In The Tempest, Prospero is served by both Ariel and Caliban. But whereas the former serves willingly, Caliban despises—and ultimately betrays—his master. Both seek their freedom, but they do so through very different means. One way to think about them is as foils for one another: comparing and contrasting them can help us better understand the relationship between the mind and the body in the play.
Ariel, we learn, is a sort of disembodied spirit capable of invisibility and even taking on multiple forms at once; in contrast, Caliban is associated with materiality and the body. Prospero calls Caliban not only a “slave” but also “earth,” establishing a hierarchical relationship between his two servants that is at least in part informed by their material conditions.
Caliban’s focus appears to be largely driven by his own bodily needs: when he comes on stage for the first time, he announces that he “must eat [his] dinner,” and we learn that Prospero used to treat Caliban better until Caliban attempted to rape his daughter. Caliban’s character is shaped by his bodily needs, whereas Ariel needs no body at all.
While Caliban betrays Prospero in an attempt to gain his freedom, Ariel instead serves willingly, hoping that his master will honor his promise to release the spirit. Whether Ariel serves out of true appreciation or as a strategy is a question of some debate, although ultimately Prospero does free Ariel in 5.1.