IS THE BODY A MACHINE? 47 



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 them- 

 selves 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 ab- 

 ruptly. We may trace the external force to the 

 sensory organ, we may trace this force into a nerv- 

 ous 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 con- 

 verted 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, 



