  The properties of knowledge and belief have been a subject for formal logic at least since von Wright and Hintikka, (Hintikka 1962, von  Wright 1951). Knowledge and belief are both treated as modalities in  a way that is now very familiar—Epistemic Logic. But  of Plato’s three criteria for knowledge, justified, true,  belief, (Gettier 1963, Hendricks 2005), epistemic logic really  works with only two of them. Possible worlds and indistinguishability model belief—one believes what is so under all circumstances  thought possible. Factivity brings a trueness component into  play—if something is not so in the actual world it cannot be  known, only believed. But there is no representation for the  justification condition. Nonetheless, the modal approach has been  remarkably successful in permitting the development of a rich  mathematical theory and applications, (Fagin, Halpern, Moses, and  Vardi 1995, van Ditmarsch, van der Hoek, and Kooi 2007). Still, it is not the whole picture.