DEPENDENCE ON ASSOCIATION. 83 



Psychological investigation teaches us that for 

 the causes of nervous or psychic change, we must 

 look to the forces acting from without upon the 

 organism. That all of the psychic changes are 

 accompanied by the display of energy in some form 

 of material change in the nervous structures, is the 

 most striking and far-reaching conclusion of modern 

 psychology. All the facts of physics and biology, 

 which have any bearing upon the subject, render 

 it most probable that in the nervous system, as 

 elsewhere, no energy is destroyed and none is 

 originated ; no molecule moves without due appli- 

 cation of force from without, and the whole mechan- 

 ism thus moves perfectly subservient to the law of 

 the conservation of energy. Yet we cannot observe 

 the process in exactly the same manner as we ob- 

 serve and test other simpler physical processes. 

 Forces which act upon living matter elude us in 

 the wonderful intricacy of their results. 



The general data of Biology go to show that no 

 physical change can take place in a living animal 

 without directly or indirectly affecting the psychical 

 condition of the animal. The psychical change 

 may follow immediately as a sensation, and may 

 remain as a new association in memory, or it may be 

 a subconscious nervous co-ordination ; or again the 

 psychical change may be only a gradual change of 



