Simulation is based on the vast increase in computational power that is available to researchers. This increase, however, does not by itself characterize simulation. What philosophically matters are the conceptual ramifications. Simulation modeling combines extant concepts in a new way. It intertwines different types of experiments and in this way gives rise to a new combinatorial style of reasoning. The argument in favor of this thesis discusses two exemplars of simulation, namely thermodynamics and quantum chemistry. The conclusion reflects upon some of the resulting challenges for the philosophy of science.