Full Idea
What we need to know, in order to know what is empirically necessary, is some empirical fact plus our conventions that tell us which truths are necessary given which empirical facts.
Gist of Idea
To know empirical necessities, we need empirical facts, plus conventions about which are necessary
Source
Alan Sidelle (Necessity, Essence and Individuation [1989], Ch.4)
Book Reference
Sidelle,Alan: 'Necessity, Essence and Individuation' [Cornell 1989], p.100
A Reaction
I take this attack on a posteriori necessities to be the most persuasive part of Sidelle's case, but you can't just put all of our truths down to convention. There are stabilities in the world, as well as in our conventions.