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.