Ideas from 'Existence,Ontological Commitment and Fictions' by Peter van Inwagen [2003], by Theme Structure

[found in 'The Oxford Handbook of Metaphysics' (ed/tr Loux,M /Zimmerman,D) [OUP 2005,0-19-928422-9]].

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4. Formal Logic / F. Set Theory ST / 8. Critique of Set Theory
What in the real world could ground the distinction between the sets {A,{A,B}} and {B,{A,B}}?
                        Full Idea: Nothing in the world of nominalistically acceptable things could ground or explain the non-identity of the set {A,{A,B}} with the set {B,{A,B}}.
                        From: Peter van Inwagen (Existence,Ontological Commitment and Fictions [2003], p.154)
                        A reaction: [He cites Goodman for this thought] Van Inwagen is offering this to show that the existence of sets is abstract, whereas Goodman was denying the existence of sets altogether. I'm with Goodman. Nice example.