 It was not some deficiency of imagination that led Turing to model his L.C.M.s on what could be achieved by a human computer. The purpose for which he invented the Turing machine demanded it. The Entscheidungsproblem is the problem of finding a humanly executable method of a certain sort, and, as was explained earlier, Turing’s aim was to show that there is no such method in the case of the full first-order predicate calculus.