8 THE FOUNDATIONS OF SCIENCE 



with the phenomena of mind and the influences 

 affecting it primarily, of course, with the mind of 

 man, but not exclusively so, since comparative 

 psychology is investigating also the minds of animals. 



The relations between the sciences have been the 

 subject of discussion since philosophy, as the science 

 of the sciences, began. While the term science was 

 taken to include the whole of knowledge, the problem 

 of classification was more complicated, and perhaps 

 more important, than it is if we accept the modern 

 more limited view. The problem of the classifica- 

 tion of knowledge is one of much difficulty ; and 

 any scheme, such as that which is now suggested, 

 must not be regarded as final. The branches of 

 knowledge are not only growing within themselves, 

 but their relations to each other are continually 

 changing, so that the scheme of arrangement suitable 

 for one age may not be appropriate for another. 



As regards science, the three main divisions 

 we have adopted seem to be related as follows. 

 Physical science, the knowledge of the phenomena 

 of matter and energy, on one side touches biology, 

 the science of living things. Biology, in its turn, 

 touches psychology, the science of mind, since mind, 

 as we can investigate it by scientific methods, is 

 always contained in living organisms, or, at all 

 events, is always investigated in relation to such 

 organisms. 



So far all seems simple ; and we are inclined to 



