Full Idea
A sentence is logically true if all sentences with that grammatical structure are true.
Gist of Idea
A sentence is logically true if all sentences with that grammatical structure are true
Source
Willard Quine (Philosophy of Logic [1970], Ch.7)
Book Reference
Quine,Willard: 'Philosophy of Logic' [Prentice-Hall 1970], p.95
A Reaction
Quine spends some time on the tricky question of deciding which parts of a sentence are grammatical structure ('syncategorematic'), and which parts are what he calls 'lexicon'. I bet there is a Quinean argument which blurs the boundary.