Combining Philosophers

Ideas for H.Putnam/P.Oppenheim, Robert C. Stalnaker and Peter Goldie

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4. Formal Logic / F. Set Theory ST / 4. Axioms for Sets / b. Axiom of Extensionality I
In modal set theory, sets only exist in a possible world if that world contains all of its members [Stalnaker]
     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.
     From: Robert C. Stalnaker (Mere Possibilities [2012], 2.4)
     A reaction: Does this mean there can be no set containing all of my ancestors and future descendants? In no world can we coexist.