Ṣadr al-Dīn Muḥammad Shīrāzī (d. 1640), widely known as “Mullā Ṣadrā,” crafted a comprehensive metaphysical system deeply embedded within the rich intellectual tradition of Islamic philosophy, synthesizing key elements from Avicennian, Suhrawardian, and Akbarian thought. This chapter, summarizing my doctoral dissertation titled “Squaring the Circle: The Suspended Person Thought Experiment’s Conditions Approved Apperception as an Onto-Epistemic Basis for Mullā Ṣadrā’s Existentialist Psychology,” explores Mullā Ṣadrā’s engagement with Abū ʿAlī al-Ḥusayn ibn ʿAbd Allāh ibn Sīnā (commonly known as Avicenna, d. 1037) and his “Suspended Person” (SP) thought experiment, focusing on the interplay between knowledge and existence. This chapter challenges the traditional interpretation of Avicenna’s “Suspended Person” (SP) thought experiment, which has been widely accepted as evidence of the self’s immaterial nature. I argue that when Avicenna’s conditions for the SP are strictly applied, the experiment reveals that the self (nafs) is not an immaterial substance but is instead synonymous with existence (wujūd) itself. This overlooked existential implication has significant consequences for later Islamic philosophers, particularly Mullā Ṣadrā. Ṣadrā’s Transcendent Philosophy integrates Avicennian, Suhrawardian, and Akbarian thought but retains Avicenna’s initial oversight regarding the nature of the self. By reexamining the SP through the lens of this existential perspective, this study demonstrates that Ṣadrā’s metaphysical framework, which treats the self as a transubstantial entity, needs reconsideration. The SP experiment, when properly understood, undermines the static essentialism in Ṣadrā’s ontology, suggesting instead a dynamic, existential view of the self. By tracing the development of the SP across significant Islamic philosophers—Avicenna, Shihāb al-Dīn Yaḥyā ibn Ḥabash ibn Amīrak Suhrawardī (commonly known as Suhrawardī, d. 1191), Muḥyī al-Dīn Muḥammad ibn ʿAlī ibn Muḥammad ibn ʿArabī (commonly known as Ibn ʿArabī, d. 1240), and Mullā Ṣadrā—I underscore Ṣadrā’s distinctive interpretation within his Transcendent Philosophy. This reinterpretation uncovers an oversight in Ṣadrā’s approach, stemming from his failure to recognize that the SP’s apperception does not account for the perception of substances due to similar oversights in previous interpretations. By addressing this lapse, the study brings new clarity to the fundamental concepts within Ṣadrā’s metaphysics, advocating for a more profound understanding of the existential implications of the SP. This work situates the SP within a broader philosophical context, enriching the discourse on the nature of the self in Islamic metaphysical thought.