 In one response to unlimited pancomputationalism, Copeland (1996) argues that the mappings the thesis relies on are illegitimate because they are constructed ex post facto—after the computation is already given. In the case of genuine computation—the kind normally used in science— the work of generating successive descriptions of a system’s physical dynamics is done by a computer running an appropriate program (e.g., a weather forecasting program); this is the point of using a computer in the first place. If the physical dynamics were already known—which is required for an ex post facto computational description—then using a computer would be unnecessary.