 When I recognized the incompatibility of the traditional theorems on modal propositions in 1920, I was occupied with establishing the system of the ordinary “two-valued” propositional calculus by means of the matrix method. I satisfied myself at the time that all theses of the ordinary propositional calculus could be proved on the assumption that their propositional variables could assume only two values, “0” or “the false”, and “1” or “the true”. (1970: 164) 