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Full Idea
If □φ says that φ is true by convention, then □φ would apparently turn out to be contingent, since statements about what conventions we adopt are not themselves true by convention. The main axioms of S4 and S5 would be false.
Gist of Idea
If truths are necessary 'by convention', that seems to make them contingent
Source
Theodore Sider (Writing the Book of the World [2011], 12.1)
Book Ref
Sider,Theodore: 'Writing the Book of the World' [OUP 2011], p.268
Related Idea
Idea 15000 'It is raining' and 'it is not raining' can't be legislated, so we can't legislate 'p or ¬p' [Sider]