Computer Science has witnessed the emergence of a plethora of different logics, models and paradigms for the description of computation. Yet, the classic Church–Turing thesis may be seen as indicating that all general models of computation are equivalent. Alan Perlis referred to this as the ‘Turing tarpit’, and argued that some of the most crucial distinctions in computing methodology, such as sequential versus parallel, deterministic versus non-deterministic, local versus distributed disappear if all one sees in computation is pure symbol pushing. How can we express formally the difference between these models of computation?