Full Idea
Not all events involve change. We cannot afford to count the unchanges as nonevents, for the unchanges may be needed to complete causal histories.
Gist of Idea
Some events involve no change; they must, because causal histories involve unchanges
Source
David Lewis (Events [1986], VI)
Book Reference
Lewis,David: 'Philosophical Papers Vol.2' [OUP 1986], p.261
A Reaction
You end up calling non-changes 'events' if you commit to a simplistic theory that all causal histories consist of events. Why not allow conditions as well as events? Lewis concedes that he may be abusing language.