The relation of self-knowledge and experience of the unity of the self's own and absolute ground is a central thought in the philosophy of Plotinus. In a movement of inner "ascent" thinking becomes aware of its own self – the thinking One in it. Plotin's imperative: "Let go of everything" points to the essentially processual character of philosophizing: at the end of the comprehending self-awareness developing in the course of the universal movement of abstraction stands the self-transgression of thinking as its utmost possibility, coming to it from its own ground of unity. This book analyzes this basic idea on the basis of an in-depth interpretation of Plotinus' Ennead V 3, gives a new translation of the Greek text, and explains the concepts and forms of thought that essentially determine it. It elaborates the historical context of the concept of "self-knowledge" in view of the Platonic tradition, suggests a reasonable concept of philosophical mysticism and highlights important cues for the medieval and modern discussion of "self-consciousness".