THE PERCEPTION OF SIGHT. 301 



fields would be completely established by direct sensation, 

 as soon as the observer had got his two fixed points to 

 coincide and a single raeridian of one eye to coincide 

 with the corresponding one of the other. 



The reader sees how this series of facts has driven us 

 by force to the empirical theory of vision. It is right to 

 mention that lately fresh attempts have been made to 

 explain the origin of our perception of solidity and the 

 phenomena of single and double binocular vision by the 

 assumption of some ready-made anatomical mechanism. 

 We cannot criticise these attempts here : it would lead 

 us too far into details. Although many of these hypo- 

 theses are very ingenious (and at the same time very 

 indefinite and elastic), they have hitherto always proved 

 insufiicient ; because the actual world offers us far more 

 numerous relations than the authors of these attempts 

 could provide for. Hence, as soon as they have arranged 

 one of their systems to explain any particular phe- 

 nomenon of vision, it is found not to answer for any 

 other. Then, in order to help out the hypothesis, 

 the very doubtful assumption has to be made that, in 

 these other cases, sensation is overcome and extinguished 

 by opposing experience. But what confidence could we 

 put in any of our perceptions if we were able to extinguish 

 our sensations as we please, whenever they concern an 

 object of our attention, for the sake of previous concep- 

 tions to which they are opposed ? At any rate, it is clear 

 that in every case where experience must finally decide, 

 we shall succeed much better in forming a correct notion 

 of what we see, if we have no opposing sensations to 

 overcome, than if a correct j udgment must be formed in 

 spite of them. 



It follows that the hypotheses which have been suc- 

 cessively framed by the various supporters of intuitive 



