Full Idea
A may be the cause of B even if there actually are cases of B not following A. Striking a match will be the cause of its igniting, in spite of the fact that some matches are damp and fail to ignite.
Gist of Idea
Striking a match causes its igniting, even if it sometimes doesn't work
Source
Bertrand Russell (On the Notion of Cause [1912], p.185)
Book Reference
Russell,Bertrand: 'Mysticism and Logic' [Unwin 1989], p.185
A Reaction
An important point, although defenders of the constant conjunction view can cope with it. There is a further regularity between dampness of matches and their failure to strike.