In the 1930s, “computer” was a job description: someone, usually a woman of mathematical bent, with an adding machine and a big sheet of columnar paper who performed a rigorous routine of hand calculations, using paper and pencil, slide rules and tables of logarithms. Stone knives and bearskins weren’t involved, but to modern eyes they might as well have been.
Large research organizations and the Department of War had a few special purpose mechanical computers intended to integrate differential equations. Vannevar Bush (who deserves his own article someday) brought a young grad student to MIT to work on the differential analyzer, a relatively advanced version of these. This video shows a version of the differential analyzer being applied to a problem for which it was utterly unsuited in Earth vs. the Flying Saucers:
This young man, a recent graduate of the University of Michigan, was named Claude Shannon, Jr. Shannon, while working on the differential analyzer, had the insight that these same computations could be done using combinations of a few simple circuits that performed basic logical operations on true and false values. He described how this could be done, and invented the whole concept of digital circuits, which derive from from Shannon’s thesis on what he called switching theory.
His Master’s thesis.