ARIEL: I prithee,
Remember I have done thee worthy service,
Told thee no lies, made thee no mistakings, served
Without or grudge or grumblings. Thou didst promise
To bate me a full year.
PROSPERO: Dost thou forget
From what a torment I did free thee?

– William Shakespeare

The Tempest, Act 1, Scene 2. Ariel wants his freedom from servitude to Prospero after serving him well for a year. But his reluctant master grumbles that he had freed him from a life of torment. This suggests that Prospero treats Ariel more like a slave than simple servant.