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# 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*.