 Last but not least, the axiomatic study of epistemic notions has greatly benefited from application of techniques used for proving incompleteness and indefinability results since the early sixties: they have yielded negative results (Kaplan and Montague 1960, Montague 1963, Thomason 1980) and established an interesting link with the surprise test paradox. The situation may have also been changed by the study of possible world semantics for modal notions, conceived as predicates in Halbach, Leitgeb and Welch 2003. However this is open to debate and experimentation: for instance it is argued in Halbach and Welch 2009 that the predicate approach to necessity is a viable route — insofar as the expressive power is considered — provided one resorts to languages that involve both a truth predicate and the necessity operator.