identical; with all this the physiologist, as such, has nothing
whatever to do; his sole concern is with the fact that matter and
consciousness are functions one of the other.

By the help of this hypothesis of the functional interdependence of
matter and spirit, modern physiology is enabled to bring the
phenomena of consciousness within the domain of her investigations
without leaving the terra firma of scientific methods. The
physiologist, as physicist, can follow the ray of light and the wave
of sound or heat till they reach the organ of sense. He can watch
them entering upon the ends of the nerves, and finding their way to
the cells of the brain by means of the series of undulations or
vibrations which they establish in the nerve filaments. Here,
however, he loses all trace of them. On the other hand, still
looking with the eyes of a pure physicist, he sees sound waves of
speech issue from the mouth of a speaker; he observes the motion of
his own limbs, and finds how this is conditional upon muscular
contractions occasioned by the motor nerves, and how these nerves are
in their turn excited by the cells of the central organ. But here
again his knowledge comes to an end. True, he sees indications of
the bridge which is to carry him from excitation of the sensory to
that of the motor nerves in the labyrinth of intricately interwoven
nerve cells, but he knows nothing of the inconceivably complex
process which is introduced at this stage. Here the physiologist
will change his standpoint; what matter will not reveal to his
inquiry, he will find in the mirror, as it were, of consciousness; by
way of a reflection, indeed, only, but a reflection, nevertheless,
which stands in intimate relation to the object of his inquiry. When
at this point he observes how one idea gives rise to another, how
closely idea is connected with sensation and sensation with will, and