PERSON
Alonzo Church
The Princeton logician who gave computation a rigorous definition before a single working computer existed—inventor of the lambda calculus, prover of undecidability, and the man whose fence around the computable still marks the outer limit of what any machine, however intelligent, can ever do.
Alonzo Church is the thinker who never trends and without whom none of this would be possible. He spent four decades at Princeton erasing blackboards with soap and water until they were spotless, writing in a hand so careful that his students compared it to a grade-school teacher’s, coating his important papers in cement so they would survive. He was the least dramatic man in any room. And in 1936, before there was a single working computer anywhere on earth, he defined what it means for a machine to compute—more precisely than anyone before or since. His lambda calculus, a formal system in which everything is a function and nothing else exists, captured exactly the same class of calculations that
Turing’s imagined tape machine could perform and that the theory of recursive functions independently described. Three completely different formalizations, three different intuitions, one identical set: the convergence is why “computable” is not