Alonzo Church

Alonzo Church (1903-1995) was an American logician and mathematician who made major contributions to mathematical logic and the foundations of theoretical computer science. He is best known for Lambda Calculus, Frege-Church ontology, Church-Rosser theorem, and proving the undecidability of the Entscheidungsproblem in 1936, when he and Alan Turing independently showed that a general solution to the Entscheidungsproblem is impossible, assuming that the intuitive notion of "effectively calculable" is captured by the functions computable by a Turing machine, or equivalently, by those expressible in the lambda calculus. This assumption is now known as the Church-Turing thesis.