Any ideas on how to do this one?

Let A be a set. Show that a complement of A does not exist. So I need to show that there isn't a set of all x not in A.

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- Sep 10th 2009, 09:48 AMspaceship42Axioms
Any ideas on how to do this one?

Let A be a set. Show that a complement of A does not exist. So I need to show that there isn't a set of all x not in A. - Sep 10th 2009, 09:55 AMPlato
- Sep 10th 2009, 09:57 AMspaceship42
- Sep 10th 2009, 10:13 AMPlato
- Sep 10th 2009, 12:15 PMspaceship42
Oh, I'm sorry. I forgot that I'd called this thread "Axioms". This problem is about sets, not axioms. I mislabeled it accidentally.

- Sep 10th 2009, 12:28 PMTaluivren
Hi,

is a set and ?

Assuming axioms of ZF set theory, can't be a set: If were a set, then since is a set, axiom of pairing gives that is a set. Axiom of union then tells us that that there exist a set whose elements are elements of and elements of . But from the definition of we get that is the universal class of all sets, which is a proper class, i.e. is not a set. This is a contradiction, thus can't be a set.

Can you see why the class of all sets is not a set?