THE PERCEPTION OF SPACE. Ill 



2. The Measurement of the sense-spaces against eojch other. 



What precedes is all we can say in answer to the problem 

 of discrimination. Turn now to that of measurement of the 

 several spaces against each other, that being the first step 

 in our constructing out of our diverse space-experiences the 

 one space we believe in as that of the real world. 



The first thing that seems evident is that we have no 

 immediate power of comparing together with any accuracy 

 the extents revealed by different sensations. Our mouth- 

 cavity feels indeed to itself smaller, and to the tongue 

 larger, than it feels to the finger or eye, our tympanic 

 membrane feels larger than our finger-tip, our lips feel 

 larger than a surface equal to them on our thigh. So much 

 comparison is immediate ; but it is vague ; and for anything 

 exact we must resort to other help. 



The great agent in comparing the extent felt by one sensory 

 surface luiih that felt by another, is superposition — superposition 

 of one surface upon another, and superposition of one outer 

 thing upon many surfaces. Thus are exact equivalencies and 

 common measures introduced, and the way prepared for 

 numerical results. 



Could we not superpose one part of our skin upon an- 

 other, or one object on both parts, we should hardly suc- 

 ceed in coming to that knowledge of our own form which 

 we possess. The original differences of bigness of our dif- 

 ferent parts would remain vaguely operative, and we should 

 have no certainty as to how much lip was equivalent to so 

 much forehead, how much finger to so much back. 



But with the power of exploring one part of the surface 

 by another we get a direct perception of cutaneous equiva- 

 lencies. The primitive differences of bigness are over- 

 powered when we feel by an immediate sensation that a 

 certain length of thigh-surface is in contact with the entire 

 palm and fingers. And when a motion of the opposite finger- 

 tips draws a line first along this same length of thigh and 



measuring of objects in terms of some common standard we shall see pres- 

 ently how movements help also ; but no more in this case than the other 

 do they help, because the quality of extension itself is contributed by ihe 

 'muscular sense.' 



