^ AN ESSAY ON 



as equivalent to the following one : Whence 

 comes it, that the mind should be affected with 

 only one perception from two impressions upon 

 the external organs of sight, since either of 

 those impressions is, of itself, sufficient to 

 produce a similar perception ? Their universal 

 answer has been : Because the two impressions 

 are united before they are communicated to 

 the mind. And the only difference among 

 these authors, has been with respect to the 

 manner in which such an union takes place. 

 To the second class are to be referred the 

 opinions of those, who hold it as certain, that 

 an object is seen single by both eyes, because 

 it is seen by each of them in the same external 

 place ; and who profess to point out some law, 

 or constant rule of vision, from which this 

 sameness of place is to be derived as a neces- 

 sary consequence. Aguilonius, I believe, first 

 gave this view of the question, which has since 

 been adopted by Dechales, Dr. Porterfield, 

 Dr. Smith of Cambridge, and Dr. Reid of 

 Glasgow. 



In opposition to the opinions of the first 

 class, more especially as they have been re- 

 peatedly examined by others, I think I need 

 only say, that they must all be considered as 

 mere conjectures, founded upon certain sup- 

 posed changes in the brain and nerves, the 



