Full Idea
The extension of a property in any given world is just a contingent fact about that world; its extension is not the essence of the property.
Gist of Idea
The extension of a property is a contingent fact, so cannot be the essence of the property
Source
Brian Ellis (Scientific Essentialism [2001], 2.07)
Book Reference
Ellis,Brian: 'Scientific Essentialism' [CUP 2007], p.87
A Reaction
The Quinean idea, common among logicians, that a predicate is just a set defined for some model, may be useful in the logic, but is preposterous as an account of what a property actually is in nature, even if the set covers possible worlds.