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 unlikeness of mental and physical phenomena is so absolute that we must hesitate about drawing any connection between them.  It is impossible to conceive the mental side of a sensation as a form of wave motion.  If, further, we take into consideration the other phenomena associated with the nervous system, the more distinctly mental processes, we have absolutely no data for any comparison.  We can not imagine thought measured by units, and until we can conceive of such measurement we can get no meaning from any attempt to find a correlation between mental and physical phenomena.  It is true that certain psychologists have tried to build up a conception of the physical nature of mind; but their attempts have chiefly resulted in building up a conception of the physical nature of the brain, and then ignoring the radical chasm that exists between mind and matter.  The possibility of describing a complex brain as growing parallel to the growth of a complex mind has been regarded as equivalent to proving their identity.  All attempts in this direction thus far have simply ignored the fact that the stimulation of a nerve, a purely physical process, is not the same thing as a mental action.  What the future may disclose it is hazardous to say, but at present the mental side of the living machine has not been included within the conception of the mechanical nature of the organism.

==The Living Body is a Machine.==—­Reviewing the subject up to this point, what must be our verdict as to our ability to understand the running of the living machine?  In the first place, we are justified in regarding the body as a machine, since, so far as concerns its relations to energy, it is simply a piece of mechanism—­complicated, indeed, beyond any other machine, but still a machine for changing one kind of energy into another.  It receives the energy in the form of chemical composition and converts it into heat, motion, nervous wave motion, etc.  All of this is sure enough.  Whether other forms of nervous and mental activity can be placed under the same category, or whether these must be regarded as belonging to a realm by themselves and outside of the scope of energy in the physical sense, can not perhaps be yet definitely decided.  We can simply say that as yet no one has been able even to conceive how thought can be commensurate with physical energy.  The utter unlikeness of thought and wave motion of any kind leads us at present to feel that on the side of mentality the comparison of the body with a machine fails of being complete.