This dissertation examines the possible interchangeable connection between the medieval ontological relationship of essence/existence and the modern epistemological relationship of subject/object. Rather than investigating the many medieval and modern philosophers, this dissertation focuses on two philosophers as study cases: One is the Islamic philosopher Mulla Sadra, and the other is the German philosopher George. W. F. Hegel. While the medieval relationship of essence/existence finds its highest development in Sadra, the modern relationship of subject/object finds its highest summit in Hegel. Sadra's break with the metaphysical essentialism of both Greek philosophy and Arabic philosophy results in the moment of Absolute Existentialism. This moment is not explicitly mentioned by Hegel-- neither in the historical development of the Philosophical Idea in the Lectures on The History of Philosophy, nor in the conceptual development of the Logical Idea in the Science of Logic. If the moment of the Absolute Existentialism is contained within Hegel's Absolute Idealism at all, it is so implicitly, as a moment within the Hegelian Idea's self-dialectical process of determining. In this dissertation, I unveil Sadra's moment of the Absolute Existentialism within Hegel's moment of the Absolute Idealism. I argue that, although Sadra's Absolute Existentialism and Hegel's Absolute Idealism are different, both challenge the traditional views of metaphysical essentialism. I provide the logical connection between Sadra's and Hegel's critiques of metaphysical essentialism. I show that, within Hegel's Absolute Idealism, Sadra's Absolute Existentialism emerges in opposition to quality as affirmative reality in the metaphysical essentialism of the Aristotelian tradition. / McAnulty College and Graduate School of Liberal Arts; / Philosophy / PhD; / Dissertation;