96 THE FOURTH DIMENSION. 



we can conceive of its two dimensional surfaces, of its twelve one- 

 dimensional edges, and of its eight no-dimensional corners. For we 

 reach the perception of its surface, for example, solely by reason of 

 the fact that the material particles which form the cube prevent the 

 transmission of light, and reflect it, whereby a part of the light re- 

 flected from every material particle strikes our eye. Now, by think- 

 ing exclusively of those material particles which are reflected, in 

 contrariety to the empty space without and the hidden and there- 

 fore non-reflected particles within, we form the notion of a surface. 

 It is evident from this, that all that we perceive is three-dimen- 

 sional, that we cannot reach anything two-dimensional without 

 an intellectual abstraction, and that, therefore, we cannot conceive 

 of anything two-dimensional exerting effects upon material things. 

 But this fact is a refutation of the retinal argument of Zollner. If 

 vision consisted wholly and exclusively in the creation of a two- 

 dimensional image, the things which take place in the world could 

 never come into our consciousness. The child, therefore, does not 

 originally apprehend the world, as Zollner says, as two-dimensional; 

 on the contrary, it apprehends it either not at all, or it apprehends 

 it as three-dimensional. Of course the child must first "learn how" 

 to see. It is found from the observation of children during the first 

 months of their lives, and of the congenitally blind who have sud- 

 denly acquired the power of vision by some successful operation, 

 that seeing does not consist alone in the irritations which arise in 

 the optic nerves, but also in the correct interpretation of these irri- 

 tations by reason. This correct interpretation, however, can be 

 accomplished only by the accumulation of a considerable stock of 

 experience. Especially must the recognition of the distance of the 

 object seen be gradually learned. In this, two things are especially 

 helpful ; first, the fact that we have two eyes and, consequently, 

 that we must feel two irritations of the optic nerves which are not 

 wholly alike ; and, secondly, the fact that we are enabled by our 

 power of motion and our sense of touch to convince ourselves of 

 the distance and form of the bodies seen. The question now arises, 

 what sort of an intuition of space would a creature have that had 

 only one eye, that could neither move itself nor its eye, and also 





