Full Idea
The strict conditional implies the counterfactual conditional: □(A⊃B) ⊃ (A□→B) - suppose that A would not have held without B holding too; then if A had held, B would also have held.
Gist of Idea
Strict conditionals imply counterfactual conditionals: □(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.81
A Reaction
[He then adds a reading of his formula in terms of possible worlds] This sounds rather close to modus ponens. If A implies B, and A is actually the case, what have you got? B!