HERMANN VON HELMHOLTZ 



eyes, the theory of corresponding points, and the 

 mathematical investigation of the geometrical form 

 of the horopter in different circumstances, led 

 Helmholtz to question Kant's doctrine of space. 

 The philosopher of Konigsberg taught that space 

 and time are forms of intuition. Space is the form of 

 external sensibility, that is our sense of the relative 

 positions of things in the outer world ; while time 

 is in the form of internal and external sensibility 

 jointly, that is our sense of the relative sequences 

 of events. On the a priori nature of space depends 

 the validity of geometrical judgments. On the a 

 priori nature of time arithmetical judgments depend. 

 Things in themselves related neither to space nor 

 to time are unknowable to man. Co-existence and 

 succession are only in phenomena and are only in 

 the perceiving subject. Now Helmholtz found in 

 the study of the horopter, that two different sensa- 

 tions arising from the picture on each retina are 

 blended together in consciousness, that this blending 

 cannot be anatomically explained, and, indeed, has 

 no anatomical foundation, and that it is due to a 

 mental act, which is the result of experience. We 

 cannot separate the part due to the immediate sen- 

 sation from the part due to experience. There is 

 not a perfect agreement between the external object 

 and the mental representation, except in a mathe- 

 matical sense. The feeling of localisation in space 

 is not inborn, but is the result of an act of reason 

 260 



