  It is false that real experiments always manipulate exactly their targets of interest. In fact, in both real experiments and simulations, there is a complex relationship between what is manipulated in the investigation on the one hand, and the real-world systems that are the targets of the investigation on the other. In cases of both experiment and simulation, therefore, it takes an argument of some substance to establish the ‘external validity’ of the investigation – to establish that what is learned about the system being manipulated is applicable to the system of interest. Mendel, for example, manipulated pea plants, but he was interested in learning about the phenomenon of heritability generally. The idea of a model organism in biology makes this idea perspicuous. We experiment on Caenorhabditis elegans because we are interested in understanding how organism in general use genes to control development and genealogy. We experiment on Drosophila melanogaster, because it provides a useful model of mutations and genetic inheritance. But the idea is not limited to biology.  Galileo experimented with inclined planes because he was interested in how objects fall and how they would behave in the absence of interfering forces—phenomena that the inclined plane experiments did not even actually instantiate.