Stuart_Armstrong comments on The Cartoon Guide to Löb's Theorem - Less Wrong

12 Post author: Eliezer_Yudkowsky 17 August 2008 08:35PM

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Comment author: Will_Sawin 07 January 2011 06:55:10PM 2 points [-]

It think the provability of consistency would be:

"There is a proof that there is no proof of the canonical false proposition F"

Comment author: Stuart_Armstrong 07 January 2011 07:15:48PM *  0 points [-]

I'm using Peter Smith's definition (see http://www.logicmatters.net/resources/pdfs/gwt/GWT.pdf , Godel without too many tears).

In definition 59 (on page 1 of part 10), he identifies consistency with "not a proof of the canonical false statement". This is a valid statement within Peano arithmetic. And it's logical consequences are... anything.

Comment author: Will_Sawin 07 January 2011 07:22:32PM 2 points [-]

You're confusing consistency with a proof of consistency.

Theorem 56: Consistency implies no proof of consistency.

Which is of course where you get:

Proof of consistency implies inconsistency.

Which gives you:

Proof of consistency implies anything.

Comment author: Stuart_Armstrong 07 January 2011 10:33:08PM 2 points [-]

I think you're right.... I was commiting the same mistake is above, using the first derivability condition and assuming that Peano arithmetic could treat it as a statement in Peano arithmetic - which it isn't.