INTRODUCTION 



Biology at the present day claims not merely a certain 

 domain of the will, but also the possession of a peculiar 

 theoretical basis of its own, which is in no way 

 deducible from the fundamental concepts of physics 

 and chemistry. 



The need for elaborating the theory of biology made 

 itself felt relatively late. So long as biological studies, 

 such as zoology and botany, confined themselves to 

 description, they needed, it is true, special methods 

 for attaining to a clear arrangement of the great mass 

 of facts, but they did not require a special theoretical 

 foundation. 



The investigation of the processes in the living 

 organism followed the description of forms ; and for 

 that the basis furnished by chemistry, physics and 

 mechanics sufficed. And so it came about that men 

 learnt to regard the living organism as a physico- 

 chemical machine. 



The correctness of this view has been questioned 

 indeed more than once by those investigators who have 

 studied the connection between subjective processes 

 and objective phenomena. In the course of their work 

 they met with life-factors that would not permit of 

 subordination to physico-chemical laws. But the name 

 which, following the trend of the times, men gave to 

 this science, expressed the hope that this ideal might 

 be reached in the future. Physiological psychology 

 means that psychology is to be treated according to 

 physiological principles. 



What turned the scales in this direction was the 

 weighty opinion of a physicist of genius. Helm- 

 holtz, by perfectly logical methods, reduced all the 

 objects surrounding us to sense-qualities pure and 



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