THE PERCEPTION OF SPACE. Ill 



tion wliicli is particularly exciting, and tliat is the feeling 

 of motion over any of our surfaces. The erection of this 

 into a separate elementary quality of sensibility is one of 

 the most recent of psychological achievements, and is 

 worthy of detaining us a while at this point. 



Tlie Sensation of 3Iotion over Surfaces. 



The feeling of motion has generally been assumed by 

 physiologists to be impossible until the positions of terminus 

 a quo and termimis ad quern, are severally cognized, and the 

 successive occupancies of these positions by the moving 

 body are perceived to be separated by a distinct interval of 

 time.'^ As a matter of fact, however, we cognize only the 

 very slowest motions in this way. Seeing the hand of a 

 clock at XII and afterwards at VI, we judge that it has 

 moved through the interval. Seeing the sun now in the 

 east and again in the west, I infer it to have passed over 

 my head. But we can only infer that which we already 

 generically know in some more direct fashion, and it is ex- 

 perimentally certain that we have the feeling of motion 

 given us as a direct and simple sensation. Czermak long ago 

 pointed out the difference between seeing the motion of the 

 second-hand of a watch, when we look directly at it, and 

 noticing the fact of its having altered its position when we 

 fix our gaze upon some other point of the dial-plate. In 

 the first case we have a specific quality of sensation which 

 is absent in the second. If the reader will find a portion 

 of his skin — the arm, for example — where a pair of com- 

 pass-points an inch apart are felt as one impression, and if 

 he will then trace lines a tenth of an inch long on that spot 

 with a pencil-point, he will be distinctly aware of the point's 

 motion and vaguely aware of the direction of the motion. 

 The perception of the motion here is certainly not derived 

 from a pre-existing knowledge that its starting and ending 

 points are separate positions in space, because positions in 

 space ten times wider apart fail to be discriminated as such 



* This is only another example of what I call ' the psychologist's fal- 

 lacy ' — thinking that the mind he is studying must necessarily be conscious 

 of the object after the fashion in which the psychologist himself is con- 

 scious of it. 



