196 SCIENCE AND THE HUMAN MIND 



held sway till the middle of the nineteenth century. 

 Then the change of view begun by the synthesis of 

 urea was reinforced on the physical side by the applica- 

 tion by Mayer and Helmholtz to the living organism 

 of the principle of the conservation of energy. All 

 the manifold activities of the body were thus traced 

 ultimately to the chemical and thermal energy of the 

 food taken in, and it was natural, if not strictly logical, 

 to conclude that, if the total output of energy was 

 governed by physical laws, the intermediate pro- 

 cesses could be described completely by those 

 laws also. 



The naturalistic standpoint was further strengthened 

 by the work of Schleiden and of Schwann, who showed 

 that all organisms were built up of living cells as 

 biological units, and by other investigations on similar 

 lines into cell-structure and cell-function. The know- 

 ledge of physical phenomena associated with solution, 

 and especially with the solution of jelly-like sub- 

 stances or colloids, was rapidly applied to physio- 

 logical problems, while the phenomena of nerve were 

 found to have electrical concomitants. It came to be 

 believed that physiology was but a special case of 

 " the physics of colloids and the chemistry of the 

 proteids." And, whatever be the truth about the 

 whole physiological problem, and the psychological 

 and metaphysical questions that underlie it, it is 

 necessary for the increase of knowledge to assume 

 that, at each step, the process is understandable 

 by those natural principles which have already 

 been elucidated, and find their best ultimate state- 

 ment in the fundamental concepts of physics and 

 chemistry. 



