Alonzo Church, an American logician and philosopher, was born in Washington, D.C. He received his PhD from Princeton in 1927, having written his dissertation under Oswald Veblen on alternatives to the axiom of choice. He spent a year at Harvard and then a year in Europe, studying first at Göttingen and then at Amsterdam with L.E.J. Brouwer. He returned to Princeton where he was professor of mathematics from 1929 to 1967, after which he moved to UCLA to become professor of mathematics and philosophy. He retired from teaching at UCLA in 1990. Church's most important contributions to logic were his analysis of the concept of effective computability and his proof of the undecidability of first-order logic (Church's theorem).

A function of natural number is effectively computable if there is an algorithm—a surefire method requiring no ingenuity to follow—that will yield the value...