Deductive Reasoning in First-order Logic 221 Leon Albert Henkin (19.04.1921–1.11.2006) was an American logician and mathematician, best known for the Henkin Completeness Proof for axiomatic deductive systems of first-order logic This proof is much simpler and intuitive than the original Gödel proofs, and has become the standard method for semantic completeness proofs since then After studying mathematics and philosophy at Columbia College, Henkin was a doctoral student of Alonzo Church at Princeton University, where he received his PhD in 1947 He then became Professor of Mathematics at the University of California, Berkeley in 1953 where he worked until his retirement in 1991 He was a collaborator of Alfred Tarski and a keen popularizer of logic and of mathematics In his doctoral dissertation The completeness of formal systems, Henkin originally proved the completeness of Church’s higher-order logic using the now-called general Henkin semantics, where the higher types are interpreted not by the full space of functions but by a suitably rich subset of it He then adapted his method to obtain a new completeness proof for first-order logic, introducing the now-called Henkin constants and Henkin models His proof method is now used for completeness proofs of a wide variety of logical systems Henkin also studied extensions of first-order logic with branching quantifiers and with infinite strings of quantifiers and proposed game-theoretic semantics for them Henkin was not only a distinguished academic and promoter of logic and mathematics, but also a passionate social activist, instrumental in designing special programs and scholarships for talented students from disadvantaged minorities