Quis custodiet ipsos custodes?

Quis custodiet ipsos custodes? is a Latin phrase found in the Satires (Satire VI, lines 347–348), a work of the 1st–2nd century Roman poet Juvenal. It may be translated as "Who will guard the guards themselves?" or "Who will watch the watchmen?".

The original context deals with the problem of ensuring marital fidelity, though the phrase is now commonly used more generally to refer to the problem of controlling the actions of persons in positions of power, an issue discussed by Plato in the Republic. It is not clear whether the phrase was written by Juvenal, or whether the passage in which it appears was interpolated into his works.

Original context
The phrase, as it is normally quoted in Latin, comes from the Satires of Juvenal, the 1st–2nd century Roman satirist. Although in its modern usage the phrase has wide-reaching applications to concepts such as tyrannical governments, uncontrollably oppressive dictatorships, and police or judicial corruption and overreach, in context within Juvenal's poem it refers to the impossibility of enforcing moral behaviour on women when the enforcers (custodes) are corruptible (Satire 6, 346–348):

Modern editors regard these three lines as an interpolation inserted into the text. In 1899 an undergraduate student at Oxford, E. O. Winstedt, discovered a manuscript (now known as O, for Oxoniensis) containing 34 lines which some believe to have been omitted from other texts of Juvenal's poem. The debate on this manuscript is ongoing, but even if the verses are not by Juvenal, it is likely that it preserves the original context of the phrase. If so, the original context is as follows (O 29–33):

Reference to political power
This phrase is used generally to consider the embodiment of the philosophical question as to how power can be held to account. It is sometimes incorrectly attributed as a direct quotation from Plato's Republic in both popular media and academic contexts. There is no exact parallel in the Republic, but it is used by modern authors to express Socrates' concerns about the guardians, the solution to which is to properly train their souls.

Several 19th-century examples of the association with Plato can be found, often dropping "ipsos". John Stuart Mill quotes it thus in Considerations on Representative Government (1861), though without reference to Plato. Plato's Republic though was hardly ever referenced by classical Latin authors like Juvenal, and it has been noted that it simply disappeared from literary awareness for a thousand years except for traces in the writings of Cicero and St. Augustine. In the Republic, a putatively perfect society is described by Socrates, the main character in this Socratic dialogue.

Socrates proposed a guardian class to protect that society, and the custodes (watchmen) from the Satires are often interpreted as being parallel to the Platonic guardians (phylakes in Greek). Socrates's answer to the problem is, in essence, that the guardians will be manipulated to guard themselves against themselves via a deception often called the "noble lie" in English. As Leonid Hurwicz pointed out in his 2007 lecture on accepting the Nobel Memorial Prize in Economic Sciences, one of Socrates's interlocutors in the Republic, Glaucon, even goes so far as to say "it would be absurd that a guardian should need a guard."

The issue of the accountability of political power, traced back to different passages of the Old and New Testaments, received great attention in medieval and early modern Christian thought, especially in connection with the exercise of authority in the Church and in church-state relations. In the Protestant tradition it also animated the debate about who was to be the final arbiter in the interpretation of the Scriptures.

In his 2013 report to the UN Human Rights Council, Alfred-Maurice de Zayas, the United Nations Independent Expert on the Promotion of a Democratic and Equitable International Order, elucidated Juvenal's continued relevance: “Crucial remains the conviction that the government should serve the people and that its powers must be circumscribed by a Constitution and the rule of law. Juvenal's question quis custodiet ipsos custodes (who guards the guardians?) remains a central concern of democracy, since the people must always watch over the constitutional behaviour of the leaders and impeach them if they act in contravention of their duties. Constitutional courts must fulfil this need and civil society should show solidarity with human rights defenders and whistleblowers who, far from being unpatriotic, perform a democratic service to their countries and the world.”

In popular culture

 * The question "Who watches the watchmen?" often partially appears as graffiti scrawled in the background of Alan Moore and Dave Gibbons' graphic novel Watchmen, but the phrase is never seen in its entirety. Moore stated in an interview that the title of the series related directly to this question, although at the time of the interview Moore did not know where the sentence originated.
 * It appears frequently in Terry Pratchett's Discworld novels, usually heard from Sir Samuel Vimes, commander of the City Watch. He answers it in Thud!, though very briefly, with the line "I do". When asked who watches over him, he follows it up with "I do, too". It also appears in Feet of Clay and I Shall Wear Midnight. It first appears in Guards! Guards! from a citizen, also addressed to Vimes, as: "Quis custodiet custard?"
 * "Who Watches the Watchers" is an episode of Star Trek: The Next Generation involving a group of anthropologists who are observing a primitive culture from a concealed location, but are revealed following an accident.
 * "Who watches the watchmen?" is spray painted on a Electronics store window at the end of the opening sequence in the film Watchmen (2009), in another sequence, a rioter protesting masked vigilantes is shown to be carrying a sign that says "WHO WATCHES THE WATCHMEN?".
 * In Batman v Superman: Dawn of Justice, the phrase is handwritten on a floor support near the staircase as Batman carries a weakened Superman over his shoulder at the climax of their duel.
 * Who Monitors the Birds, twelfth episode Space: Above and Beyond, makes reference to the saying.
 * In "America Decides," the eighth episode of Succession season four, Democratic political strategist Nate Sofrelli asks this question of Siobhan Roy when she informs him of her plans for election night at the headquarters of the American Television Network (ATN).
 * The 1980 album For How Much Longer Do We Tolerate Mass Murder? by English post-punk band The Pop Group features the song Justice with a chorus that contains the lyrics "Who guards the guards? Who polices the police?".
 * The 1991 album Out of Order by American thrash metal band Nuclear Assault contains the song Quocustodiat [sic], which critiques corruption and the abuse of power.
 * It appears in Frank Herbert’s Heretics of Dune.