Talk:Set of Gödel Numbers of Arithmetic Theorems Not Definable in Arithmetic

It is not clear which notion of definability is in play here. Ordinarily in this context, one is concerned with subsets of some model of a theory T -- the set A is definable in the structure M if and only if there is a formula F(x) in the language of T and A is the set of members a of M such that M satisfies F(a). -- 03:32, 30 April 2014‎ Ratboy