The output of a simulation model does not, prima facie, appear to have an objective status comparable with data captured by observation or experiment. Counter to this Winsberg, Humphreys and others emphasise parallels between experiment and simulation in practices which are said to "carry with them their own credentials" (Winsberg, 2010). Humphreys holds that that the physical implementation of computer models places constraints on simulation methods not present in traditional mathematics, creating essential epistemic opacity. By this he means that it is impossible for a cognitive agent, given its characteristics, to know all of the epistemically relevant elements of a computational process. Humphreys views essential epistemic opacity as reflecting the limitations not of the simulation method itself but of the human agent, and thus as evidence for a "non-anthropocentric epistemology" recognising computational tools as a superior epistemic authority.The possibility of testing a highly parameterised simulation model via the hypothetico-deductive method can indeed be open to doubt; moreover empirical measurements are often not available on the scale needed to evaluate model outputs. Even were appropriate data available, Lenhard & Winsberg (2010) argue that climate simulation models face epistemological challenges associated with a novel kind of "confirmation holism": it is impossible to locate the sources of the failure of any complex simulation to match known data, so that it must stand or fall as a whole. This is because of three interrelated characteristics which they regard as intrinsic to the practice of complex systems modelling -"fuzzy modularity", "kludging" and "generative entrenchment". In "fuzzy modularity", different modules simulating different parts of the complex system are in continual interaction, thus it is difficult to define clean interfaces between the components of the model. A kludge is an inelegant, 'botched together' piece of program, very complex, unprincipled in its design, ill-understood, hard to prove complete or sound and therefore having unknown limitations, and hard to maintain or extend. Generative entrenchment refers to the historical inheritance of hard-to-alter features from predecessor models.
Is confirmation holism(1) essential to and unavoidable in complex systems modelling?(2) embedded in specific disciplinary practices of climate science? or (3) does it exemplify a failure to observe, recognise and apply available and wellestablished sound Software Engineering practices when developing simulation software (as promoted, for example, by the Sustainable Software Institute)?Belief in the essential epistemic opacity of computational science points to (1) but we shall argue for (3) on two main grounds.