THE PERCEPTION OF SIGHT. 267 



It is plain that, by the experience which we collect in the 

 way I have been describing, we are able to learn as much cf the 

 meaning of sensible 'signs' as can afterwards be verified by 

 further experience ; that is to say, all that is real and positive in 

 our conceptions. 



It has been hitherto supposed that the sense of touch confers 

 the notion of space and movement. At first, of course, the only 

 direct knowledge we acquire is that we can produce by an act 

 of volition, changes of which we are cognisant by means of touch 

 and sight. Most of these voluntary changes are movements, or 

 changes in the relations of space; but we can also produce 

 changes in an object itself. Now, can we recognise the move- 

 ments of our hands and eyes as changes in the relations of space 

 without knowing it beforehand 1 and can we distinguish them 

 from other changes which affect the properties of external 

 objects ? I believe we can. It is an essentially distinct cha- 

 racter of the relations of Space that they are changeable rela- 

 tions between objects which do not depend on their quality 

 or quantity, while all other material relations between objects 

 depend upon their properties. The perceptions of sight prove 

 this directly and easily. A movement of the eye which 

 causes the retinal image to shift its place upon the retina always 

 produces the same series of changes as often as it is repeated, 

 whatever objects the field of vision may contain. The effect is 

 that the impressions which had before the local signs , a 1? 2 

 3 , receive the new local signs b , 6,, b 2 ,b 3 , and this may always 

 occur in the same way, whatever be the quality of the impres- 

 sions. By this means we learn to recognise such changes as 

 belonging to the special phenomena which we call changes in 

 space. This is enough for the object of Empirical Philosophy, 

 and we need not further enter upon a discussion of the 

 question, how much of universal conceptions of space is de- 

 rived a priori, and how much a posteriori. 1 



of progressive science to Zoology, has been published by M. Lacaze Duthkrs, 

 in the first number of his Archives de Zoologie. TK. 



1 The question of the origin of our conceptions of space is discussed by Mr. 

 Bain on empirical principles in his treatise on The Senses and the Intellect, pp. 

 114-118, 189-194, 245, 363-392, <tc. TR. 



