Thesis advisor: Olivier Boulnois / Thesis advisor: Stephen F. Brown / The Dissertation Text has Three Parts. Each paragraph is referred at the end to the Part it summarizes. My dissertation places Saint Anselm’s Ontological Argument within its original Neoplatonic context that should justify its validity. The historical thesis is that Anselm’s epistemology, underlying the Proslogion, the Monologion and De Veritate, was a natural, often unaccounted for, reflection of the essentially Neoplatonic vision that defined the pre-thirteenth century mental culture in Europe. (Introduction and Part I) This thesis is shown through the reception of Anselm’s argument by 27 XIIIth-XIVth century thinkers, whose reading of it exhibits a gradual weakening of Neoplatonic premises up to a complete change of paradigm towards the XIVth century, the first reason being the specificity of the Medieval reception of Aristotle’s teaching on first principles that is the subject of Posterior Analytics (Part II), and the second reason being the specificity of the Medieval reception of Dionysius the Areopagite (Part III, see sub-thesis 4 below). The defense of this main historical thesis aims at proving three systematic sub-theses, including a further historical sub-thesis. The Three Systematic Sub-Theses: 1) The inadequacy of rationalist and idealist epistemology in reaching and providing apodictic truths (the chief one of which is God’s existence) with ultimate ontic grounding, as well as the inadequacy of objectivistic metaphysics that underlies these epistemologies, calls for another, non-objectifying epistemic paradigm offered by the Neoplatonic (Proclian theorem of transcendence) apophatic and supra-discursive logic (kenotic epistemology) that should be a better method to achieve certainty, because of its ability to found logic in its ontic source and thus envisage thought as an experience and a mode of being in which it is grounded. Within such a dialectic, there cannot be any opposition or division either between being and thought, or between faith and reason, faith being an ontic ground of reason’s activity defined as self-transcendence. The argument of the Proslogion is thus an instance of logic that transcends itself into its own principle – into ‘that than which nothing greater can be conceived’. Such an epistemological vision is also supported by contemporary epistemology (Russell’s Paradox and Gödel’s Incompleteness Theorem) (Introduction and Part I) 2) In virtue of this apophatic and supra-discursive vision, God’s existence, thought by human mind (as expressed in the argument of the Proslogion), happens to be a common denominator between God’s inaccessible essence and the created essence of human mind, so that human consciousness can be defined as ‘con-science’ – the mind experiencing its own being as co-knowledge with God that forges being as such. (Part I) 3) However, God’s existence as a common denominator between God’s essence and the created essence of human mind cannot be legitimately accommodated within the XIIIth-XIVth century epistemology and metaphysics because of the specificity of relation between God’s essence and His attributes, typical of Medieval scholasticism and as stated by Peter Lombard and Thomas Aquinas. If this relation is kept, while at the same time God’s existence is affirmed as immanent to the human mind (God as the first object of intellect), God’s transcendence is sacrificed and He becomes subject to metaphysics (Scotus’ nominal univocity of being). In order to achieve real univocity between the existence of human thinking and God’s existence, one needs a relation between God’s essence and His attributes that would allow a real participation of the created in the uncreated. The configuration of such a relation, however, needs the distinction between God’s essence and His energies that Western Medieval thought did not know, but that is inherent to the Neoplatonic epistemic tradition persisting through the Eastern Church theologians and Dionysius the Areopagite up to Gregory Palamas. (Part III) Another Historical Sub-Thesis: 4) One of the reasons why Medieval readers of Anselm’s Proslogion misread it in the Aristotelian key, was that they did not have access to the original work of Dionysius the Areopagite, in which the said distinction between God’s essence and His energies is present. This is due to the fact that the Medievals read Dionysius through Eriugena’s translation. However, Eriugena was himself influenced by Augustine’s De Trinitate that exhibits an essentialist theology: in fact, it places ideas within God’s essence, which yields the notion of the created as a mere similitude, not real participation, and which ultimately makes the vision (knowledge) of God possible only in the afterlife. Since already with Augustine the relation between grace and nature is modified (grace becomes a created manifestation of God, instead of being His uncreated energy), God’s essence remains incommunicable. Similarly, God’s existence is not in any way immanent to the created world, of which the created human intellect is a part, so that it remains as transcendent to the human mind as is His incommunicable essence. This should explain why for the Medievals analogy, and eventually univocity, was the only way to say something about God, and also why they mostly could not read Anselm’s Proslogion otherwise than either in terms of propositional or modal logic. (Part III) The dissertation concludes that whilst Anselm’s epistemology in the Proslogion is an instance of Neoplatonic metaphysical tradition, the question of the possibility of certainty in epistemology, as well as the possibility of metaphysics as such, depends on the possibility of real communicability between the immanence of human predicating mind and the transcendence of God’s essence through His trans-immanent existence.