This article shows that a forgotten source of Marsilius’ scientia of law-making in the Defensor Pacis is the Lucidator, the main astrological work of Peter of Abano. A compared analysis of these two works demonstrates that the theories of experientia and scientia that Marsilius considers necessary to make laws in the first dictio of the Defensor Pacis entirely draw on Peter of Abano’s views on the epistemological status of ‘the science of the stars’. It is shown that the purpose of (...) Marsilius’ reinstatement of Peter of Abano’s theory of scientia is to build a science of law-making that does not need natural law and is based on experience and certain demonstrations. (shrink)