Quis custodiet ipsos custodes?
Quis custodiet ipsos custodes? (Who will guard the guards themselves?) is a quotation from one of the satirical poems of the Roman poet Juvenal (?60-?140 CE) (Satire VI, 347-348). It is sometimes abbreviated to Quis custodiet? (‘Who will guard?) on the assumption that the addressee will recognise the quotation and know how to complete it.
The quotation is used, typically in political or similar contexts, as a way of asking how those in authority are to be held to account for their actions and what safeguards are available to protect those under their authority against any abuse of power.
This way of understanding and using the quotation has almost certainly been influenced by the assumption that it implicitly refers to Plato’s account of the Ideal State in Republic II-VII, 367-541, a natural assumption inasmuch as the philosophically enlightened rulers in that Ideal State are known as Guards or Guardians (φύλακες (phulakes), i.e., in Latin custodes). In Juvenal’s Satire VI, however, the question Quis custodiet? is asked in a very different context. The theme of this satire is marital fidelity: Juvenal deplores the tendency of wives to cheat on their husbands and considers how they may be prevented from doing so. One possibility, briefly discussed in the passage of which Quis custodiet? is a part, is that wives be kept at home in a state of virtual imprisonment – a proposal Juvenal immediately rejects on the grounds that a clever wife will win over and corrupt those who are supposed to be her gaolers. Here are the lines:
audio quid ueteres olim moneatis amici, 'pone seram, cohibe.' sed quis custodiet ipsos custodes? cauta est et ab illis incipit uxor.
(I hear what you, my old friends, have been advising me for some time - keep her at home, put her under lock and key. But who will guard the guardians? A wife can be crafty and will begin with them.)
Finally, and incidentally, whether Juvenal is actually the author of Quis custodiet? has been disputed, many commentators believing that the passage of which it is a part, i.e., the three lines quoted above, are an interpolation by another author.