axiom of his system of investigation, the prudent psychologist, on
the other hand, will investigate the laws of conscious life according
to the inductive method, and will hence, as much as the physicist,
make the existence of fixed laws his initial assumption. If, again,
the most superficial introspection teaches the physiologist that his
conscious life is dependent upon the mechanical adjustments of his
body, and that inversely his body is subjected with certain
limitations to his will, then it only remains for him to make one
assumption more, namely, THAT THIS MUTUAL INTERDEPENDENCE BETWEEN THE
SPIRITUAL AND THE MATERIAL IS ITSELF ALSO DEPENDENT ON LAW, and he
has discovered the bond by which the science of matter and the
science of consciousness are united into a single whole.

Thus regarded, the phenomena of consciousness become functions of the
material changes of organised substance, and inversely--though this
is involved in the use of the word "function"--the material processes
of brain substance become functions of the phenomena of
consciousness. For when two variables are so dependent upon one
another in the changes they undergo in accordance with fixed laws
that a change in either involves simultaneous and corresponding
change in the other, the one is called a function of the other.

This, then, by no means implies that the two variables above-named--
matter and consciousness--stand in the relation of cause and effect,
antecedent and consequence, to one another. For on this subject we
know nothing.

The materialist regards consciousness as a product or result of
matter, while the idealist holds matter to be a result of
consciousness, and a third maintains that matter and spirit are