A pioneering figure in logic and computer science who gave a negative answer to Hilbert’s decision problem in 1935, a year before Turing independently showed the same. He proved this via his ‘lambda calculus’, a model of computation still important today. Church’s thesis (or the Church-Turing thesis) states that any notion of ‘effectively calculable’ will lead to the same notion of computable functions that Church and Turing arrived at.