I've heard Paul Simon say that he starts playing and he simply decides to go one way instead of another.
Billy Joel claims that there's no muse. He just plays until he hears something he likes.
Other musicians seem more instinctive about it. I've heard Paul McCartney say that while there are a few little tricks he's learned over the years, mostly he doesn't understand how a song comes to be and he's content to leave it that way. In a Rolling Stone interview I read with him in the early seventies, he called himself "a primitive." And to maintain that status, he has steadfastly refused to learn to read music.
Many musicians claim to "hear" the songs they write in their heads before they've played or written down a note. Brian Wilson, the creative force and frequent lead singer of the Beach Boys, falls into this category.
In an article for The Guardian from Britain, where Wilson is revered in ways that seem to surpass the respect in which he's held in America, Victoria Williamson, a psychologist who specializes in a frield called "musical memory," who is a fellow for music at the University of Sheffield, seeks to understand and analyze what Wilson "heard" when he composed songs like God Only Knows or Good Vibrations. While loving the music, Williamson suggests that there may be a natural, rather than supernatural, explanation for it: musical memory.
According to Williamson:
...Brian Wilson had music going through his head almost continuously. Of course, many of us are able to activate a musical memory when required – for instance, if I asked you whether the third note in Happy Birthday was higher or lower than the fourth, you would probably be able to summon up the tune – but to have it playing constantly like that is rare: one survey we did suggests that less than 5% of people experience it.Some have better musical memories than others, Williams suggests. They have more raw material to play around with, recast, re-imagine, set off into different tunes and combinations. For the 5% who have music playing in their heads constantly, the possibilities appear to be infinite, even though everyone who composes is ultimately dealing with just seven notes. (That's more confining writing haiku poetry, which gives you three lines in a pattern of five syllables, seven syllables, and five syllables.)
What music was going through Brian’s mind? We don’t know. It may have been re-runs of old 1940s and 50s songs, or it may have been fragments of all sorts of different tunes. But it’s possible to theorise that, if someone experiences this constant musical background, they might become more adept at playing around with those sounds – hearing links between them and thinking about how they fit together.
Williamson, who points to research being done on the subject, says that one scene in a recent movie about the Beach Boys, Love & Mercy, shows just talents like Wilson use their musical memory to invent something new:
At one point one of his musicians says “Hey Brian, I think you might have screwed up here. You’ve got Lyle playing in D and the rest of us are in A major.” “Yeah that’s right,” he replies. “How does that work? Two bass lines and two different keys?” she asks, to which Brian replies: “It works in my head”. That’s because he can already hear it. It’s the same way that Beethoven could lose his hearing but still be able to compose his ninth symphony. And the same way a conductor such as Arturo Toscanini would be able to scan two hours of music in a minute and be able to reassure a bassoonist in his orchestra that the broken key on their instrument wouldn’t matter as that particular note didn’t appear in the performance.To which the only appropriate response, I suppose, is wow. Read the whole thing.