The defendant wants to hide the truth because he's generally guilty. The defense attorney's job is to make sure the jury does not arrive at that truth.
“There is only one difference between a madman and me. The madman thinks he is sane. I know I am mad.”
“Most people today still believe, perhaps unconsciously, in the heliocentric universe every newspaper in the land has a section on astrology, yet few have anything at all on astronomy.”