not really
understood as a cause of those sound sensations of the word. We have no
insight into the connection of those two happenings. But the situation
is entirely changed, if we consider the smell effect from the point of
view of the parallelistic theory. Now the association of facts would
indicate that we got the first two impressions together, because two
brain processes were going on at the same time. My nose brought me the
smell stimulus, my ear gave me the sound stimulus, each going on in a
particular center, or, to express it in a simplified schematic way, each
reaching particular brain cells, and the excitement of these brain cells
being accompanied by the particular sensations. The physiologist has
many possibilities of conceiving the further stages of the process, in
order to satisfy the demand of explanation. He may say the excitement of
each of these two brain cells, the one in the olfactory center, the
other in the auditory center, irradiates in all directions through the
fine branches of the brain fibers. Each cell has relations to every
other cell in the brain; thus there is also one connecting path between
those two cells which were stimulated at once. Now if the two ends of an
anatomical path are excited at the same time, the path itself becomes
changed. The connecting way becomes a path of least resistance, and that
means that if, in future, one of the two brain cells becomes excited
again, the overflow of the nervous excitement will not now go on easily
in all directions, but only just along that one channel which leads to
that other brain cell. A theory like this explains in real explanatory
terms, in ways which physics and chemistry can demonstrate as necessary,
that any excitement of the odor cell runs over into the sound cell and
vice versa. In short, the psychological association of ideas, which we
should simply have to accept as inexplainable fact, is thus transformed
into a connection which we understand as necessary; and the fact is
really explained.

This simple scheme of the physiology of association for a hundred years
has given a most decided impulse to the progress of psychology. As the
association process can so easily be expressed in physiological terms,
the aim was prevalent to understand the interplay of mental life more
and more as the result of association. The underlying thought of this
whole association psychology was thus a conviction that whenever two
mental experiences occur