THE MECHANICS OF THE LIVING MACHINE 317 



gunpowder, and a nervous impulse is produced. In all of this 

 we are plainly dealing with nothing more than the transforma- 

 tion of one type of energy into another. In the same way the 

 optic nerve has at its end, in the eye, a bit of mechanism that 

 is easily excited by the light waves, and when such waves strike 

 the eye there will be started in the optic nerve a series of 

 impulses which pass towards the brain. Thus each sensory 

 nerve has at its end a bit of machinery designed for trans- 

 forming certain kinds of external force into nervous impulses. 



The second phase of the sensation is, on the other hand, 

 not explainable by any mechanical principle. When the im- 

 pulse started in the ear reaches the brain, it is converted into 

 what we call a sensation, i. e., a consciousness, a perception, a 

 distinct feeling. In our attempt to trace external forces we can 

 follow the stimulus to the brain, but there we must stop. We 

 have no idea how a nervous impulse is converted into sensation. 

 By no means of thinking can we conceive of the correlation 

 of the sensation itself with any form of physical energy. It 

 is true that the mental sensation is excited by the nervous 

 impulse, and true also that in the development of the individual 

 the mental powers develop parallel with the growth of the 

 nerves and brain. Moreover, certain visible changes occur in 

 the brain cells when they are excited into mental activity. 

 All of these facts point to a close association between the mental 

 side of sensation and the physical structure of the machine. 

 But they do not prove any correlation between them. The 

 unlikeness between the mental and physical phenomena is so 

 absolute that we must hesitate about drawing any connection 

 between them. It is impossible to conceive of the mental side 

 of sensation as a form of wave motion. 



Mental functions. If we go farther and try to consider the 

 other phenomena associated with the nervous system the more 

 distinctive mental processes we have absolutely no ground for 

 comparison. We cannot imagine thought measured by units; 

 and until we conceive of some such measurement we can get 



