Double and Inverted Images on the Retince. 479 



Now, no disciple of Reid or Stewart can have any hesitation 

 about admitting, that this principle of Intuition is part of the 

 cause of all the information we derive from this or any other 

 sense. I hold it to be equally certain, that we learn some things 

 intuitively, we know not how, as that we do some things instinc- 

 tively, we know not why. And, admitting the principle of Intui- 

 tion, it is impossible for us to say a priori, without special inves- 

 tigation of any alleged case, how far it may extend, or how much 

 of the information which we habitually acquire by the senses, is 

 explicable in no other way. But it is obvious, that this must be 

 our last resource in attempting to account for these phenomena ; 

 and it is unphilosophical to assume, that the limit to our curio- 

 sity is to be found on the very threshold of our inquiry. 



Dr Reid has stated this with his usual candour and preci- 

 sion. After observing that he could trace the phenomenon of cor- 

 rect vision by inverted images no farther than to the law of visi- 

 ble direction above stated, he adds the following words, which 

 may be taken as the groundwork of any farther speculations on 

 this subject. " We acknowledge that the retina is not the last 

 and most immediate instrument of the mind in vision. If ever 

 we come to know the structure and use of the choroid membrane, 

 the optic nerves and brain, and what impressions are made on them 

 by means of the pictures on the retince, some more links of the chain 

 may be brought within our view, and a more general law of vision 

 be discovered." — Inquiry, Sfc. ch. vi. § 12. 



I apprehend, then, that two facts are established, — are not to 

 be explained by experience or association, — and, not being neces- 

 sarily ultimate facts, afford a fair subject of physiological inquiry. 

 1. That images formed 071 corresponding points of the retince of the 

 human eyes, and on those only, naturally affect our minds in the 

 same manner as a single image formed on the retina of one eye ; 

 and, 2. That impressions made on different points of the retina of 

 the eye are naturally followed by inferences, as to the relative posi- 

 tion of the objects producing these impressions, exactly opposite to 



