 In Łukasiewicz’s logic textbook of 1929, after treating propositional calculus he does not go on, as one would nowadays, to expound predicate logic, but gives a brief formal account of Aristotle’s categorical (non-modal) syllogistic, presupposing twelve theorems of propositional calculus. This foreshadowed his 1951 book, Aristotle’s Syllogistic, by 22 years. This book, which revolutionized the study of Aristotle’s logic, had a long and interrupted genesis. A talk on the subject given at Kraków in 1939 was published in Polish only in 1946. In 1939 Łukasiewicz prepared a Polish monograph, but the partial proofs and manuscript were destroyed in the bombing of Warsaw. In 1949 he was invited to lecture on Aristotle’s syllogistic at University College Dublin, and those lectures formed the basis of the book, completed in 1950 and published the following year, his first in English. The first edition dealt only with categorical syllogistic. For the second edition, completed in 1955, less than a year before his death, Łukasiewicz added three chapters on the modal syllogistic, employing the modal logic Ł that he had developed in the meantime. The second edition was proof-read and indexed by Lejewski, and appeared in 1957.