Because Tereza's mother blamed Tereza for the way her life turned out, she insisted on keeping Tereza trapped with her in a world of immodesty, "a vast concentration camp of bodies," as punishment.
This is why Tereza looks in the mirror as she does; she longs to be "a body unlike other bodies," to find within her body her "sad, timid, self-effacing soul" (2.8.2).
Consider the day that she met Tomas. She was working as a waitress, her body was tired, and her soul lay somewhere in the pit of her stomach.
And then she saw Tomas. He seemed special to her because he had an open book on the table.
Books are special to Tereza; in her eyes, they are "emblematic of a secret brotherhood" (2.8.4). When she was a child, they were a way of escaping from the world of her mother. Reading them made her different from others.
The narrator notes that the books indeed made Tereza different, but they also made her old-fashioned.
Anyway, when Tomas appeared to Tereza, her soul rose from the pit of her stomach to show itself to him.