This dissertation discusses how being and unity are related in the metaphysical systems of Aristotle and Plotinus. I suggest that Aristotle's metaphysical position contrasts with what I call the Platonic metaphysical hierarchy, a general trend in Platonism to place being in a dependent relationship to unity, and particular things in a dependent relationship to being. Aristotle, by contrast, sees being and unity as dependent on particulars. Understanding Aristotle against the backdrop of the Platonic metaphysical hierarchy is of some assistance in (...) understanding his critique of Plato, and his own position in the Metaphysics regarding substance, cosmology and first principles. Aristotle's Unmoved Mover is substance par excellence , and stands as an exemplary cause for the First Moved Mover, guaranteeing the motion necessary for the generation of other particulars, but it does not provide them with being and unity. This is because being and unity are dependent on, and logically posterior to, particulars. I also examine some of the difficulties in Aristotle's system which Plotinus takes up in the Enneads . Plotinus, in trying to remain true to his understanding Platonism, rejects Aristotelianism, and posits instead, a revised version of the Platonic metaphysical hierarchy. In addition to examining Plotinus, critique of Aristotle, I examine some of Plotinus, other influences. These include Parmenides, Plato, Albinus and Numenius, in order to provide some grounding in understanding Plotinus' own philosophy. I conclude with an examination of Plotinus' metaphysics that shows its consistency with the general direction of Platonism, if in a different, hypostatic system. Plotinus I first principle, the One, is a synergy of negative and positive theology, grounded in the belief that being and thinking are extensionally the same in his second principle, Nous. That being and thinking are multiple necessitates the positing of a principle of unity which is "[Special characters omitted.] ," (beyond being) a phrase which Plotinus takes over from Plato's Republic in the service of his own philosophy. (shrink)