You're right to question my bald comment. My answer: history. I think the history of music (and more generally of art) is the story of the breaking-down of exis

You're right to question my bald comment. My answer: history. I think the history of music (and more generally of art) is the story of the breaking-down of existing borders, while the history of those who would try to enforce the borders is the story of Canute on the beach. Some early music used 5-note scales and regarded 2nds and 7ths as unusable (I'm referring to music in the west; folk music elsewhere often still uses 5-note scales). Early music was monophonic, but that limitation was overcome by the 12th century I think. Bach was genteel, but Beethoven was emotional. Brahms was tonal, but Debussy allowed atonality in music. Cage allowed aleatoric sounds to be music. Glass allowed excessive repetition of simple patterns to be music.