“But who will guard the guardians themselves?”
VI, line 347
Variant translations:
But who is to guard the guards themselves?
Translated by Lewis Evans, in The Satires of Juvenal, Persius, Sulpicia, and Lucilius (1861), p. 51
Who watches the watchmen?
Famous variant used in the graphic novel, Watchmen by Alan Moore and Dave Gibbons.
The original context is that a husband might lock his wife in the house to prevent her adulteries, but she is cunning and will start with the guards; hence, who guards the guards? The phrase has come to be applied broadly to people or organisations acting against dishonesty or corruption, esp. in public life. See Quis custodiet ipsos custodes? at Wikipedia.
Satires, Satire VI
Original
Sed quis custodiet ipsos custodes?
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Juvenal 24
ancient roman poet 50Related quotes

Independence Day speech (1828)
Context: Is there a thought can fill the human mind
More pure, more vast, more generous, more refined
Than that which guides the enlightened patriot's toll:
Not he, whose view is bounded by his soil;
Not he, whose narrow heart can only shrine
The land — the people that he calleth mine;
Not he, who to set up that land on high,
Will make whole nations bleed, whole nations die;
Not he, who, calling that land's rights his pride
Trampleth the rights of all the earth beside;
No: — He it is, the just, the generous soul!
Who owneth brotherhood with either pole,
Stretches from realm to realm his spacious mind,
And guards the weal of all the human kind,
Holds freedom's banner o'er the earth unfurl'd
And stands the guardian patriot of a world!

Source: Minor Characters: A Beat Memoir

As quoted in Hindu Psychology : Its Meaning for the West (1946) by Swami Akhilananda, p. 204

Footnote: He does this because of his altruistic (parental) instinct. The higher one rises in the vertebrate scale the more altruistic one becomes. The higher vertebrates are just one mass of altruism.
The Three-Spined Stickleback
How to Become Extinct (1941)

“The people who guard the rainbow don't like those who get in the way of the sun.”
Source: Going Postal

“Honor to those who in the life they lead
define and guard a Thermopylae.”
Thermopylae http://www.cavafy.com/poems/content.asp?id=69&cat=1
Collected Poems (1992)
Context: Honor to those who in the life they lead
define and guard a Thermopylae.
Never betraying what is right,
consistent and just in all they do
but showing pity also, and compassion;
generous when they are rich, and when they are poor,
still generous in small ways,
still helping as much as they can;
always speaking the truth,
yet without hating those who lie.