Full Idea
The counterfactual conditional transmits possibility: (A□→B) ⊃ (◊A⊃◊B). Suppose that if A had held, B would also have held; the if it is possible for A to hold, it is also possible for B to hold.
Gist of Idea
Counterfactual conditionals transmit possibility: (A□→B)⊃(◊A⊃◊B)
Source
Timothy Williamson (Modal Logic within Counterfactual Logic [2010], 1)
Book Reference
'Modality', ed/tr. Hale,B/Hoffman,A [OUP 2010], p.82