From: Dusko Pavlovic <dusko@kestrel.edu>
To: CATEGORIES@mta.ca
Subject: Re: Is Zermelo-Fraenkel set theory inconsistent?
Date: Thu, 01 Apr 1999 12:23:09 -0800 [thread overview]
Message-ID: <3703D5AD.CEE89138@kestrel.edu> (raw)
In-Reply-To: <199904011210.NAA21705@wax.dcs.qmw.ac.uk>
> Let L(0) be Zermelo set theory (or the axioms for an elementary topos).
>
> For each n, let L(n+1) be L(n) plus
> as much of the axiom-scheme of replacement as is needed
> to justify the gluing construction that shows that
>
> L(n+1) |- ``L(n) is consistent.''
>
> Now let L(infinity) be the union of L(n) over n:N.
>
> If L(infinity) |- false then L(n) |- false for some n.
>
> But L(infinity) |- ``L(n) is consistent,''
>
> so L(infinity) proves its OWN consistency,
> contradicting Godel's theorem.
>
> However, L(infinity) has a standard non-trivial interpretation
> in Zermelo--Fraenkel set theory, which is therefore inconsistent.
i think there is a gap is in the step
L(infinity) |- "L(n) is consistent"
so L(infinity) proves its OWN consistency
formalized in a suitable category of theories and interpretations, paul's
construction, if i understand it correctly, refers to the colimit of the tower
L(0) --> FL(0) --> FFL(0) -->...
where FX = X + replacement_X, so that FX |- "X is consistent".
IF the colimit of this tower, paul's L(infinity), were a fixpoint of F, THEN it
would indeed prove its own consistency. but it doesn't seem to be a fixpoint:
the restrictions of the replacement to L(0), FL(0) etc. do not imply the
replacement for L(infinity).
btw, i bought paul's book and warmly recommend it.
-- dusko
prev parent reply other threads:[~1999-04-01 20:23 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
1999-04-01 12:10 Paul Taylor
1999-04-01 15:52 ` Mike Oliver
1999-04-01 20:23 ` Dusko Pavlovic [this message]
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