and suppressed.
Here were always the real difficulties of the association theories; they
seemed so entirely unable to explain from their own means why certain
states become foremost in our minds and others fade away, why some have
the power to grow and others are neglected. These facts of attention and
vividness, inhibition and fading, worked almost as a temptation to give
up the physiological explanation altogether and to rely on some mystical
power, some mental influence which could pull and push the ideas without
any interference and help from the side of the brain. Yet since we have
seen that the truth of psychophysical parallelism has the meaning of a
postulate which we cannot escape unless we want to give up explanation
altogether, it is evident that such falling back into un-physiological
agencies would be just as inconsistent as if the naturalist should posit
miracles in the midst of chemistry or astronomy. If the facts which
cluster about attention cannot be understood by the simple scheme of
associationism, the demand must be for a better physiological theory.

The development of physiological psychology in recent years has indeed
shown the way to such a wider theory, which furnishes the physiological
accompaniment also for those experiences of attention and vividness
which form the weakness of associationism. This new development has come
up with the growing insight that the brain's mental functions are
related not only to the sensory impressions, but at the same time to the
motor expressions. The older view, still prevalent to-day in popular
writings, made the brain the reservoir of physical stimuli, which come
from the sense organs to the cortex of the brain hemispheres. There the
perceptions arose and th