Full Idea
One principle of modal set theory should be uncontroversial: a set exists in a given possible world if and only if all of its members exist at that world.
Gist of Idea
In modal set theory, sets only exist in a possible world if that world contains all of its members
Source
Robert C. Stalnaker (Mere Possibilities [2012], 2.4)
Book Reference
Stalnaker,Robert C.: 'Mere Possibilities' [Princeton 2012], p.38
A Reaction
Does this mean there can be no set containing all of my ancestors and future descendants? In no world can we coexist.