276 RECENT PROGRESS OF THE THEORY OF VISION. 



The correspondence, therefore, between the external world 

 and the Perceptions of Sight rests, either in whole or in part, 

 upon the same foundation as all our knowledge of the actual 

 world on experience, and on constant verification of its accuracy 

 by experiments which we perform with every movement of our 

 body. It follows, of course, that we are only warranted in ac- 

 cepting the reality of this correspondence so far as these means 

 of verification extend, which is really as far as for practical pur- 

 poses we need. 



Beyond these limits, as, for example, in the region of 

 Qualities, we are in some instances able to prove conclusively 

 that there is no correspondence at all between Sensations and 

 their Objects. 



Only the relations of time, of space, of equality, and those 

 which are derived from them, of number, size, regularity of 

 coexistence and of sequence ' mathematical relations,' in short 

 are common to the outer and the inner world, and here we 

 may indeed look for a complete correspondence between our con- 

 ceptions and the objects which excite them. 



But it seems to me that we should not quarrel with the 

 bounty of Nature because the greatness, and also the emptiness, 

 of these abstract relations have been concealed from us by the 

 manifold brilliance of a system of signs; since thus they can be 

 the more easily surveyed and used for practical ends, while yet 

 traces enough remain visible to guide the philosophical spirit 

 aright, in its search after the meaning of sensible Images and 

 Signs. 



