Plato, part 2

Plato's Republic, book III

414c How, then, could we devise one of those useful falsehoods we were talking about a while ago, (see 382a ff) one noble falsehood that would, in the best case, persuade even the rulers, but if that's not possible, then the others in the city?
What sort of falsehood?
Nothing new, but a Phoenician story which describes something that has happened in many places. At least, that's what the poets say, and they've persuaded many people to believe it too. It hasn't happened among us, and I don't even know if it could. It would certainly take a lot of persuasion to get people to believe it.

now let's skip ahead a bit...
423e These orders we give then, Adeimantus, are neither as numerous nor as important as one might think. Indeed, they are all insignificant, provided, as the saying goes, that they guard the one great thing, though I'd rather call it sufficient than great.
What's that?
Their education and upbringing, for if by being well educated they become reasonable men, they will easily see things for themselves, as well as all the other things we are omitting... (emphasis mine ~s)

424b To put it briefly, those in charge must cling to education and see that it isn't corrupted without their noticing it, guarding it against everything. Above all, they must guard as carefully as they can against any innovation in music and poetry or in physical training that is counter to the established order. And they should dread to hear anyone say:

People care most for the song

that is newest from the singer's lips.
(Odyssey 1.351-2)

Someone might praise such a saying, thinking that the poet meant not new songs but new ways of singing. Such a thing shouldn't be praised, and the poet shouldn't be taken to have meant it, for the guardians must beware of changing to a new form of music, since it threatens the whole system. As Damon says, and I am convinced, the musical modes are never changed without change in the most important of a city's laws.