The Didascalia in Rethoricam Aristotilis ex glosa Alpharabii (henceforth Didascalia) constitute the prologue, in its Latin version, of al-Fārābī’s Great Commentary on Aristotle’s Rhetoric, which itself is otherwise lost. The purpose of this contribution is to describe the way al-Fārābī dispelled ambiguities that affect Aristotle’s Rhetoric in regard to the differences between rhetoric and dialectic and to the relative weight of rhetoric's philosophical value and its political value. Al-Fārābī first emphasizes the epistemological frame in which persuasion (persuasio) can be grasped and distinguished from certitude (certitudo), in which both persuasion and certitude are understood as modes that constitute conviction (creditio, ar. taṣdīq) and are opposed to the « representation of a simple thing in mind » (rei incomplexe in mente formatio, ar. taṣawwur). He then determines more precisely what rhetoric is and how rhetorical arguments are to be determined ; and he identifies the many values that can be attributed to rhetoric, in particular the heuristic method that can applied under certain circumstances in science. In following and developing these views which were probably inherited from the Alexandrian School, al-Fārābī underlines the prominent role played by rhetoric as the most useful and most necessary tool for governing cities.