302 RECENT PROGRESS OF THE THEORY OF VISION. 



theories of vision, in order to suit one phenomenon after 

 another, are really quite unnecessary. No fact has 

 yet been discovered inconsistent with the Empirical 

 Theory: which does not assume any peculiar modes 

 of physiological action in the nervous system, nor any 

 hypothetical anatomical structures ; which supposes no- 

 thing more than the well known association between the 

 impressions we receive and the conclusions we draw from 

 them, according to the fundamental laws of daily ex- 

 perience. It is true that we cannot at present offer any 

 complete scientific explanation of the mental operations 

 involved, and there is no immediate prospect of our doing 

 so. But since these operations actually exist, and since 

 hitherto every form of the intuitive theory has been 

 obliged to fall back on their reality when all other 

 explanation failed, these mysteries of the laws of thought 

 cannot be regarded from a scientific point of view as con- 

 stituting any deficiency in the empirical theory of vision. 



It is impossible to draw any line in the study of our 

 perceptions of space which shall sharply separate those 

 which belong to direct Sensation from those which are 

 the result of Experience. If we attempt to draw such 

 a boundary, we find that experience proves more minute, 

 more direct and more exact than supposed sensation, 

 and in fact proves its own superiority by overcoming the 

 latter. The only supposition which does not lead to any 

 contradiction is that of the Empirical Theory, which 

 regards all our perceptions of space as depending upon 

 experience, and not only the qualities, but even the 

 local signs of the sense of sight as nothing more than 

 signs, the meaning of which we have to learn by ex- 

 perience. 



We become acquainted with their meaning by com- 

 paring them with the result of our own movements, with 



