XVI 



THE TEST OF TRUTH 



THE first of the three subjects dealt with in the 

 Principles of Psychology that I propose to discuss 

 at some length is an idea to which expression was 

 first given in The Westminster Review, in an essay 

 called "The Universal Postulate." I choose this 

 particular idea for discussion here, not only because 

 of its influence in the production of Spencer's 

 second book, but also because of its profound im- 

 portance. Not that this idea has in any way in- 

 fluenced subsequent psychology, as have such con- 

 ceptions as that which refers the origin of will 

 to reflex action; but the philosophic importance 

 of the question greatly transcends that of any de- 

 partment in psychology proper, and notably it 

 bears upon the proposition of the conservation of 

 energy, upon which the evolutionary philosophy 

 is based. 



Spencer asked himself this question: By what 

 criterion, in the last resort, can we judge of the 

 truth of any proposition? Since his answer no 

 other has been given, though destructive criticism 

 has of course been essayed. Yet few will dispute 

 the following : 



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