Alan M. Turing’s scientific contributions

... Not only has the Turing-machine model proved basic for theory, but the work of these pioneers presaged many aspects of computational practice that are now commonplace." For Davis this quite dramatic change of conceptual perspective from recursive function theory to computability theory was based on the fact that Post completely captured Turing's way of analyzing the mechanical calculations of a human being via "production systems" given in [94]. Indeed, in [95], Turing extended Post's techniques to establish the undecidability of the word problem for semigroups with cancellation; then in [96], Turing used Post's concept of deterministic production systems (and dubbed them "substitution puzzles") for two goals: (1) to discuss the methodological problem of articulating a mathematically precise and adequate concept of computation and (2) to prove central undecidability results. ...

Reference:

Martin Davis: An Overview of his Work in Logic, Computer Science, and Philosophy