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Incorrect comments on Gap in understanding of Logical Pinpointing - Less Wrong Discussion

6 Post author: Incorrect 12 November 2012 05:33PM

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Comment author: elseif 13 November 2012 02:57:01AM *  0 points [-]

Perhaps LessWrong is a place where I can say "Your question is wrong" without causing unintended offense. (And none is intended.)

Yes, for ω-consistency to even be defined for a theory it must interpret the language of arithmetic. This is a necessary precondition for the statement you quoted, and does not contradict it.

Work in PA, and take a family of statements P(n) where each P(n) is true but independent of PA, and not overly simple statements themselves---say, P(n) is "the function epsilon n in the fast growing hierarchy is a total function". (The important thing here is that the statement is at least Pi2---true pure existence statements are always provable, and if the statements were universal there would be a different ω-consistency problem. The exact statement isn't so important, but not that these statements are true, but not provable in PA.)

Now consider the statement T="there is an n such that P(n) is false". PA+T has no standard model (because T is false), but PA+T doesn't prove any of the P(n), let alone all of them, so there's no ω-consistency problem.

Comment author: Incorrect 13 November 2012 11:23:05PM 0 points [-]

Thanks, can you recommend a textbook for this stuff? I've mostly been learning off Wikipedia.

I can't find a textbook on logic in the lesswrong textbook list.

Comment author: elseif 14 November 2012 03:25:16PM 1 point [-]

I'm a fan of Enderton's "A Mathematical Introduction to Logic". It's short and very precisely written, which can make it a little difficult to learn from on its own, but together with a general familiarity with the subject and using wikipedia for additional examples elaboration, it should be perfect.