In the book Uncle Tom's Cabin how does George demonstrate that he is not the kind of man who can “become a thing”?

George is put to the hardest of work, He is treated like an animal but George refuse to act like one. He refuses to become a mere possession, a "thing". Although he said nothing, his face and body language showed he was still a human being that is forced to be a slave,

"George was taken home, and put to the meanest drudgery of the farm. He had been able to repress every disrespectful word; but the flashing eye, the gloomy and troubled brow, were part of a natural language that could not be repressed, – indubitable signs, which showed too plainly that the man could not become a thing."