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.

At the beginning of The Tempest, Caliban and Ariel are both the slaves of Prospero. Both are magical creatures whose precise identity is not clear. Caliban is generally portrayed as a monster, the offspring of the witch Sycorax, from whom Prospero wrested control of the island. Ariel is a spirit of the air who was imprisoned by Sycorax and rescued by Prospero. It is not surprising, therefore, that the two are treated differently by Prospero and have different attitudes toward him.

Ariel represents faithful service and obedience. In postcolonial readings of the play, Ariel is the compliant native who welcomes the conqueror as a liberator and serves him willingly, though still with an idea of eventual independence. Caliban is the unwilling slave and has generally been treated unsympathetically by critics and audiences as a violent brute. However, from a postcolonial perspective, he may be seen as the victim of Prospero's oppression.

There are various different readings of Prospero's character. If you regard him as a benevolent, wise old magician, you will probably see him as the protector of Ariel and stern but fair master of Caliban, keeping the monster in check. If you take the postcolonial perspective, however, then he is an oppressive and untrustworthy figure who tricks Caliban into helping him and then enslaves him as soon as the opportunity presents itself. With Ariel, he is almost equally duplicitous, promising freedom, which he withholds until the very end of the play, when he is about to leave the island in any case.

According to Caliban himself in Act I, Scene 2, he is the only inhabitant of the island that Prospero and Miranda land on:

This island's mine, by Sycorax my mother, 
Which thou takest from me. When thou camest first, 
Thou strokedst me and madest much of me, wouldst give me 
Water with berries in't, and teach me how
To name the bigger light, and how the less, 
That burn by day and night: and then I loved thee 
And show'd thee all the qualities o' the isle, 
The fresh springs, brine-pits, barren place and fertile: 
Cursed be I that did so! All the charms
Of Sycorax, toads, beetles, bats, light on you! 
For I am all the subjects that you have, 
Which first was mine own king: and here you sty me 
In this hard rock, whiles you do keep from me 
The rest o' the island.

In this speech, Caliban indicates both that the island is his because it was his mother's and he is its sole inhabitant, "For I am all the subjects that you have."

In Act 3, Scene 2, Ariel the fairy torments Caliban by pretending to be Trinculo, a jester on the shipwrecked vessel whose crew lands on the island.  For example, Caliban tells Stephano and Trinculo:

I am subject to a tyrant,/A sorcerer, that by his cunning hath/Cheated me of the island

When Ariel enters invisibly to mimic Trinculo and proclaim "Thou liest," Caliban's response indicates his tormented and enraged state:

Thou liest, thou/jesting monkey, thou!/I would my valient master would destroy thee./I do not lie.

Ariel continues to repeat the accusation and further plots against Caliban by telling Prospero of Caliban's attempt on his life: "This will I tell my master."

Prospero uses his magic to control both Ariel and Caliban, albeit in different ways. In that sense his attitude towards them is inherently exploitative. Ariel and Caliban exist to serve him, and how they perform their duties will largely determine how they are treated. Caliban, for instance, is treated by Prospero in a malicious, vengeful manner. Prospero's magic is used to control Caliban, to keep him in a permanent state of subjugation. It's no wonder that Caliban is so surly and resentful towards his master. But ultimately he has no choice; through the exercise of his magic powers Prospero has a hold on him:

I must obey. His art is of such pow’r It would control my dam’s god, Setebos, And make a vassal of him (Act I Scene ii).

It's different with Ariel. Whereas Prospero uses his magic to subjugate Caliban, he uses it to set Ariel free from the curse of Sycorax, the evil witch. Because of this, Ariel adopts a submissive posture towards Prospero; he feels forever in his debt. Even so, Ariel makes so bold as to ask his master if he might one day be released from his authority:

Is there more toil? Since thou dost give me pains, Let me remember thee what thou hast promised, Which is not yet performed me . . . My liberty (Act I Scene ii).

Yet Prospero immediately puts Ariel back in his box. He threatens to imprison him in an oak tree for twelve years. Wisely, Ariel reverts to his supine attitude:

Pardon, master. I will be correspondent to command And do my spiriting gently (Act I Scene ii).

So even though Prospero has a more benevolent attitude towards Ariel, he still makes it abundantly clear that he is firmly in charge. Until he's finally granted his liberty, Ariel's relationship to Prospero is characterized by domination and control no less than Caliban's.