  But verification and validation are not, strictly speaking, so cleanly separable. That is because most methods of validation, by themselves, are much too weak to establish the validity of a simulation. And most model equations chosen for simulation are not in any straightforward sense “the right equations”; they are not the model equations we would choose in an ideal world. We have good reason to think, in other words, that there are model equations out there that enjoy better empirical support, in the abstract. The equations we choose often reflect a compromise between what we think best describes the phenomena and computational tractability. So the equations that are chosen are rarely well “validated” on their own. If we want to understand why simulation results are taken to be credible, we have to look at the epistemology of simulation as an integrated whole, not as cleanly divided into verification and validation—each of which, on its own, would look inadequate to the task.