This article analyzes Masdar Farid Mas'udi's view on the reconstruction of the concepts of qaṭ'iy and zanniy in the epistemology of Islamic law. Masdar categorizes benefits and teachings that are universal as qaṭ'iy (absolute) categories, while technical and elaborative teachings as zanniy (assumptive) categories. Based on this concept, Masdar formulated a methodology of Islamic law reasoning that makes benefit as the main reference and puts technical teachings as its support. Consequently, the standard of teachings (qaṭ'iy) that are technical in nature can be modified and annulled if they are deemed to be contrary to the benefit. Masdar's view is quite contrasted with the concepts of qaṭ'iy and zanniy in the view of ushul fiqh experts which do not refer to mere benefit, but the reference in determining qaṭ'iy and zanniy in the views of experts is the semantic aspect (dalâlah) and source authenticity (tsubût). a religious text. The results of this study conclude that there are essential differences in definition and implementation between the concepts of qaṭ'iy and zanniy carried by Masdar and the concepts of ushul fiqh experts in general