What are Atticus' reasons for defending Tom Robinson in a case he thinks he can't win?

When thinking about this question you must take into account the racism that is in Maycomb at the time in which the book is set. Some characters in the story believe that black people are below white people and deserve the horrible treatment they get. They think that Atticus should not defend Tom simply because of the colour of his skin. There is no real reason why Atticus should not defend Tom Robinson.