Knowing that one knows and the classical definition of knowledge

Abstract

In his Essay in Modal Logic, G. H. von Wright suggested that the logic of knowledge or epistemic logic is a branch of modal logic.1 E. J. Lemmon has recommended a system of epistemic logic based on the Feys-von Wright modal system M,2 and in Knowledge and Belief Jaakko Hintikka proposed a stronger system which corresponds to Lewis's S4.3 The distinctive axiom of S4 is 'Np^NNp',* and the epistemic counter part of this formula is the principle that knowing implies knowing that one knows, that is,