SPACE 21 



direction-signs tell us about movement in the outside world ; 

 they come into being with the sounding out and dying away 

 of adjacent local signs: 



Helmholtz's experiment shows that the two kinds of 

 direction-sign which we have distinguished from one another, 

 are really identical, for only identical but opposed signs can 

 annul one another. 



We also arrived at this conclusion from our discussion of 

 what happened when we slid the balls of our fingers along 

 the edge of the table ; but with the movements of our limbs, 

 the sensations in the muscles and tendons play so large a part 

 that we cannot lose the feeling of our own movement. With 

 the eye, however, the subsidiary feelings become quite incon- 

 spicuous ; and so here the opposed signs can completely 

 cancel one another, and we get an impression of rest. 



It is only through the eye that we become certain that 

 motion is not merely a derived concept, but that, just as 

 space is the form of the co-existence of the direction-signs, 

 so motion represents the form of the sequence of the direc- 

 tion signs ; accordingly the appearance of the first direction- 

 signs presupposes space as well as time. 



The production of a second kind of illusory movements 

 of objects depends on the assumption that each retina is like 

 a chess-board on which a game of chess is being played. 

 How is it that the two games simultaneously played come to 

 our consciousness as one ? This is explained by identical 

 fields of the two chess-boards having the same local signs ; 

 and two different things can never happen simultaneously at 

 the same spot. Now, while looking at an object, turn one 

 eye slightly outwards (it is not every one who can do this) ; 

 the fields of sight will shift towards one another, and the 

 outline of the object, which in the one eye keeps its original 

 place, in the other begins to shift. The shifting is not per- 

 ceived through the outward movement of the eye that has 



