In il nome della rosa William of Baskerville tells us that concepts are signs and words are "signs of signs". In math logic we can define systems of symbols an

In il nome della rosa William of Baskerville tells us that concepts are signs and words are "signs of signs". In math logic we can define systems of symbols and logical connectives and deductive rules. We can argue for the correctness of a logical system but we can not do it in the same system; we have to go "meta". Similarly a formal system could be a model of something "real", but the correspondence would be beyond mere logic.