How we cite our quotes: All quotations are from The Dark Knight.
Quote #1
THE JOKER: I believe whatever doesn't kill you simply makes you… stranger.
This is a riff on a quote from philosopher Friedrich Nietzsche, who famously said "that which does not kill us makes us stronger." The Joker changes one little letter and alters the entire meaning. In his words, he's basically saying that if life doesn't kill you, it will drive you nuts, which is the only rule he seems to adhere to.
Quote #2
VIGILANTE: What gives you the right? What's the difference between you and me?
BATMAN: I'm not wearing hockey pads!
An early salvo in the movie's main philosophical meditation: what's the difference between good and evil? Why does Batman get to do what he does and nobody else get to join him? He has a very practical answer here, but the rule's going to be put to the test when the Joker enters the picture.
Quote #3
THE JOKER: Batman has no jurisdiction.
A little dig here from the Joker, suggesting that Batman breaks the rules that he finds inconvenient, too. Considering the context—he's warning the mob that Batman is going to bring them down—it also shows the Joker's admiration for the power that kind of lawlessness can bring. That's scary, man.