The Problem of Existence in Western Philosophy: Aristotle - Thomas Aquinas Maria Dimarogkona Abstract: This thesis consists in the presentation of Thomas Aquinas' ontology in relation to Aristotle's account of the nature of being. In the first chapter an account of Presocratic thought is presented, beginning from Thales and ending with Parmenides, who is often characterized as the founder of metaphysics or ontology. The second chapter consists in a brief presentation of Aristotle's account of the nature of being. Finally Aquinas' ontology is presented in the third chapter, which closes with a brief discussion of the connection between his approach and modern philosophy. Thomas Aquinas was an immensely influential philosopher and theologian in the tradition of scholasticism. In De Ente et Essentia, which is thought to be his most personal work, Aquinas gives his own answer to one of the most fundamental problems of Western philosophy, which was formulated by Aristotle as the question "What is being? ". As we will see, Thomas' answer leads to a reformulation of the problem of being which becomes the problem of existence (or the question "what is existence?"). Étienne Gilson1, in his work L'être et l'essence, presents the solutions offered to this problem by Aquinas' predecessors, as well as those who succeeded him, and concludes that only Thomas managed to face the paradoxes born from it, and succeeded in building a coherent system. In other words, Gilson claims that Thomas' solution is the most complete. The examination of this claim is a very difficult endeavor, as it requires the critical examination of the answers of all great Western thinkers to the question of being, or existence, since the beginning of philosophy. The much more feasible objective of this thesis is to offer a better understanding of Thomas' answer, by examining it in relation to the Aristotelean one, on which it is heavily based.