In this chapter and the next, we present two articles by Robert Feys (1889-1961), one published in 1937 and the other in 1950. Feys 1937, ‘The new logics of modalities’, is perhaps the first article that presented modal logic as a series of systems based on the classical truth-functional logic. Since not all of the many systems studied by Lewis contain the S4 axiom (i.e., Becker’s 1.92 or Gödel’s Bp → BBp,) Feys simply drops it, keeping the rest of Gödel’s axiomatization. He was aware that all of Lewis’s systems contained necessitated versions of the classical propositional calculus. But he may not have appreciated that the rule of necessitation —that if α is a theorem, so is □α— was stronger than the principle that tautologies are necessary and that the necessitation rule is not in any Lewis system weaker than S4. Contrary to Russell and Quine, Feys believed that modal notions were essential to philosophy and required a precise formal study. This is done by the axiomatic method, whose systems presentation is purely formal, i.e., not requiring any meaning of the formulae to be taken into account. Moreover, he criticized the restriction of logic to the logic of truth and falsity and in addition to the classical Aristotelian modalities, he presented logics originating in intuitionism and multi-valued logics.