The relationship between burhān (demonstration), mystical intuition (kashf), and the Qur'an constitutes one of the central issues in the epistemology of Transcendent Theosophy, as it clarifies the role of demonstrative reason within Mullā Ṣadrā's intellectual system. Employing a descriptive-analytical approach and drawing on Mullā Ṣadrā's principal works, this article examines the epistemological functions of burhān in relation to mystical intuition and the Qur'an. The findings show that, in relation to mystical intuition, burhān is not merely a criterion for evaluating mystical disclosures; rather, by providing their rational articulation, justification, and epistemic completion, it transforms them into coherent and defensible knowledge. In relation to the Qur'an, burhān performs distinct functions in both tafsīr and taʾwīl. In tafsīr, it serves as the criterion for valid interpretation of the Qur'an and as the rational explication of its teachings. In taʾwīl, it provides both the criterion for the validity of esoteric interpretation and the rational justification for the transition from the outward meaning of the verses to their deeper realities. Thus, burhān neither replaces mystical intuition and the Qur'an nor stands alongside them as an independent source of knowledge. Rather, by performing distinct epistemological functions in relation to each, it plays an essential role in validating, rationally explicating, and systematizing the understanding of knowledge derived from mystical intuition and the Qur'an.