6 Psychophysical Evolution 



this series we have taken mainly the ' psychic ' or sub- 

 jective point of view, going over to the psychological, 

 however, when we had occasion to reach interpretations 

 of a biological or sociological sort. Now the main re- 

 source is psychological and biological, the facts of mind 

 and those of life standing on the same objective footing. 

 We are now distinctly in the spectator's shoes, observing 

 facts in the evolution or development of minds and or- 

 ganisms together, at any grade in the series of forms from 

 lower to higher. 



Such a distinction, it is evident, is not possible to the 

 biologist as such : all of his facts are simply vital as con- 

 trasted with the non-vital ; there is no question of a sub- 

 jective over against an objective point of view. So as 

 regards the relation of biology to psychology we have a two- 

 fold distinction : that of the vital as distinguished from the 

 psychological ; and, on the other hand, that of the vital as 

 distinguished from the psychic. As to the first of these 

 distinctions, there is a broad truth which may be stated at 

 this point. 



In another place ^ it is pointed out that the entire hierar- 

 chy of the sciences is run through by a form of interdepen- 

 dence as between contiguous departments of research. 

 The concept of force, when strictly construed, becomes the 

 touchstone for the differentiation of the sciences. A force 

 is whatever is present when one stage of a process suc- 

 ceeds necessarily upon another stage. A force always 

 shows itself in a change, one aspect of process succeed- 

 ing another ; and the passing over of one set of phenomena 



1 Psychological Review, January, 1902, pp. 57 f., and Social and Ethical 

 Jnterpretationst 3d ed., 1902, Introduction, § 2; cf. also below Chap. XIX. 

 §8. 



