Plato's theory of the tripartite soul can be found in his work "The Republic". Reason (logos) - rational part of the soul that enables us to think, reflect, and make decisions based on logic and evidence. Spirit (thumos) - spirited or emotional part of the soul that drives our desires, passions, and ambitions. Appetite (epithumia) - instinctual or appetitive part of the soul that is responsible for our basic needs and desires, such as hunger, thirst, and sexual desire.