What is the theme in Citizen of the Galaxy by Robert A. Heinlein?

Slavery is a major recurring theme in Citizen of the Galaxy. The novel opens with a slave auction, and we are made to see how our hero is stripped of his humanity via the institution of slavery. The auctioneer does not even bother to address our hero by name (or even to acknowledge that he has a name). He is merely referred to as "lot 97". Throughout the course of the auction, the "piece of merchandise" is spoken of as if he weren't present. Thorby has clearly suffered greatly during his time as a slave, and it requires a lot of time and effort from Baslim to heal the young boy's physical and psychological wounds (and Thorby never completely recovers. Even at the end of the novel, when he has returned to his family fortune, Thorby carries the scars of the whip on his back and is haunted with nightmares of physical abuse).