SINGLE VISION. 27 



as follows : Since visible place contains in it 

 both visible distance and visible direction, it is 

 not necessary that the single appearance of an 

 object, to both eyes, should depend altogether 

 either upon custom, or an original principle of 

 our constitution ; for its visible distance to each 

 eye may be learned from feeling, and its visible 

 direction be given by nature ; in which case, 

 the unity of its place to the two eyes, will be 

 owing to neither of those causes singly, but to 

 a combination of both ; and this I regard as a 

 sufficient reply. 



