Full Idea
To claim that mathematical truths are conventional in the sense of following logically from definitions is the claim that mathematics is a part of logic.
Gist of Idea
If mathematics follows from definitions, then it is conventional, and part of logic
Source
Willard Quine (Truth by Convention [1935], p.79)
Book Reference
Quine,Willard: 'Ways of Paradox and other essays' [Harvard 1976], p.79
A Reaction
Quine is about to attack logic as convention, so he is endorsing the logicist programme (despite his awareness of Gödel), but resisting the full Wittgenstein conventionalist picture.