Full Idea
Events have their essences built in, in the form of necessary conditions for their occurrence.
Gist of Idea
Events have inbuilt essences, as necessary conditions for their occurrence
Source
David Lewis (Events [1986], III)
Book Reference
Lewis,David: 'Philosophical Papers Vol.2' [OUP 1986], p.247
A Reaction
Revealing. He thinks the essence of an event is something which precedes the event. I take it as obvious that if an event has an essence, it will be some features of the event that occur in it and during it. They need to be intrinsic.