An attempt to understand the notion better led
Robin Gandy (Turing's student and friend) in 1980 to analyze
machine computation (as opposed to human-computation acted out by a Turing machine). Gandy's curiosity about, and analysis of, "cellular automata", "Conway's game of life", "parallelism" and "crystalline automata" led him to propose four "principles (or constraints) ... which it is argued, any machine must satisfy."
[30] His most-important fourth, "the principle of causality" is based on the "finite velocity of propagation of effects and signals; contemporary physics rejects the possibility of instaneous action at a distance."
[31] From these principles and some additional constraints -- (1a) a lower bound on the linear dimensions of any of the parts, (1b) an upper bound on speed of propagation (the velocity of light), (2) discrete progress of the machine, and (3) deterministic behavior -- he produces a theorem that "What can be calculated by a device satisfying principles I-IV is computable
