Frederic Fitch
Frederic Brenton Fitch (September 9, 1908, Greenwich, Connecticut – September 18, 1987, New Haven, Connecticut) was an American logician, a Sterling Professor at Yale University. Education and career. At Yale, Fitch earned his B.A in 1931 and his Ph.D. from Yale in 1934 under the supervision of F. S. C. Northrop. From 1934 to 1937 Fitch was a postdoc at the University of Virginia. In 1937 he returned to Yale, where he taught until his retirement in 1977. His doctoral students include Alan Ross Anderson, Ruth Barcan Marcus, and William W. Tait. Work. Fitch was the inventor of the  Fitch-style calculus for arranging formal logical proofs as diagrams. In his 1963 published paper "A Logical Analysis of Some Value Concepts" he proves "Theorem 5" (originally by Alonzo Church), which later became famous in context of the knowability paradox. Fitch worked primarily in combinatory logic, authoring an undergraduate-level textbook on the subject (1974), but he also made significant contributions to intuitionism and modal logic. He was interested in the problem of the consistency, completeness, categoricity, and constructivity of logical theories, especially nonclassical logics, and contributed to the foundations of mathematics and to inductive probability. He dealt with the  theory of references in "The Problem of the Morning Star and the Evening Star" (1949). He also contributed to the philosophy of how logic relates to language.