20 AN ESSAY ON 



which is a sufficient reason for rejecting every 

 conclusion that depends upon its truth. 



Admitting, however, that objects are repre- 

 sented single, when their pictures fall upon the 

 centres of the retinas, or upon any other two 

 points which are equally distant from the cen- 

 tres, and both upon the same side, it appears to 

 me, notwithstanding, to be in violation of all 

 analogy, to ascribe this effect, with respect to 

 the points, at least, on the right and left sides 

 of the centres, to any peculiar property which 

 they possess from nature. For when anatomists 

 find, in a new species of animals, organs similar 

 in structure to those of others they are already 

 acquainted with, they immediately conclude, 

 that they are also similar in regard to their use. 

 In animals of the same species, they believe 

 with certainty, that the organs they see in one 

 have the same properties, as the corresponding 

 organs of another ; and, if it be possible, they 

 attribute with greater certainty the same pro- 

 perties to two organs of the like kind, which 

 are found in the same individual. Such is the 

 influence of the rule, that resemblance of pro- 

 perty is implied by resemblance of structure. 

 Now it is an universal fact, that if an animal 

 be divided into a right and left half, the cor- 

 responding parts of those organs, which exist in 

 pairs, are found at equal distances from the plaae 



