Full Idea
It is easy to see that intersubstitutability salva veritate is not a sufficient condition for synonymy. 'Renate' (with kidney) and 'cordate' (with heart) can be substituted in a purely extensional language, but are plainly not synonymous.
Clarification
'Intersubstitutability salva veritate' meaning swapping without changing the truth value
Gist of Idea
'Renate' and 'cordate' have identical extensions, but are not synonymous
Source
report of Willard Quine (Two Dogmas of Empiricism [1953]) by Alexander Miller - Philosophy of Language 4.2
Book Reference
Miller,Alexander: 'Philosophy of Language' [UCL Press 1998], p.117
A Reaction
This seems to be a key example (along with Hesperus, and many others) in mapping out synonymy, meaning, analyticity, sense, reference, extension, intension, and all that stuff.
Related Idea
Idea 20295 All analytic truths can become logical truths, by substituting definitions or synonyms [Frege, by Rey]