"I belong," I answered in voice shrill and tight, "to the nation of whosoever—without profit—pursues the good and the right."
"Then," said Mr. Sharpe, turning from me, "you are a member of an even more bedraggled and inconsequential diaspora than I had imagined." He poured his tea into his saucer and sipped it loudly. I rose from my chair and, like one distempered, began shouting, "This is insupportable!"
"Octavian," said Mr. Gitney, with a note of warning.
"They are not crimes," said Mr. Sharpe. "Your escape is a crime."
"It is theft of my property. Your labor belongs to me."
"When did I sell it?"
"Your body belongs to me."
"When did I—"
"Good God! How!" yelled Mr. Sharpe, striding to the door and unlocking it. "Put the mask back on him! I do not need to argue points with a specimen." (4.12.47-57)
We know this conversation doesn't seem like the most positive way to show Octavian's growth, since Mr. Sharpe does put the mask back on Octavian's head after this exchange, but it's important to note that this is the first time Octavian has verbally stood up to the people who own him.
It's the first time that he has expressed his opinions and his impeccable logic in a way that is succinct and powerful, which is why Mr. Sharpe has to shut him up with the mask. There's just no other way Mr. Sharpe could ever overcome Octavian's supremely rational line of argument.