1926 – 2015
John C. Shepherdson (1926–2015) was a British mathematician and logician at the University of Bristol, best known for foundational contributions to computability theory. He co-developed the Unlimited Register Machine (URM) model and wrote critically on the epistemological status of Church's Thesis, arguing that it admits no formal mathematical proof.
Co-developed the Unlimited Register Machine (URM) with H.E. Sturgis as an alternative model of computation equivalent to Turing machines
Argued that Church-Turing Thesis is an empirical or conceptual claim, not provable within formal mathematics
Contributed to the theory of inductive inference and machine learning foundations
Worked on subrecursive hierarchies and the limits of formal systems