TIME 63 



THRESHOLD 



In order to expose the causes of these laws, we must 

 have recourse to the idea of threshold, as originated by Weber. 

 Threshold means the just perceptible difference between two 

 intensities of a quality. It can be used in the same 

 way, however, to mean the difference perceptible be- 

 tween two qualities. If we compare together two adjacent 

 local signs, it appears that the difference between them is so 

 slight as to be inappreciable, i.e. it lies below the threshold. 

 If that were not so, and if each local sign lay alongside the 

 next without there being any intermediate steps, the whole 

 world would consist of coloured points. It is only because 

 the difference between two adjacent local signs is imper- 

 ceptible that the world-picture is continuous, for continuity 

 means nothing but an imperceptible transition, in contrast 

 to one that is abrupt. 



The same holds good for direction-signs, whether they be 

 objective or subjective. It is merely because the difference 

 between two adjacent direction-signs lies below the threshold 

 that a movement appears to be continuous. 



The continuity of time depends also on this state of things. 

 If the moment-signs were perceptibly separated from one 

 another, our life would proceed by tiny, separate jerks. 



For a continuous movement to be appreciable at all, 

 several conditions must be fulfilled. Not only must the 

 movement include more than two direction-signs if it is to be 

 perceptible, but the direction-signs must stand in the right 

 relation to the moment-signs. If all the direction-signs are 

 comprehended within two moment-signs, then the whole path 

 traversed is interpreted as a synchronous unity, and the move- 

 ment is suppressed. 



But even if there be a sufficiently large number of moment- 

 signs as well as of direction-signs, the movement may yet pass 



