Full Idea
One event is the cause of another iff there exists a causal chain leading from the first to the second.
Gist of Idea
One event causes another iff there is a causal chain from first to second
Source
David Lewis (Causation [1973], p.200)
Book Reference
'Causation', ed/tr. Sosa,E. /Tooley,M. [OUP 1993], p.200
A Reaction
It will be necessary to both explain and identify a 'chain'. Some chains are extremely tenuous (Alexander could stop a barrel of beer). Go back a hundred years, and the cause of any present event is everything back then.