Atticus first tells Jem that any jury in their part of the world is either going to convict or acquit. Atticus does not come right out and say the jury was prejudiced. Searching for some defense of Tom Robinson, Jem suggests that maybe rape shouldn't be a capital offense. Atticus recognizes Jem is just trying to figure out a way for Tom to receive a lighter sentence, or be freed somehow, but Atticus adds that he has no problem with rape being a capital offense. The problem he has is with a jury that convicts a man based on circumstantial evidence. After discussing the efficiency of juries, Atticus says:
“If you had been on that jury, son, and eleven other boys like you, Tom would be a free man,” said Atticus. “So far nothing in your life has interfered with your reasoning process. Those are twelve reasonable men in everyday life, Tom’s jury, but you saw something come between them and reason. You saw the same thing that night in front of the jail. When that crew went away, they didn’t go as reasonable men, they went because we were there. There’s something in our world that makes men lose their heads—they couldn’t be fair if they tried. In our courts, when it’s a white man’s word against a black man’s, the white man always wins. They’re ugly, but those are the facts of life.”
Jem responds that this isn't fair. This marks a significant moment in Jem's life when he learns that children are more open-minded than adults and the adult world is full of injustices.