The project inquires into Hannah Arendt's thinking of the political in order to develop from it a possible new thread towards a different philosophy beyond metaphysical legacy. Arendt's notion of human existence as always necessary doing to the world rather than just being is read here from her understanding of man as beginning. It is argued that, as such, it uncovers the existentialist dimension of Arendt's work, by and large neglected in Arendtian literature, while also influencing Arendt's understanding of the political as working freedom out of necessity and taking up of existence. This ultimately constitutes the unexplored contribution by Arendt to contemporary ontology: If ontology if to overcome and move beyond its metaphysical roots, it must ask political questions as the fundamental philosophical questions since it concerns the being that is always necessarily (in) doing. This reading of Arendt's project is founded upon the parallels of her thought with Heidegger's work that proceeds in the same philosophical direction. In order to understand and develop the implications of Arendt's thinking of the political in that direction, the project engages with Arendt's work on the source of action, which is interpreted here as a conceptual effort to overcome the metaphysical dualism of world. Arendt's theory of mind is analysed in relation to two fundamental principles of action, plurality and freedom, in order to argue that none of the three mind faculties - thinking, willing, judging - can generate action. The theory is then expanded through reference to the fragments on imagination in Arendt's writing, elaborated and developed in dialogue with Heidegger's and Castoriadis' work on the concept of imagination. Developing the concept of originary imagination as the source of action from these intuitions in Arendt's thought, the project uncovers the ontological fundament of opposition of Arendt's work to Heidegger's philosophy and establishes the ground to assert that Arendt's work offers an opening to post-metaphysical philosophy. While Heidegger's project is arrested by the notion of Dasein as being-in-the-world, unable to transcend givenness of existence and finally affirming it, Arendt puts forth the notion of human existence as primarily being to the world, always bringing about the new and resisting the givenness. This notion of human existence suggests that the fundamental questions of ontology ought to be political questions, the questions of doing rather than being.