On 09 May 2012, at 13:19, R AM wrote:

On Wed, May 9, 2012 at 12:48 PM, Bruno Marchal <marc...@ulb.ac.be> wrote: The empty set is the absence of elements (nothing) in that set. It is the set { }. The empty set is not nothing. For example, the set is { { } } is not empty. It contains as element the empty set.
Just to be precise.

Well, I guess that the empty set is more like an empty box.

"Nothing", in set theory, would be more like an empty *collection* of sets, or an empty "universe" (a model of set theory), except that in first order logic we forbid empty models (so that AxP(x) -> ExP(x) remains valid, to simplify life (proofs)).



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