Atticus discusses the reasons why he is defending Tom Robinson to Scout in chapter 9 of Lee's To Kill a Mockingbird. Scout asks him why he continues to fight for the case when he knows he will lose and everyone thinks he shouldn't take it in the first place. Atticus responds by saying, "Simply because we were licked a hundred years before we started is no reason for us not to try to win" (76).
When Scout hears this coming from her father, it reminds her of Cousin Ike Finch saying something similar:
The Missouri Compromise was what licked us, but if I had to go through it agin I'd walk every step of the way there an' every step back jist like I did before an' furthermore we'd whip 'em this time (76).
Not only does Cousin Ike mention the word "licked," which means getting beaten down or losing a battle, but the principle behind what he says is the same as what Atticus implies. Both men are saying that just because the odds are stacked against a person doesn't mean the battle isn't worth fighting. Atticus believes in this principle so much that he is willing to risk people hating him because of it.
One other person who fights a battle, even though she may lose, is Mrs. Dubose. Atticus thinks she is the bravest person he knows because she wins the fight against her addiction to morphine before she dies in chapter 11. Atticus explains to his children what it means to fight important battles even though a person knows he or she might lose in the following passage:
I wanted you to see what real courage is, instead of getting the idea that courage is a man with a gun in his hand. It's when you know you're licked before you begin but you begin anyway and you see it through no matter what. You rarely win, but sometimes you do. Mrs. Dubose won, all ninety-eight pounds of her. According to her views, she died beholden to nothing and nobody. She was the bravest person I ever knew (112).
Notice again that the word "licked" is used, followed by the explanation that a brave person is one who commits to a specific goal and sticks to it no matter how hard it is to accomplish. Atticus specifically says Mrs. Dubose won her fight because she died having fought a battle for herself and she achieved her goal. The same is true for Atticus when he defends a black man in a white court. Even though the odds are stacked against him and he most likely won't win, the battle still needs to be fought.