[FOM] Proli's Question about Set Theory
Aatu Koskensilta
aatu.koskensilta at xortec.fi
Tue Feb 28 03:46:26 EST 2006
On Feb 27, 2006, at 4:57 PM, Andrea Proli wrote:
> Two questions:
> 1) Why does the non-existence of a set of all sets create troubles with
> the fact that the domain (I suppose by "domain" you mean the universe
> of
> discourse) of a model is a set?
In the intended interpretation of the language of set theory the
quantifiers range over all sets so the universe of the intended
interpretation is the collection of all sets. However, in model theory
one usually requires the universe of a model to be a set, and there is,
provably in ZF(C), no set of all sets (and there are no structures
containing non-set collections, or indeed such collections at all, in
ZF(C)). Thus the intended interpretation of ZF(C) is not a model. ZF(C)
does have natural models which are of form <V_kappa, {<x,y> | x in y,
x,y in V_kappa}> where V_kappa is the kappath level of the cumulative
hierarchy with kappa strongly inaccessible. The existence of these
models is not provable in ZF(C), and they do not define the
interpretation of the language of set theory.
> 2) What do you say that ZF implies that there is "no set of ordered
> pairs
> that can serve as the interpretation of the membership predicate"?
It just means that the collection E={<x,y> | x in y} is not a set. E is
the intended interpretation of the epsilon predicate symbol in the
langauge of set theory. If E were a set, the collection of all sets
would also be set, which it, provably in ZF(C), is not.
Aatu Koskensilta (aatu.koskensilta at xortec.fi)
"Wovon man nicht sprechen kann, darüber muss man schweigen"
- Ludwig Wittgenstein, Tractatus Logico-Philosophicus
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