1 42 COSM W FHILOSOPH 1. [pt. ii. 



be, I think, quite ready to admit that, if it does not contain 

 the whole truth, it must at least contain a foreshadowing of 

 the truth. 



For we have now to note that, by a deduction from an 

 established law of molecular motion, this hypothetical law 

 of nervous action can be shown to explain that law of 

 association which subjective analysis proclaims as the 

 fundamental law of intelligence. In the chapter on Life 

 and Mind, we saw that the chief business of psychology is 

 to answer the question how there comes to be established in 

 the mind a relation between two subjective states X and Y, 

 answering to a relation between two phenomena A and b in 

 the environment. How is it that there is a subjective rela- 

 tion between the idea of sweetness and the group of ideas 

 comprised in the visual perception of a peach, answering in 

 some way to the objective relation between the coexistent 

 properties of the peach, so that tlie presentation of the one 

 to the cephalic ganglia is inevitably accompanied by the 

 representation of the other ? This question lies at the bottom 

 of psychology, and we have now to see how it is to be 

 answered. The answer will lead us through a portion of the 

 domain of molecular physics, and will incidentally give us a 

 hint concerning the genesis of nervous systems. 



In the chapter on Matter, Motion, and Force, it was shown 

 that all motion takes place along the line of least resistance, 

 whether the motion be the movement of a mass of matter 

 through a resisting medium, or the passage of a series of 

 undulations through the molecules of an aggregate. Let us 

 reconsider this truth in one of its concrete applications. 



When a wave of molecular motion traverses a mass of 

 matter for the first time, the line of least resistance will of 

 course be determined by the intimate structure of the mass. 

 But now mark what happens. Immediately after the passage 

 of the wave, the intimate structure of the mass, in the 

 vicinity of the line along which the wave has travelled, is 



