'Who guards the guardians' is a phrase lifted from the satirical poet Juvenal *Satires*, (and not as I thought from Plato). The context here is marital infidelity, but Juvenal being a (satirical) poet - it wouldn't take a great deal to convince me that he's talking politics too.

Its often used in reference to the section of Platos Republic where he tackles the question of political corruption.

Is it fair to say that in Platos *Republic* he answers this by relying on *virtue*, that is the guardians refrain from *selfishness* (they guard themselves from themselves) by relying on the *noble lie*? That is they are taught by the educators of the city to not act in their own interests, but the interests of the whole. 

Could one answer this by setting up two opposing sets of guardians - each watches the other. But then what stops them from acting in concert? If they do we're back to one set of guardians, except of course they've allowed themselves to have been corrupted because they're acting in concert - so the situation is worse than the original context.