[FrankHudson]

I'm an eclectic when composing. I start and use all sorts of things to make another piece. Starting with theory or even "What if we bend the theory's rules or use a more advanced scheme?" ideas are sometimes the genesis of things I do. If nothing else it can help break you out of ruts by showing you a pattern you're stuck in and giving you recipes to try as alternatives.

Of course, another common compositional starting point is copying something else, perhaps with some modifications. We have IP concerns these days about that -- something that George Harrison and John Lennon helped pioneer with "She's So Fine"/"My Sweet Lord" and "Come Together" and Chuck Berry cases. But of course composers have always done this.

For the best of the Beatles though I suspect the method was more at I don't know the rules or the theories, but I've just been shown a new chord, or found one fooling around on an instrument. How does it sound after this other chord? Or before it? Hey, that sounds cool, now what's next? I particularly think that's the case with John Lennon who breaks rules and expectations fairly often.

We can use theory to explain why it works. Or we can listen and enjoy.