Full Idea
Just as a sentence is necessary if it holds in all worlds, so it is causally necessary if it holds in all worlds compatible with the laws of nature.
Gist of Idea
Causal necessities hold in all worlds compatible with the laws of nature
Source
David Lewis (Counterpart theory and Quant. Modal Logic [1968], V)
Book Reference
Lewis,David: 'Philosophical Papers Vol.1' [OUP 1983], p.37
A Reaction
I don't believe in the so-called 'laws of nature', so I'm not buying that. Is there no distinction in Lewis's view between those sentences which must hold, and those which happen to hold universally?