Frederic Fitch

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Frederic Brenton Fitch (September 9, 1908 – September 18, 1987) was an American logician and a professor at Yale University. He was born in Greenwich, Connecticut, and earned his B.A. in 1931 and his Ph.D. in 1934 at Yale, studying under F. S. C. Northrop. After a postdoctoral period at the University of Virginia (1934–1937), he returned to Yale and taught there until 1977. His doctoral students included Alan Ross Anderson, Ruth Barcan Marcus, and William W. Tait.

Fitch invented the Fitch-style calculus, a way of organizing formal proofs as diagrams. In his 1963 paper “A Logical Analysis of Some Value Concepts,” he proved Theorem 5 (originally by Alonzo Church), a result that is well known in discussions of the knowability paradox.

He worked mainly in combinatory logic and wrote an introductory textbook on the subject (1974). He also made important contributions to intuitionism and modal logic, and he explored foundational questions in mathematics and inductive probability. He examined the theory of references in “The Problem of the Morning Star and the Evening Star” (1949) and wrote about how logic relates to language.