Quote:
Originally Posted by PairTheBoard
"In general it is always possible to prove a sentence is true and unprovable but not in an algorithmic way."
However, I think this must be in the context where all true and unprovable sentences have been adjoined as axioms to the axiomatic system. He had just mentioned such a case.
I didn't listen to the talk, but this is called
True arithmetic and there are no unprovable and true statements. Of course there won't be a mechanical set of prove verification operations like you have for PA etc.
Quote:
In that interval he was asked what it means for an unprovable sentence to be "true". He said he would get to that in a minute but then he never did.
There are a couple of ways this can happen....depending on exactly what is meant by unprovable and true. Godel statements are sort of an example...."PA cannot prove this statement" which is true if PA is consistent, but this can be proven in ZF so unprovable only means "unprovable in PA".
Another way would be if something like Goldbach was independent of PA....that could mean that Goldbach is true for the standard natural numbers 1,2,3,4.....but you could construct a non standard model of PA 1',2',3',4'.... where Goldbach would be false. But this would basically amount to a proof that Goldbach is true, because Goldbach is a statement about the standard integers and not about all systems that are models of PA.
Last edited by dessin d'enfant; 08-29-2015 at 01:04 PM.