Many philosophers today approach important psychological phenomena, such as weakness of the will and moral motivation, using a broadly Humean distinction between beliefs, which aim to represent the world, and desires, which aim to change the world. On this picture, desires provide the ends or goals of action, while beliefs simply tell us how to achieve those ends. In the Republic, Socrates attempts to explain the phenomena using a different distinction: he argues that the human soul or psyche consists in reason, spirit, and appetite. It is initially tempting to assimilate Socrates’ picture to the standard belief/desire model, and to think that reason’s role in motivating action is restricted to calculating the best means for satisfying spirited and appetitive desires. But this would be a mistake, since Socrates thinks that each element in the soul is capable of setting the ends of action. But then how exactly should we understand these elements? My aim in this essay is to introduce the reader to Plato’s theory of the tripartite psychology. In part 2, I present Socrates’ argument for the claim that the soul has three elements. In part 3, I provide a general characterization of reason, spirit, and appetite, respectively. I then turn to discuss two central interpretive issues. In part 4, I discuss the sense in which Socrates considers the appetitive and spirited elements to be non-rational. And in the final part of the essay, I discuss the issue of how we ought to conceive of the parts of the soul, and more specifically, whether we should think of them as agent-like parts, or in some other way.