After a presentation of the main onto-theological theses contained in the Liber de causis, the author explains how they were received and interpreted by Thomas Aquinas and Siger of Brabant in their respective commentaries on the short treatise “de primis causis rerum.” Starting from a mistranslation of the word “yliathim,” Thomas “injects” into the De causis his own doctrine of the distinction between being and essence. As for Siger, while he is often regarded as an adversary of Thomas Aquinas, his (...) ideas about the ontological difference between the Prime Cause and the created beings are on the very lines of the Thomasian lecture of the Liber de causis. (shrink)