Seventh-century Persian king Khosrow II is said to have tested the loyalty of courtiers whom he believed were becoming too close. Telling one of his decision to execute the other, he would swear the man to secrecy and then watch the friend’s behavior. If it went unchanged, he knew the first man was loyal and had kept silent; if different, he was a traitor and dealt with accordingly. 

Michael Hayden, former director of the CIA and NSA, claimed while discussing the NSA’s collection of telephone-call metadata, “We kill people based on metadata,” quickly qualifying, “But that’s not what we do with this metadata.” When declining an interview about alleged U.S. cyberattacks on Iran, he sent a one-line email that read, “Don’t know what I would have to say beyond what I read in the papers.”

Union general William T. Sherman believed newspaper correspondents to be liabilities. “A spy is one who furnishes an enemy with knowledge useful to him and dangerous to us,” Sherman wrote in an 1863 letter. “I say—in giving intelligence to the enemy, in sowing discord and discontent in an army—these men fulfill all the conditions of spies.”

New York Giants pitcher Christy Mathewson wrote in 1912 that he had heard the Philadelphia Athletics “had a spy” who stole signs and “tipped the batters by raising and lowering an awning a trifle.” In Philadelphia for the World Series the year before, Mathewson had looked for the culprit. “In the enemy’s camp, I kept watching the windows of the houses just outside the park for suspicious movements,” he wrote. “But I never discovered anything wrong.”

Charles d’Éon de Beaumont went to Russia in 1755 as a secret correspondent of French king Louis XV. Disguised as a woman, he obtained an appointment as reader to a Romanov empress; he returned for a mission the next year dressed as a man. By the 1770s speculation about his gender reached such fervor that odds were quoted by London brokers. An 1810 postmortem finally confirmed that he was anatomically male; Marie Cole, his companion of fourteen years, reportedly “did not recover from the shock for many hours.”