This study examines the relationship between Jalāl al-Dīn Dawānī's theory of tasha''un (manifestation) and Mullā Ṣadrā's account of mental existence by investigating the connection between ontology and epistemology in the Islamic philosophical tradition. Using an analytical-comparative method and drawing upon the works of Dawānī and Mullā Ṣadrā, the study argues that several central elements of Dawānī's thought—particularly his doctrine of Dhawq al-Ta'alluh and his concept of tasha''un—provide a theoretical framework for a unity-oriented interpretation of the relationship between being and knowledge. Within this framework, intelligible forms are understood not as independent entities but as modes and levels of the manifestation of reality within the realm of cognition. The study further shows that Mullā Ṣadrā reformulated these conceptual capacities within a more systematic philosophical framework through the principles of the primacy of existence (aṣālat al-wujūd), the doctrine of Basīṭ al-Ḥaqīqah, and his distinctive account of mental existence. Accordingly, mental existence in Transcendent Philosophy is not merely a theory of the mental representation of external objects; rather, it should be understood within the ontological foundations of Ṣadrian philosophy and in light of the logic of unity-in-diversity. The findings suggest that certain theoretical elements in Dawānī's thought constitute important conceptual foundations for understanding Mullā Ṣadrā's account of mental existence and the structural relationship between ontology and epistemology in Transcendent Philosophy.