[The detective story] keeps in some sense before the mind the fact that civilisation itself is the most sensational of departures and the most romantic of rebellions... It is the agent of social justice who is the original and poetic figure, while the burglars and footpads are merely placid old cosmic conservatives, happy in the immemorial respectability of apes and wolves. [The police romance] is based on the fact that morality is the most dark and daring of conspiracies.