The legitimacy of governance refers to the rightful exercise of authority by the government and the obligation of citizens to comply with its rule. To elucidate this concept, it is essential to determine the criteria for the legitimacy of governance. One prominent perspective identifies "reason" as the criterion for legitimacy. This study examines the role and scope of reason in governance through the perspectives of Plato, Fārābī, and the modernist approach. Plato believes that the rule of reason necessitates the governance of philosophers, asserting that only philosophers, by connecting with the realm of forms, can comprehend the principles of justice and proper governance, thus guiding society towards progress. Fārābī considers reason a criterion for legitimacy only when it is connected to divine revelation. If this connection is direct, the highest levels of rationality and governance are achieved, whereas indirect connection leads to subsequent levels of legitimate governance, which manifests in the form of the governance of "representative ruler" or "analogous president", and at the next level, in the form of tradition-based governance. In the modernist approach, on the other hand, the reason that is the criterion for legitimizing governance is instrumental reason. It neither relates to the realm of forms nor to revelation. This reason is autonomous, confined to the natural world, and derives its raw material from empirical experience.