Full Idea
An 'event' (in a statement of the 'law of causation') is intended to be something that is likely to recur, since otherwise the law becomes trivial. It follows that an 'event' is not some particular, but a universal of which there may be many instances.
Gist of Idea
In causal laws, 'events' must recur, so they have to be universals, not particulars
Source
Bertrand Russell (On the Notion of Cause [1912], p.179)
Book Reference
Russell,Bertrand: 'Mysticism and Logic' [Unwin 1989], p.179
A Reaction
I am very struck by this. It may be a key insight into understanding what a law of nature actually is. It doesn't follow that we must be realists about universals, but the process of abstraction from particulars is at the heart of generalisation.