ON THE PSYCHO-PHYSICAL VIEW OF NATURE. 517 



subject I shall have to take up Wundt's theories where 

 I now leave them. 



Through the efforts and widespread influence of Prof. 

 Wundt, the inner or psychical phenomena have been 

 drawn into the circle of exact research ; a large portion 

 of psychology has become natural science. It is quite 

 consistent with this that some of the disciples of the 

 modern school should have assumed towards the new 

 branch of natural science the attitude which has be- 

 come habitual among those who cultivate other natural 

 sciences. All these sciences are based upon observation, 

 aided if possible by experiment ; none of them, however, 

 has succeeded in rising to the rank of an exact science 

 without the aid of some generalisation which admitted 

 of clear expression in a few definite conceptions, being 

 the more valuable in the degree that it lent itself to 

 a clothing of mathematical language. In the course of 

 the last centuries, notably the nineteenth, several of 

 these fundamental principles such as the laws of 

 motion, gravitation, atomism, vibratory motion, the con- 

 ception of energy, natural selection, metabolism have 

 attained in various degrees, some almost perfectly, to 

 this state of definiteness, and the sciences built up 

 by their aid have accordingly acquired the character 

 of certainty. Psycho - physics having through Weber, 

 Lotze, Fechner, and Wundt gradually evolved the notion 

 of a partial parallelism of physical and psychical pheno- 

 mena, the conception of a mathematical dependence or 

 of function could be introduced between the measur- 

 able external processes and the hidden internal events 

 which we term mental ; the whole of the latter being 



