Of the important articles in modal logic between 1937 and 1950, McKinsey 1945 and Carnap 1946 are among the most significant advances. Each of them was aiming at an account of modal logic that was more than simply a collection of axioms. McKinsey had made a great deal of progress on the semantics of modal notions, contrary to Feys, who doesn’t seem to have worked on the topic. Feys and McKinsey were to write a book in which McKinsey’s contributionwould have been the semantics of modal logic. Feys’s part has subsequently been published in Feys 1965. As in his 1937 article, in the 1950 article, ‘Formalized systems of Aristotelain modalities’, Feys presents modal logic in a purely formal way, ‘formal’ meaning a way that makes no reference to the meaning of modal formulae. In both his articles, Feys summaries and attempts to understand what lay behind Lewis’s studies. Here, he presents a survey of the systems in Lewis and Langford 1932. As in the 1937 article, a large amount of the 1950 article is to list all the various respects in which the theorems of the various systems of strict implication do or do not match up with the theorems of the ordinary non-modal propositional calculus.