ow will the Guardians get hold of the knowledge they need? To answer this, Socrates is to start with completely negative. He totally rejects the traditional Athenian education, which was based on Homer and the poets. His theories owe much to contemporary Spartan practice.
The Guardians will need mousike and gymnastike (training for the mind and the body respectively - not "music" and "gym").
Mousike
Homer and the poets misrepresent gods and heroes:
God is good and can't be responsible for evil. (p.135)
Gods can't change - if they're already perfect this would have to be a change for the worse.
Existing poetry encourages moral weakness:
It doesn't inspire a fighting spirit.
It encourages insubordination, disobedience and arrogance (e.g. Iliad I).
It shows gods as fond of laughter.
Gods and heroes are supposed to be models to copy - Homer's are totally unsuitable.
Poetry (esp Homer and Tragedy) encourages you to "be" someone else (a form of lying) - and thus break the "1 man 1 job" rule. How can Homer "be" a priest, then Agamemnon, then Achilles....? Drama also encourages men to act like women or slaves or criminals.
And so all poets, even Homer (especially Homer), are sent packing - like the quacks and impostors from Cloudcuckooland in Aristophanes' Birds.
Music too will need censorship - certain "modes" encourage immoral behaviour.
Gymnastike
This is Physical Education, and like mousike, its purpose is to train the character: especially during the ages of 18 - 20. It overlaps with military training. As mousike trains the intellect, so gymnastike develops spirit (energy and initiative). The Guardians will need both: their education must be a balance between the two.
(Later (in Part 10) Plato returns to the idea of censorship with an additional argument - all art must be banned because it deals with imitation - the Guardians must deal with what is real, not imitations of reality.)
So our Ideal State is based on censorship? Hey, isn't that a bit like, well, lying?