You're looking at Less Wrong's discussion board. This includes all posts, including those that haven't been promoted to the front page yet. For more information, see About Less Wrong.

Quinn comments on Second order logic, in first order set-theory: what gives? - Less Wrong Discussion

10 Post author: Stuart_Armstrong 23 February 2012 12:29PM

You are viewing a comment permalink. View the original post to see all comments and the full post content.

Comments (19)

You are viewing a single comment's thread. Show more comments above.

Comment author: Quinn 25 February 2012 11:05:37AM 4 points [-]

Well, models can have the same reals by fiat. If I cut off an existing model below an inaccessible, I certainly haven't changed the reals. Alternately I could restrict to the constructible closure of the reals L(R), which satisfies ZF but generally fails Choice (you don't expect to have a well-ordering of the reals in this model).

I think, though, that Stuart_Armstrong's statement

Often, different models of set theory will have the same model of the reals inside them

is mistaken, or at least misguided. Models of set theory and their corresponding sets of reals are extremely pliable, especially by the method of forcing (Cohen proved CH can consistently fail by just cramming tons of reals into an existing model without changing the ordinal values of that model's Alephs), and I think it's naive to hope for anything like One True Real Line.

Comment author: Stuart_Armstrong 27 February 2012 02:20:09PM 0 points [-]

Thanks for that elucidation.

Comment author: cousin_it 25 February 2012 11:28:07AM *  0 points [-]

Thank you for helping me fill my stupid gap in understanding!