Full Idea
The Humean idea, developed by Lewis, is that rather than being sufficient for their effects, causes are (counterfactual) necessary for their effects.
Gist of Idea
The counterfactual view says causes are necessary (rather than sufficient) for their effects
Source
report of David Lewis (Causation [1973]) by Alexander Bird - Causation and the Manifestation of Powers p.162
Book Reference
'The Metaphysics of Powers', ed/tr. Marmodoro,Anna [OUP 2013], p.162