Full Idea
Typically philosophers say that for P to entail Q is for the proposition that all P's are Q's to be necessary. I think this analysis is backwards, and that necessity rests on entailment, not vice versa.
Gist of Idea
Entailment does not result from mutual necessity; mutual necessity ensures entailment
Source
Michael Jubien (Possibility [2009], 4.4)
Book Reference
Jubien,Michael: 'Possibility' [OUP 2009], p.92
A Reaction
His example is that being a horse and being an animal are such that one entails the other. In other words, necessities arise out of property relations (which for Jubien are necessary because the properties are platonically timeless). Wrong.