What does Atticus tell Scout about his reasons for defending Tom?

Atticus explains that he is defending Tom Robinson becasue he values equailty and justice. He believes that everyone is equal and therefore just because Tom Robinson is black, doesn't mean that he should not defend him. He says to Scout many times, if he did not defend this man, he could never tell Jem or her (Scout) not to do something. He has to set a good example for his children and if he expects them to do something or follow certain values, then he must display them first.