Full Idea
A necessary causal condition is closely related to a counterfactual conditional: if no-cause then no-effect, and a sufficient causal condition is closely related to a factual conditional (Goodman's phrase): since cause-here then effect.
Gist of Idea
Necessary conditions are like counterfactuals, and sufficient conditions are like factual conditionals
Source
J.L. Mackie (Causes and Conditions [1965], §4)
Book Reference
'Causation', ed/tr. Sosa,E. /Tooley,M. [OUP 1993], p.48
A Reaction
The 'factual conditional' just seems to be an assertion that causation occurred (dressed up with the logical-sounding 'since'). An important distinction for Lewis. Sufficiency doesn't seem to need possible-worlds talk.