ced for its dependence
upon the physical. For what is true of all other parts of the
organisation, of the building up and perfecting of every member and every
system of organs, the bony skeleton, the circulatory system, the
alimentary canal, that they can be referred back to very simple
beginnings, and that their evolution may be traced through all its
stages--is equally true of the nervous system in general and of the brain
in particular. It increases more and more in volume and in intricacy of
structure, it expands the cranial cavity and diversifies its convolutions.
And the more it grows, and the more complex it becomes, the more do the
mental capacities increase in perfection, so that here again it seems once
more apparent that the psychical is an accompaniment and result of the
physical.

Popular naturalism usually stops short here, and contents itself with
half-truths and inconsequences, for it naively admits that psychical
processes, sensation, perception, will, have a real influence upon the
physical, and, not perceiving how much the admission involves, it does not
trouble itself over the fact that, for instance in the so-called voluntary
movements of the body, in ordinary behaviour, the psychical, and the will,
in particular, is capable of real effect, and can move hand and foot and
the whole body, and thus has a real reciprocal relation with the physical.
This form of popular naturalism sometimes amuses itself with assuming a
psychical inwardness even in non-living matter, and admitting the
co-operation of psychical motives even in regard to it.

But it is far otherwise with naturalism in the strict sense, which takes
its fundamental principles and its method of investigation seriously. It
is aware that such half-and-half measures interrupt the continuity of the
system at the most decisive point. And therefore with the greatest
determination it repeats along psychological lines the same kind of
treatment that it has previously sought to apply to biological phenomena:
the corporeal must form a sequence of phenomena complete in itself and not
broken into from without. All processes of movement, all that looks as if
it happened "through our will," through a resolve due to the intervention
of a psychical motive, every flush of shame that reddens the cheek, every
stroke executed by the hand, every sound-wave caused by tongue and lips,
must be the result of conditions of stimulation and tension in the energy
of the