Substitution salva veritate of co-extensional terms can fail in opaque contexts.
[1] The literal translation of the Latin "salva veritate" is "with (or by) unharmed truth", using ablative of manner: "salva" meaning "rescue," "salvation," or "welfare," and "veritate" meaning "reality" or "truth".
Quine takes substitutivity salva veritate to be the same as the "indiscernibility of identicals".
In fact, the whole quantified modal logic of necessity is dependent on context and empty otherwise; for it collapses if essence is withdrawn.
[3] For example, the statements: are true; however, replacement of the name 'Giorgione' by the name 'Barbarelli' turns (2) into the falsehood: Quine's example here refers to Giorgio Barbarelli's sobriquet "Giorgione", an Italian name roughly glossed as "Big George."