Prospero treats Caliban as a slave. Caliban's speech, found in 1.2.334-47, establishes Caliban's point of view of his treatment by Prospero early on in the play, and the audience needs to keep this in mind throughout the remainder of it. The general complaint by those who have read the play, including most college professors, use the alleged complaint of rape as a justifiable reason for the poor treatment Caliban receives at the hands of all who come into contact with him. But this is taking political correctness too far, in my opinion. Caliban, it must be remembered, is a "natural" creature and does not hold to or even understand a society's ideology about sexual relations. I have argued this in papers and, in effect, set myself up as Caliban's defense attorney. :o) In reading the play from such a defense attorney's point of view, there are other places throughout in which can be found good evidence to argue the case. I shall let you have the pleasure of reading through to find those parts :o)