Full Idea
A counterfactual is non-vacuously true iff it takes less of a departure from actuality to make the consequent true along with the antecedent than it does to make the antecedent true without the consequent.
Gist of Idea
For true counterfactuals, both antecedent and consequent true is closest to actuality
Source
David Lewis (Causation [1973], p.197)
Book Reference
'Causation', ed/tr. Sosa,E. /Tooley,M. [OUP 1993], p.197
A Reaction
Almost every theory proposed by Lewis hangs on the meaning of the word 'close', as used here. If you visited twenty Earth-like worlds (watch Startrek?), it would be a struggle to decide their closeness to ours in rank order.