Full Idea
We may define the cause of a phenomenon to be the antecedent, or the concurrence of the antecedents, on which it is invariably and unconditionally consequent.
Gist of Idea
A cause is an antecedent which invariably and unconditionally leads to a phenomenon
Source
John Stuart Mill (System of Logic [1843], 3.05.6)
Book Reference
Mill,John Stuart: 'System of Logic (9th ed, 2 vols)' [Longmans, Green etc 1875], p.392
A Reaction
This ignores the possibility of the world ending just before the effect occurs, the 'ceteris paribus' clause. If it only counts as a cause if the effect has actually occurred, we begin to suspect tautology.