is thus clear that we aim to extend our experimental work over the
whole field of psychology and to avoid one-sideness. Nevertheless
there is no absence of unity in our work; it is not scattered work as
might appear at a first glance; for while the choice of subjects is
always made with relation to the special interests of the students,
there is after all one central interest which unifies the work and has
influenced the development of the whole laboratory during the years of
my direction.

I have always believed--a view I have fully discussed in my 'Grundzüge
der Psychologie'--that of the two great contending theories of modern
psychology, neither the association theory nor the apperception theory
is a satisfactory expression of facts, and that a synthesis of both
which combines the advantages without the defects of either can be
attained as soon as a psychophysical theory is developed which shall
consider 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 rôle 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 rôle 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