The problem of method, which involves the application of
experiment, and the problem of a psychophysical supplement, which
involves a knowledge of the bodily substrates of the mental
life.
The problem of method is more significant.As an experimental
science, physiological psychology seeks to accomplish a reform in
psychological investigation comparable with the revolution brought
about in the natural sciences by the introduction of the
experimental method.
It is of the essence of experiment that we can vary the
conditions of an occurrence at will and, if we are aiming at exact
results; in a quantitatively determinable way Hence, even in the
domain of natural science the aid of the experimental method
becomes indispensable whenever the problem set is the analysis of
transient and impermanent phenomena, and not merely the observation
of persistent and relatively constant objects.
The psychological experiment proceeds very differently. In the
first place, it creates external conditions that look towards the
introduction of a determinate mental process at a given moment. In
the second place, it makes the observer so far master of the
general situation, that the state of consciousness accompanying
this process remain approximately unchanged. The great importance
of the experimental method, therefore, lies in the fact that, it
enables us arbitrarily to vary the conditions of our observations,
and also it makes observation itself possible for us. The results
of this observation may then be fruitfully employed in the
examination of other mental phenomena, whose nature prevents their
own direct experimental modification.