Full Idea
It is a widely accepted thesis that the logic of metaphysical necessity is S5.
Gist of Idea
The logic of metaphysical necessity is S5
Source
Ian Rumfitt (Logical Necessity [2010], §5)
Book Reference
'Modality', ed/tr. Hale,B/Hoffman,A [OUP 2010], p.61
A Reaction
Rumfitt goes on to defend this standard view (against Dummett's defence of S4). The point, I take it, is that one can only assert that something is 'true in all possible worlds' only when the worlds are all accessible to one another.