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. 

Ariel and Caliban are markedly different in nature and appearance. Ariel as his name suggests is a spirit, ethereal, while Caliban is gross and bestial. Both are connected to the witch Sycorax who ruled the island before Prospero: Ariel was imprisoned by her, before being rescued by Prospero, while Caliban is her offspring. Both are servants of Prospero. Ariel performs a lot of magical feats at his master's command while Caliban is a more lowly and menial servant.

Caliban grumbles all the time at his servitude; he resents Prospero deeply and eventually leaves him for another master, and plans to murder him, although he is foiled in this. Ariel appears to be far more highly valued as a servant by Prospero than is the savage Caliban, but Ariel, too shows some signs of discontent with his servitude, reminding Prospero quite forcefully at one point of Prospero's promise to free him. However when Prospero is annoyed by this, he quickly apologises.  Caliban frets constantly at being kept under, while Ariel generally puts up with it. 

Caliban has often been taken as being representative of the New World natives often oppressed and enslaved by European conquerors, and he does have grounds for complaint against Prospero's brusque treatment of him. But he does not appear very favourable in himself; he apparently tried to rape Miranda and is capable of plotting murder. He is very much tied to his passions and instincts whereas Ariel, being a spirit, is more detached from things in general.