  For example, if we study some object because we believe it is sufficiently dynamically similar to a basin of fluid for us to learn about basins of fluid by studying the it, then it provides a simulation of basins of fluid. This is in line with the definition of simulation we find in Hartmann: it is something that “imitates one process by another process. In this definition the term ‘process’ refers solely to some object or system whose state changes in time” (1996, 83).  Hughes (1999) objected that Hartmann’s definition ruled out simulations that imitate a system’s structure rather than its dynamics. Humphreys revised his definition of simulation to accord with the remarks of Hartmann and Hughes as follows: