The main claim of this project is that in his tri-partite psychology, Plato theorizes spirit (thumos) as the defense of one’s own (to oikeion). More specifically, my claim is that as understood by Plato, the spirited part of the soul distinguishes between what is one’s own and alien (allotrion) and motivates action aimed at defending what it takes to be one’s own from alien threats. It is a central commitment of this reading that Plato conceives of to oikeion as a form of value, or way of being good. In the first person, one’s own is mine, or more perspicuously, me. Thus, I am arguing that Plato sees the spirited part as defined by an identification of the self (to oikeion) with the good. Plato also, and importantly, believes that this spirited sense of self can extend to other people and things insofar as they too are taken to be oikeion. However, the concern for others, and “us vs. them” thinking Plato sees as resulting from the extension of the spirited part’s sense of self, is by Plato’s lights the expression of a distinctive form of self-concern wherein what is good is me, and the bad an essentially threatening other. This reading of Plato’s conception of thumos has two primary virtues. First, it explains Plato’s sense of the spirited part of the soul as a limited—and violent— principle of psychic unity. Second, it shifts and deepens our understanding of Plato’s conception of spirit, by revealing that Plato conceives of thumos as a non-rational form of thought constituted by a limited understanding of the good as the self. Coordinate with these two virtues of my reading, the dissertation has two parts. In the first part of the dissertation (comprised of chapters 1-3), I argue for reading thumos as the defense of one’s own on the grounds that it explains Plato’s characterization of the spirited part as a limited and violent principle of unity. In the second part of the dissertation (comprising chapters 4 and 5), I further develop the content of Plato’s understanding of thumos as the defense of one’s own. My aim in doing so, is to both firmly establish that Plato conceives of spirit as on par with appetite and reason in terms of its significance for human life, and to show that his univocal account constitutes a nuanced and illuminating interpretation of that aspect of our psychology he labels thumos.