Full Idea
Lewis gives an account of causation in terms of counterfactual conditionals (roughly, an event c causes an event e iff if c had not happened then e would not have happened either).
Clarification
'Counterfactuals' are statements beginning "If.." - events that might have happened, but didn't
Gist of Idea
An event causes another just if the second event would not have happened without the first
Source
report of David Lewis (works [1973]) by Stathis Psillos - Causation and Explanation Intro
Book Reference
Psillos,Stathis: 'Causation and Explanation' [Acumen 2002], p.5
A Reaction
This feels wrong to me. It is a version of Humean constant conjunction, but counterfactuals are too much a feature of our minds, and not sufficiently a feature of the world, to do this job. Tricky.