Double and Inverted Images on the Retine. 479 
Now, no disciple of Rerp 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 @ 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 Rex 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 retine, some more links of the chain 
may be brought within our view, and a more general law of vision 
be discovered.” —Inquiry, Sc. 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 on corresponding points of the retine 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 
