Galileo's discovery of Jupiter's four moans is not the result of an experimental control carried out on a forecast deduced from a theory relative to the structure of Jupiter's system. His discovery is, instead, the result of operations which do not have their historical roots in the background knowledge of Galileo's era. This state of affairs stimulates the research of models in the history of science that permit the exploration of contexts of discovery in the framework of an evolutionary theory of human knowledge.