Atticus has many reasons for deciding to defend Tom Robinson in Harper Lee's novel, To Kill a Mockingbird, but "the main one is, if I didn't I couldn't hold my head up in this town." Scout has nearly found herself fighting again, this time with Cecil Jacobs over his claim that Atticus "defended niggers." So, Scout asks Atticus directly. After scolding her not to use the "N" word--"That's common"--he explains why he has undertaken the unpopular decision to defend Robinson, a black man, at his trial for raping a white woman.
"I couldn't represent this county in the legislature, I couldn't even tell you or Jem not to do something again... I could never ask you to mind me again... every lawyer gets at least one case in his lifetime that affects him personally. This one's mine, I guess."
Scout also discovers on the day of the trial that Atticus did not volunteer to defend Tom: He was asked by Judge Taylor himself.