This book exhibits the richness and sophistication of Plotinian and Dionysian apophatic theologies by explicating their respective internal "logics." It articulates the unique metaphysical status and explanatory role that the One and God, respectively, play in Plotinus's and Dionysius's reflections, showing the way in which apophasis is generated and sustained by the metaphysical-explanatory lines of thought in which the One (Plotinus) and God (Dionysius) function as the ultimate, unconditioned source of everything else. In the context of explanation, negation serves to convey the incomparable reality of the One or of God as beyond being. However, the metaphysical and explanatory lines of thought are themselves situated within the broader context of the soul's ascent to mystical union with the One or with God. From this broader perspective, the discursive practice of negation constitutes the basis of preparing the soul for mystical union. Preparation for mystical union involves the cognitive and trans-cognitive practice of negation, which enables the soul to progress towards and become united with the One or God. This study is motivated by the desire to more deeply understand apophasis as deployed in different philosophical, theological, and religious contexts, including the work of contemporary thinkers such as Jean-Luc Marion.