Paul Isaak Bernays (born October 17, 1888, London, England—died September 18, 1977, Zürich, Switzerland) was a Swiss mathematician whose work in proof theory and axiomatic set theory helped create the new discipline of mathematical logic.

After obtaining his doctorate from the University of Göttingen in Germany under Edmund Landau in 1912, Bernays taught for five years at the University of Zürich before returning to Göttingen. There he collaborated closely with the prominent mathematician David Hilbert, who in the twilight of his career sought to overcome the challenges to classical mathematics posed by L.E.J. Brouwer’s intuitionism. Bernays’s own philosophical views remained in the background during the “foundations crisis” of the 1920s (see mathematics, foundations of: The quest for rigour). Nevertheless, he served as a strong pillar of support for Hilbert’s program to formalize mathematics (see formalism). Taking Hilbert’s name as coauthor, he wrote the classic study Grundlagen der Mathematik, 2 vol. (1934–39; reissued 1968–70; “Foundations of Mathematics”). In 1956 Bernays also revised Hilbert’s Grundlagen der Geometrie (1899; The Foundations of Geometry), which went through several editions.

After the Nazi takeover in 1933, Bernays was compelled to give up his post and moved to Switzerland. In Zürich he delved into the realm of set theory, trying to streamline the Zermelo-Fraenkel system of axioms (see logic, history of: 20th-century set theory). This work appeared in a series of articles under the title “A System of Axiomatic Set Theory” (1937–54), from which the principal theses were published as Axiomatic Set Theory (1958). In it Bernays simplified and refined the work of John von Neumann on logic and set theory; these modifications were further developed by the logician Kurt Gödel.

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