98 Physical actions depend upon differences, 



space, makes no immediate impression upon us. We 

 only know of the existence of those uniformities by 

 inference from our perceptions of sequences or of 

 relative difference. Although the Earth moves at the 

 rate of 62,000 miles an hour in its orbit, consciousness 

 does not perceive it. If also there was no error, we 

 should be less immediately able to discern truth, 

 without pain we should lose much of the enjoyment 

 of pleasure. Without the contrast of imperfection we 

 could not directly appreciate perfection. 



This principle of " relativity," or of change of im- 

 pression, operates both in the phenomena of dead and 

 living matter and in those of mind ; the selenium in a 

 photophone is kept in a state of motion or activity, 

 v not by a beam of uniform light, but only by one 

 which changes; electrical action is excited by a rela- 

 tive difference of friction, of temperature, of chemical 

 action, &c. ; chemical action also often results from a 

 relative difference of property of two bodies. That 

 the most inscrutable phenomenon of mind, viz., con- 

 sciousness, is largely dependent upon relative physical 

 .and chemical conditions, is proved by the powerful 

 influence which alcohol, chloroform, opium, haschish, 

 and other substances, have in exciting or depressing 

 it. These facts prove that excitement of conscious- 

 ness or mental action depends upon precisely the 

 same general condition, viz. : change of impression, 

 as the excitement of some of the physical forces ; and 

 that mind possesses a similar property to the physical 

 forces of being changed by inequality of impression. 



