What is the relationship between music and justice for Plato?
Music and justice have a complex relationship for Plato. To be specific, both should be produced by a kind of divine proportion; the same fundamental rightness and relationships that makes music harmonious should also describe the relationship of thoughts, actions, and decisions in a just society. However, since Plato thought music could incite people to unseemly and even animalistic behavior, he would also argue that music could be a disruptive threat to justice.
Plato discusses the role of music in the development of soldiers in Book III of the Republic. Simply put, he advocates censorship of the arts in the development of the youth, as they don't have the ability yet to process what is just and unjust for their given tasks. He asks, "What modes of music are appropriate to dirges?" "Mixed or tenor Lydian, the tensed or bass Lydian, and some others." Socrates dismisses these, as they are of no service for children seeking to become warriors. Some others prohibited are Ionian, and some Lydian, as they promote drunkenness, softness, and laxity. His basis is the music written in these modes, and says that the musical model exposed to the youth should exhibit the characteristics becoming of a warrior: the modes of necessity, freedom, temperance, and braveness. He takes this approach with melody, harmony, rhythm, and musical instruments. The characteristics necessary for just soldiers are to be reflected in the music and the instruments. Check out Book III of The Republic for further examples and dialogue.