Full Idea
Armstrong says what is actual (namely a certain roster of universals) might have been impossible. Hence his modal logic is S4, without the 'Brouwersche Axiom'.
Gist of Idea
If what is actual might have been impossible, we need S4 modal logic
Source
report of David M. Armstrong (A Theory of Universals [1978]) by David Lewis - Armstrong on combinatorial possibility 'The demand'
Book Reference
Lewis,David: 'Papers in Metaphysics and Epistemology' [CUP 1999], p.202
A Reaction
So p would imply possibly-not-possibly-p.