r the central process in its dependence, not only upon the
sensory, but also upon the motor excitement. This I call the _action
theory_. In the service of this theory it is essential to study more
fully the role of the centrifugal processes in mental life, and,
although perhaps no single paper of this first volume appears to offer
a direct discussion of this motor problem, it was my interest in this
most general question which controlled the selection of all the
particular problems.

This relation to the central problem of the role of centrifugal
processes involves hardly any limitation as to the subject matter;
plenty of problems offer themselves in almost every chapter of
psychology, since no mental function is without relation to the
centrifugal actions. Yet, it is unavoidable that certain groups of
questions should predominate for a while. This volume indicates, for
instance, that the aesthetic processes have attracted our attention in
an especially high degree. But even if we abstract from their
important relation to the motor functions, we have good reasons for
turning to them, as the aesthetic feelings are of all feeling processes
decidedly those which can be produced in the laboratory most purely;
their disinterested character makes them more satisfactory for
experimental study than any other feelings.

Another group of researches which predominates in our laboratory is
that on comparative psychology. Three rooms of the laboratory are
reserved for psychological experiments on animals, under the special
charge of Dr. Yerkes. The work is strictly psychological, not
vivisectional; and it is our special purpose to bring animal
psychology more in contact with those methods which have found their
development in the laboratories for human psychology. The use of the
reaction-time method for the study of the frog, as described in the
fifteenth paper, may stand as a typical illustration of our aim.

All the work of this volume has been done by well-trained
post-graduate students, and, above all, such advanced students were
not only the experimenters but also the only subjects. It is the rule
of the laboratory that everyone who carries on a special research has
to be a subject in several other investigations. The reporting
experimenters take the responsibility for the theoretical views which
they express. While I have proposed the subjects and methods for all
the investigations, and while I can take the responsibility f