What reason does Atticus give for defending Tom Robinson?

Atticus says many of the town people think he ought not defend Tom because he is black. Scout asks why he's still doing it if people don't want him to, and Atticus responds that if he didn't take the case, he wouldn't be able to "hold up my head in town," represent his county in the legislature, or even tell his children what to do. Atticus explains that every lawyer gets at least one case in a lifetime that affects them personally, and that this one is his.