From Aristotle's "Politics", book 5 part 11, circa 350 BC, a selection chosen by the White Rose Society for their 3rd anti-nazi leaflet, translation by Benjamin Jowett, MIT Classics Archive.
"A tyrant should also endeavor to know what each of his subjects says or does, and should employ spies, like the detectives at Syracuse, and the eavesdroppers whom Hiero was in the habit of sending to any place of resort or meeting; for the fear of informers prevents people from speaking their minds, and if they do, they are more easily found out. "