100 THE RIDDLE OF THE UNIVERSE. 



the title of psycho-physics. Its founders, the physiolo- 

 gists Theodor Fechner and Ernst Heinrich Weber, 

 first of all closely investigated the dependence of 

 sensations on the external stimuli that act on the 

 organs of sense, and particularly the quantitative 

 relation between the strength of the stimulus and the 

 intensity of the sensation. They found that a certain 

 minimum strength of stimulus is requisite for the 

 excitement of a sensation, and that a given stimulus 

 must be varied to a definite amount before there is 

 any perceptible change in the sensation. For the 

 highest sensations (of sight, hearing, and pressure) 

 the law holds good that their variations are propor- 

 tionate to the changes in the strength of the stimulus. 

 From this empirical " law of Weber " Fechner 

 inferred, by mathematical operations, his " funda- 

 mental law of psycho-physics," according to which 

 the intensity of a sensation increases in arithmetical 

 progression, the strength of the stimulus in geo- 

 metrical progression. However, Fechner's law and 

 other psycho-physical laws are frequently contested, 

 and their " exactness " is called into question. In 

 any case modern psycho-physics has fallen far 

 short of the great hopes with which it was greeted 

 twenty years ago ; the field of its applicability is 

 extremely limited. One important result of its work 

 is that it has proved the application of physical laws 

 in one, if only a small, branch of the life of the 

 " soul " — an application which was long ago postu- 

 lated on principle by the materialist psychology for 

 the whole province of mental life. In this, as in 

 many other branches of physiology, the " exact " 

 method has proved inadequate and of little service. 

 It is the ideal to aim at everywhere ; but it is 



