  Indeed, this notion of parts is robust enough to make one wonder why  reason, spirit, and appetite are parts at all, as opposed to three independent subjects. But the Republic proceeds as  though every embodied human being has just one soul that comprises  three parts. No embodied soul is perfectly unified: even the virtuous person, who makes her soul into a unity as much as she can (443c–e),  has three parts in her soul. (She must, as we shall see, in order to  be just.) But every embodied soul enjoys an unearned unity: every  human’s reason, spirit, and appetite constitute a single soul that is the unified source of that human’s life and is a unified locus of  responsibility for that human’s thoughts and actions. (It is not as though a person is held responsible for  what his reason does but not for what his appetite does.) There are  questions about what exactly explains this unearned unity of the  soul (see E. Brown 2012).