From page xli of the CPBR volume 5 - in a letter of 13 June 1905 to Lucy Donnelly BR wrote:
"For a long time I have been at intervals debating this conundrum: If two names or descriptions apply to the same object, whatever is true of the one is true or the other. Now George IV wished to know whether Scott was the author of Waverly. Hence, putting "Scott" on the place of "the author of Waverly", we find that George IV wished to know whether Scott was Scott, which implies more interest in the Laws of Thought than was possible for the First Gentleman of Europe. This little puzzle was quite hard to solve; the solution, which I have now found, throws a flood of light on the foundations of mathematics and the whole problem of the relation of thought to things."
The solution was published on "On Denoting".
From "On Denoting" - page 423 of CPBR volume 4, BR wrote:
'According to the view which I advocate, a denoting phrase is essentially part of a sentence, and does not, like most single words, have any significance on its own account. If I say "Scott was a man", that is a statement of the form "x was a man", and it has "Scott" for its subject". But if I say "the author of Waverly was a man", that is not a statement of the form "x was a man", and does not have "the author of Waverly for its subject. ... , we may put, in place of "the author of Waverly was a man" the following: "One and only one entity wrote Waverly, and that one was a man".
... And speaking generally, suppose we wish to say that the author of Waverly had the property F, what we wish to say is equivalent to "One and only one entity wrote Waverly, and that one had the property F."
... "Scott was the author of Waverly" (i.e. "Scott was identical with the author of Waverly") becomes "One and only one entity wrote Waverly, and Scott was identical with that one"; or reverting to the wholly explicit form: "It is not always false of x that x wrote Waverly, that it is always true of y that if y wrote Waverly y is identical with x, and that Scott is identical with x."
From Quine - Word and Object page 142:
"(1) 'Tully was a Roman' is trochaic.
When a singular term is used in a sentence purelt to specify its object, and the sentence is true of the object, then certainly the sentence will stay true when any other singular term is substituted that designates the same object. Here we have a criterion for what may be called purely referential position: the position musy be subject to the substituitivity of identity. That position of 'Tully' in (1) is not purely referential is reflected in the falsity of what we get by supplanting 'Tully' in (1) by 'Cicero'.