Full Idea
In 'a = b' either 'a' and 'b' are names of the same thing, in which case the proposition says nothing, or of different things, in which case it is absurd. In neither case is it an assertion of a fact; it only asserts when a or b are descriptions.
Gist of Idea
Either 'a = b' vacuously names the same thing, or absurdly names different things
Source
Frank P. Ramsey (The Foundations of Mathematics [1925], §1)
Book Reference
Ramsey,Frank: 'Philosophical Papers', ed/tr. Mellor,D.H. [CUP 1990], p.179
A Reaction
This is essentially Frege's problem with Hesperus and Phosphorus. How can identities be informative? So 2+2=4 is extensionally vacuous, but informative because they are different descriptions.