Full Idea
According to Lewis, a counterfactual holds when the consequent is true in possible worlds very like our own except for the fact that the antecedent is true.
Gist of Idea
In good counterfactuals the consequent holds in world like ours except that the antecedent is true
Source
report of David Lewis (Counterfactuals [1973]) by Paul Horwich - Lewis's Programme p.213
Book Reference
'Causation', ed/tr. Sosa,E. /Tooley,M. [OUP 1993], p.213
A Reaction
Presumably the world being very like our own would make it unlikely that there would be anything else to cause the consequent, apart from the counterfactual antecedent.