nerve will always be excited when ether
vibrations chance to have an opportunity of setting the optic machinery
in motion. And so on with the other senses. Each sensory nerve has, at
its end, a bit of machinery designed for the transformation of certain
kinds of external energy into nervous energy, just as a dynamo is a
machine for transforming motion into electricity. If the machine is
broken, the external force has no longer any power of acting upon it,
and the individual becomes deaf or blind.

_Mental Phenomena_.--Thus far in our analysis we need not hesitate in
recognizing a correlation between physical and nervous energy. Even
though nervous energy is very subtle and only affects our instruments of
measurements under exceptional conditions, the fact that nervous forces
are excited by physical forces, and are themselves directly measurable,
indicates that they are correlated with physical forces. Up to this
point, then, we may confidently say that the nervous system is part of
the machine.

But when we turn to the more obscure parts of the nervous phenomena,
those which we commonly call mental, we find ourselves obliged to stop
abruptly. We may trace the external force to the sensory organ, we may
trace this force into a nervous stimulus, and may follow this stimulus
to the brain as a wave motion, and therefore as a form of physical
energy. But there we must stop. We have no idea of how the nervous
impulse is converted into a sensation. The mental side of the sensation
appears to stand in a category by itself, and we can not look upon it as
a form of energy. It is true that many brave attempts have been made to
associate the two. Sensations can be measured as to intensity, and the
intensity of a sensation is to a certain extent dependent upon the
intensity of the stimulus exciting it. The mental sensation is
undoubtedly excited by the physical wave of nervous impulse. In the
growth of the individual the development of its mental powers are found
to be parallel to the development of its nerves and brain--a fact which,
of course, proves that mental power is dependent upon brain structure.
Further, it is found that certain visible changes occur in certain parts
of the brain--the brain cells--when they are excited into mental
activity. Such series of facts point to an association between the
mental side of sensations and physical structure of the machine. But
they do not prove any correlation between them. The unlike