There are no rules. Really, there are no rules. It's a hard thing to accept, but it's true, and the difficulty of accepting this simple truth is because most of us really do want there to be rules, and that maybe by following those rules we will insure that everything works out OK.
I'm sitting downstairs on the patio at the Empress's parents' home looking out over the harbor, a cool breeze blowing off the water, activated by the wind so that it sparkles in the sun, the slight tang of diesel, the planes rocketing off in their high-G noise abatement pattern while her father dies upstairs. When someone dies, there are always questions: how old is he, what did she die of, did they eat too much meat, did they drink too much, did they love the wrong person, attempts to look for answers that will give us the rules of life that will guarantee that we never ever die. My mother believes it, that she won't really die. At 95, she believes she will see my father again, and I don't know what that would be like: an eternity where there is no hope for the future, no ambition, no pain, no fear of death.
When a beautiful piece is made, there are always questions about process, about the tools used, about how to analyze the sounds or the chords or the way the melody peaks or troughs. These questions are an attempt to understand what makes something good or bad or effective or full of longing. We hope that knowing the answers will give us the rules that will help us create, but they don't.
The Empress says I would be a bad teacher because, being in close contact with a class full of fresh-faced and excitable young people I would surely be arrested for something. But I know I would be a bad instructor because I would stand at the front of the lecture hall and simply say "Hey, guess what, there are no rules" and then I would sit back down and wait for something to happen, and that's not what they are looking for. They want someone to be their guide, to help them through, and I suppose I do know something: I can tell them if they want to sound exactly like someone else to study what that person does or did and to imitate it. That works, and it's what harmony and counterpoint books assemble: simplistic and descriptivist rules developed after the creative act, that explain some small average aspect of the group behavior of a set of composers culled from a particular time and place, aspects that are actually unimportant, beside the point, at least when it comes to the Ineffable Wonder of Composition.
Really, I'm serious, there are no rules, say it to yourself 100 times each day.