Guards! Guards! (Discworld, #8)

by Terry Pratchett

Once you’ve ruled out the impossible then whatever is left, however improbable, must be the truth. The problem lay in working out what was impossible, of course. That was the trick, all right.

“I believe you find life such a problem because you think there are the good people and the bad people,” said the man. “You’re wrong, of course. There are, always and only, the bad people, but some of them are on opposite sides.”