referential opacity
 
 
Two terms are referentially opaque if they cannot be substituted salva veritate (i.e. without changing the truth value of the statement).
 

Details:
 
Following Quine, we can say two terms are referentially opaque if they cannot be substituted salva veritate (i.e. without changing the truth value of the statement). Likewise, mental states are thought to be, at least sometimes, referentially opaque in that one description (or ascription) of the belief might be true, but another description be false. That is, a person might believe something to be the case, yet not have similar beliefs for co-referring beliefs.
 
For example, Bob might believe that George Elliot wrote The Mill on the Floss, but not believe, even disbelieve, that Mary Anne Evans wrote The Mill on the Floss. Similarly, it might be true to describe Bob's belief by saying that, "He believes that George Elliot wrote The Mill on the Floss." However, it might be false to describe Bob's belief by saying that, "He believes that Mary Anne Evans wrote The Mill on the Floss." Of course, George Elliot was just the nom de plume of Mary Anne Evans. So, the two names have the same referent. Nevertheless, they cannot be used interchangibly in belief descriptions without effecting the truth-values of the descriptions. Nor can one suppose that a person believes all or any co-referring beliefs on the basis of their believing something.
 
 
Charles Wallis