Quis custodiet ipsos custodes?
Quis custodiet ipsos custodes? is a Latin phrase found in the work of the Roman poet Juvenal from his Satires (Satire VI, lines 347–8). It is literally translated as "Who will guard the guards themselves?", though is also known by variant translations.
The original context deals with the problem of ensuring marital fidelity, though it 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.
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 universal, timeless 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):
audio quid ueteres olim moneatis amici,
"pone seram, cohibe." sed quis custodiet ipsos
custodes? cauta est et ab illis incipit uxor.
I hear always the admonishment of my friends:
"Bolt her in, constrain her!" But who will guard
the guardians? The wife plans ahead and begins with them.
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):
consilia et ueteres quaecumque monetis amici,
"pone seram, cohibes." sed quis custodiet ipsos
custodes? qui nunc lasciuae furta puellae
hac mercede silent crimen commune tacetur.
… I know
the plan that my friends always advise me to adopt:
"Bolt her in, constrain her!" But who can watch
the watchmen? They keep quiet about the girl's
secrets and get her as their payment; everyone hushes it up.
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' 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' interlocutors in the Republic, Glaucon, even goes so far as to say "it would be absurd that a guardian should need a guard." But Socrates returns to this point at 590d, where he says that the best person "has a divine ruler within himself," and that "it is better for everyone to be ruled by divine reason, preferably within himself and his own, otherwise imposed from without."
The question "Who watches the Watchmen?" frequently appears in Alan Moore and Dave Gibbons' graphic novel Watchmen, though the phrase is never seen in any one place in its entirety (barring the closing page of Issue 11). 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.
The internet comedy group Loading Ready Run made a video parodying the question of 'Who watches the Watchmen?', proposing that the Watchmen watch the city, the 'Neighbourhood Watchmen Watching Organisation' watch the Watchmen, the 'Watching The Neighbourhood Watchmen Watching Organisation Organisation' watch the Neighbourhood Watchmen Watching Organisation, and Geoff watches the 'Watching The Neighbourhood Watchmen Watching Organisation Organisation'. An unnamed person is seen to be watching Geoff.
An episode of the cartoon television series The Simpsons refers to this philosophical question. In episode 1F09, "Homer the Vigilante", when Homer is talking about having abused his vigilante powers, his elder daughter Lisa asks, "If you're the police, who will police the police?" Homer responds, "I don't know. Coast Guard?"
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?"
In the second Star Trek television series Star Trek: The Next Generation, episode #52 from season three is titled, "Who Watches the Watchers." The episode involves a group of anthropologists who are observing a primitive culture, but are discovered after an accident causes their camouflaged "duck blind" to fail, revealing their presence.
During an episode of Justice League Unlimited, Batman says the phrase to Green Arrow after Arrow talked Superman and the other founding members out of disbanding the League. Green Arrow translates the Latin to "Who guards the guardians".
In Dan Brown's novel, Digital Fortress, the phrase appears engraved in a ring owned by Ensei Tankado, a former NSA employee who disapproved of NSA's intrusion of the people's private lives. The phrase is aimed at NSA who checks for any information on emails sent over the web that endangers national security. The phrase asks who will keep the NSA in check as they do others.
In Did I Ever Tell You How Lucky You Are? by Dr. Seuss, the entire town of Hawtch-Hawtch is employed as watchers watching over other watchers leading to the first watcher who is watching the "lazy town bee" so it will work harder. Since the bee wasn't working harder, it was assumed the bee-watcher wasn't watching hard enough and needed to be watched.
In the video game Halo: Combat Evolved Anniversary, the seventh Terminal's description is "Who monitors the Monitor?", and details 343 Guilty Spark's frustration with loneliness. The replacement of "watchmen" with "Monitor" is a reference to Guilty Spark's position of Monitor of Installation 04. 
- E.O. Winstedt 1899, "A Bodleian MS of Juvenal", Classical Review 13: 201–205.
- Recently J.D. Sosin 2000, "Ausonius' Juvenal and the Winstedt fragment", Classical Philology 95.2: 199–206 has argued for an early date for the poem.
- E.g. Who Are The Watchmen?; T. Besley and J.A. Robinson, "Quis Custodiet Ipsos Custodes? Civilian Control over the Military", Journal of the European Economic Association v. 8, pp. 655–663, 2010; and P. Corning, The Fair Society: The Science of Human Nature and the Pursuit of Social Justice,, University of Chicago Press, p. 146, 2011.
- Oxenham, H.N. (1878). "Moral and Religious Estimate of Vivisection". Gentleman's Magazine and Historical Chronicle 243 (Jul. to Dec.): 732.
- Maguire, Thomas (1866). An Essay on the Platonic Idea. London: Longmans, Green, Reader, and Dyer. p. 39.
- Jayapalan, N. (2002). Comprehensive Study of Plato. Atlantic Publishers & Dist. p. 10.
- Plato (2008) [c. 380 BC]. The Republic. Benjamin Jowett, transl; EBook produced by Sue Asscher and David Widger. Project Gutenberg.
How then may we devise one of those needful falsehoods of which we lately spoke – just one royal lie which may deceive the rulers, if that be possible, and at any rate the rest of the city?
- Book III, XII, 403E, p. 264 (Greek) and p. 265 (English), in volume I, of Plato, The Republic (ΠΟΛΙΤΕΙΑ), with an English translation by Paul Shorey, London, William Heinemann Ltd.; New York: G. P. Putnam’s sons, 1930, as cited by Leonid Hurwicz," But Who Will Guard the Guardians?", Nobel Prize Lecture, December 8, 2007, Accessed 4-27-2011.
- Plato (1992). Republic. Translated by G.M.A. Grube, revised by C.D.C. Reeve. Indianapolis: Hackett.
- Atkinson, Doug. "The Annotated Watchmen". http://www.capnwacky.com/rj/watchmen/chapter1.html.
- Plowright, Frank. "Preview: Watchmen". Amazing Heroes. June 15, 1986.
- [1F09] Homer the Vigilante
- 343 Industries. Halo: Combat Evolved Anniversary. Xbox 360. Microsoft Game Studios