A/N: I should have covered this, but I skipped over it when I first got to it.
Chapter 23: Year 4 Revisited – House Elf Slavery
"Slave labour," said Hermione, breathing hard through her nose. "That's what made this dinner. Slave labour." Harry paused in his cutting up of a large roast potato to say "There's two kinds of slavery, though, Hermione. One where the slave is forced into it by someone else, and one where they become a slave to pay off a debt because they don't have any other way of paying. How do you know which it is?" Hermione stared at Harry furiously. "It's slavery!" she said. "They work and don't get paid! That's wrong!" "They don't get paid in money," he replied. "What if they get paid some other way? Like, what if they need magic to live, but they have to be working like they do in order to get it. For all you know, having a job, enough food to eat, and a place to sleep is all they want. Besides, from your definition of wrong, Professor Binns is a slave! He works, but doesn't get paid, or at least I don't think he does. He's a ghost – so what's he going to do with money?"
Hermione blinked in surprise as Ron cut in. "And you only know about two elves – Dobby and Winky. Dobby was Malfoy's, so you know they weren't nice to him, and you only saw Winky at the World Cup, when Mr Crouch was cross with her. You can't assume all elves are treated like that! If our family had one, we wouldn't!"
As Harry and Ron carried on eating, Hermione came to a decision. "Well then," she said, picking her knife and fork back up, "I know what I'll be looking up first in the library."