1920 1930 1940 1950 1960 1970 1980 1990 2000 Michael DummettMichael DummettMichael DummettMichael DummettMichael DummettMichael Dummett Edward John LemmonEdward John LemmonEdward John LemmonEdward John LemmonEdward John LemmonEdward John Lemmon Modal Logics between S4 and S5 Propositional modal logic