34 CRITICISM OF MECHANISTIC THEORY 



times, is so familiar that its extraordinary 

 character, from the physical and chemical 

 standpoint, usually escapes attention. 



It may be urged that the immense complex 

 ity and delicacy of the physical and chemical 

 conditions on which physiological responses 

 depend does not in itself furnish any reasons 

 for doubting the mechanistic theory of life. 

 This is doubtless true ; but the point must be 

 emphasised that in the case of stimulus and 

 response there is in reality no experimental 

 evidence whatsoever that the process can be 

 understood as one of physical and chemi 

 cal causation. In the case of physiological 

 \ stimulus and response no real quantitative 

 / relation can be traced between the supposed 

 I physical or chemical cause, and its effect. 

 When we attempt to trace a connection we 

 are lost in an indefinite maze of complex con 

 ditions, out of which the response emerges. It 

 is of little use to point out that in many cases 

 the determining cause of a physical or chemical 

 change may be something very small as com 

 pared with its effect. The turning of a switch, 

 or opening of a valve, or application of a tiny 

 spark, may, for instance, produce gigantic 



