how does mama argue the issue with cassie?

Mama sends the three youngest children to bed and comes in to talk to Cassie. She says that Big Ma did what she did because she didn't want Cassie to get hurt. She tells Cassie that Mr. Simms thinks Lillian Jean is better than her because she is white, even though it's not true. He is the type of person who needs to believe whites are better than blacks to make himself feel big.

Mama tells Cassie the story of slaves who were brought over from Africa, like Big Ma's great-grandparents. White people preached that people from Africa were not human so that they could make them slaves. They taught the slaves Christianity to make them obedient but even so people like Big Ma's father Papa Luke ran away three times. After the Civil War, when Papa Luke and Big Ma's mother Mama Rachel were freed, people continued to believe that blacks were not equal to whites. People like Mr. Simms hold on to that belief to make themselves feel important. Mama says that what black people give white people is fear, not respect. Cassie may call Lillian Jean "Miss" because she has to but she calls the black girls at her church "Miss" because she respects them.