together, either of them keeps the tendency to
reawaken the other at a later time. Through the endless combination
which life's impressions awaken in the mind from the first hour after
birth, the whole stream of memory images and thoughts and aims and
imaginations is thus to be explained.

The whole theory of physiological associationism works evidently with
two factors. First, there are millions of brain cells of which each one
may have its particular quality of sensation, and second, each brain
cell may work with any degree of energy, to which the intensity of the
sensation would correspond. If I distinguish ten thousand different
pitches of tone, they would be located in ten thousand different cell
groups, each one connected through a special fiber with a special string
in the ear. And each of these tones may be loud or faint, corresponding
to the amount of excitement in the particular cell group. Every other
variation must then result from the millionfold connections between
these brain cells. Indeed, the brain furnishes all possibilities for
such a theory. We know how every brain cell resolves itself into
tree-like branch systems which can take up excitements from all sides,
and how it can carry its own excitement through long connecting fibers
to distant places, and how the endings of these fibers clasp into the
branches of the next cell, allowing the propagation of excitement from
cell to cell. We know further how large spheres of the brain are
confined to cells of particular function, that for instance cells which
serve visual sensations are in the rear part of the brain hemispheres,
and so on. Finally we know how millions of connecting fibers represent
paths in all directions, allowing very well a cooeperation by association
between the most distant parts of the brain. The theories found their
richest development, when it was recognized that large spheres of our
brain centers evidently do not serve at all merely sensory states, but
that their cells have as their function only the intermediating between
different sensory centers. Such so-called association centers are thus
like complex switchboards between the various mental centers. Their own
activity is not accompanied by any mental content, but has only the
function of regulating transmission of the excitement from the one to
the other. Above all their operation would make it possible that through
associative processes, the wonderful complexity of our trains