Full Idea
Substitutivity 'salve veritate' cannot define identity since two expressions may be everywhere intersubstitutable and not refer at all.
Clarification
'Salve veritate' means truth-preserving
Gist of Idea
Substitutivity won't fix identity, because expressions may be substitutable, but not refer at all
Source
Ruth Barcan Marcus (Nominalism and Substitutional Quantifiers [1978], p.167)
Book Reference
'Philosophy of Logic: an anthology', ed/tr. Jacquette,Dale [Blackwell 2002], p.167