-Plato, The Republic
This is a question posed by Plato in The Republic, a book which discusses the idea of a Utopian government. He is asking, who will protect against the protectors of the city. The guards are analogous to government. Plato is concerned about how you would keep a government from harming the people. Plato eventually concludes that the people will guard the guards. This question has been influentially throughout history whenever a people has wished to establish a government that answers to them.
