There is no theory, you just have to listen. –Claude Debussy
There is a long history in the westworld of authoritarians fearing music and imposing constraints on it in the interests of controlling our behavior. Here is some of what Plato (in The Republic) had to say about music at the dawn of western civilization (c.400 BC).
These two harmonies I ask you to leave; the strain of necessity and the strain of freedom, the strain of the unfortunate and the strain of the fortunate, the strain of courage, and the strain of temperance; these, I say, leave. . .
Then, I said, if these and these only are to be used in our songs and melodies, we shall not want multiplicity of strings or a panharmonic scale?
I suppose not.
Then we shall not maintain the artificers of lyres with three corners and complex scales, or the makers of any other many-stringed, curiously harmonized instruments?
But what do you say to flute-makers and flute-players? Would you admit them into our State when you reflect that in this composite use of harmony the flute is worse than any stringed instrument; even the panharmonic music is only imitation of the flute?
Then let us now finish the purgation, I said. Next in order to harmonies, rhythms will naturally follow, and they should be subject to the same rules, for we ought not to seek out complex systems of metre, and a variety of feet, but rather to discover what rhythms are the expressions of a courageous and harmonious life; and when we have found them, we shall adapt the foot and the melody to words having a like spirit, not the words to the foot and melody. Any musical innovation is full of danger to the whole state, and ought to be prohibited. When modes of music change, the fundamental laws of the state always change with them.
This mentality has persisted among ruling elites ever since.
The medieval catholic church set up the first system of common musical practice in Europe with sets of allowable (and forbidden) intervals, modes (those used in plainsong) and meters (everything had to be in 3 to represent the trinity). More recently Nazi germany banned the performance of modern “degenerate” music (approving primarily of Beethoven, Wagner, and Bruckner) and the USSR banned everything that didn’t glorify the state and the communist party. Until fairly recently, women were essentially excluded from having any part in it –for a while they even castrated males to avoid having women sing women’s roles.
As ludicrous and pathetic as these attempts to mold culture seem in retrospect, they haven’t been completely unsuccessful (nor were they ludicrous and pathetic to those who had to live under those regimes). Almost all of us who consider ourselves musical are unconscious inheritors and perpetuators of ways of musical practice that have their origins in the chain of suppressive thinking running from Plato, through the Church and the 20th century totalitarians, to Richard Taruskin’s Oxford History of Western Music (pub. 2009).
Our western system of equal temperament and the major/minor system that it enables is the descendant of those two harmonies Plato was willing to allow in his republic. Our two commonly accepted meters, the simplest possible, are almost universal in the music we hear and make. The codification of the instruments has occurred as well (generally speaking, the instruments included in the philharmonic and taught in conservatories are they same as they were in the 19th century).
This system (which most western people think of as simply music) did not evolve naturally, it was deliberately imposed on the soundworld by generations of oligarchs with the expressed purpose of limiting our thoughts and behaviors. Wonderful music has and will be made with it, but it’s worth remembering that, to a large extent, it was and will be made in spite of the system, not because of it.
‘Music is born free; and to win freedom is its destiny.’ -Ferruccio Busoni