PROFESSOR AT KONIGSBERG 141 



mechanical in character, and comes under the category of 

 involuntary combinations of ideas, arising when two percepts 

 are frequently associated together. Thus in optical delusions, 

 the mechanism of which is evident, we are aware that the 

 idea called up by the sense-impression is false, but the idea 

 nevertheless persists in full vigour. When, given a certain 

 position of the eyes, any object excites a sensation of light 

 in certain nerve-fibres of our two eyes, our past experience 

 that it is necessary to stretch the arm out a certain distance, 

 or to take a certain number of steps in order to reach it, 

 has established an involuntary relation between the given 

 visual impression and its distance and direction : the judgement 

 of distance by the eyes is learned empirically. ' I distinctly 

 remember the moment at which I became aware of the law 

 of perspective, that distant objects look smaller. I was taken 

 past a high tower, on the topmost gallery of which people 

 were standing, and begged my mother to lift down the little 

 puppets, as I quite thought she could reach the gallery of 

 the tower by stretching out her arm. Afterwards I often 

 looked up at the gallery of the same tower when people 

 were standing on it, but nevermore to the eye of experience 

 \ did they look like pretty dolls/ When once we have learned 

 to see, i. e. to associate the idea of a certain object with 

 certain sensations, which we perceive, the seat of the optical 

 image upon the retina is a matter of indifference, since it 

 is only the fibres of the optic nerve which are excited that 

 are in question. 



Helmholtz does not here attempt to decide the question of 

 how far acquired associations of ideas, or such as are innate, 

 i.e. implicate in the actual organization of man, are involved 

 in the interpretation of our sense-organs. For him, sensations 

 are mere signals to consciousness, the meaning of which we 

 learn by a mental process, signals of the existence of changes 

 in the external world, but not images of these changes except 

 in so far as they represent the sequence of the same in 

 time, and may therefore be considered to give us a direct 

 representation of the temporal course of natural phenomena. 



tit was only at a later time that he came forward as the 

 champion of the empiricist, as against the nativistic theory, 

 though he had already made a great stride in this direction. 



