THE PERCEPTION OF SPACE. 269 



common measure, and adding them together into the single 

 all-including space of tlie real world. 



Both the measuring and the adding are performed by 

 the aid of things. 



The imagined aggregate of positions occupied by all the 

 actual or possible, moving or stationary, things which we 

 know, is oiir notion of ' real ' space — a very incomplete 

 and vague conception in all minds. 



The measuring of our space-feelings against each other 

 mainly comes about through the successive arousal of dif- 

 ferent ones by the same thing, by our selection of certain 

 ones as feelings of its real size and shape, and by the deg- 

 radation of others to the status of being merely signs of 

 these. 



For the successive application of the same thing to dif- 

 ferent space-giving surfaces motion is indispensable, and 

 hence plays a great part in our space-education, especially 

 in that of the eye. Abstractly considered, the motion of 

 the object over the sensitive surface would educate us quite 

 as well as that of the surface over the object. But the self- 

 mobility of the organ carrying the surface accelerates im- 

 mensely the result. 



In completely educated space-perception, the present 

 sensation is usually just what Helmholtz (Physiol. Optik, 

 p. 797) calls it, ' a sign, the interpretation of whose mean- 

 ing is left to the understanding.' But the understanding is 

 exclusively reproductive and never productive in the pro- 

 cess ; and its function is limited to the recall of previous 

 space-sensations with which the present one has been as- 

 sociated and which may be judged more real than it. 



Finally, this reproduction may in the case of certain 

 visual forms be as vivid, or almost so, as actual sensation is. 



The third dimension forms an original element of all 

 our space-sensations. In the eye it is subdivided by various 

 discriminations. The more distant subdivisions are often 

 shut out altogether, and, in being suppressed, have the 

 effect of diminishing the absolute space-value of the total 

 field of view.* 



* This shrinkage and expansion of the absolute space-value of the total 

 optical sensation remains to my mind the most obscure part of the whole 



