Somehow, suddenly, a musical idea occurs to you; either a whole phrase, or three notes, or a series of chords, something that seems pregnant with possibilities for development. Once you have the kinds of ideas that fascinate you, you're no longer in a position to decide the nature of the animal. It's going to take its essence from the musical ideas that occur to you.... Some musical ideas are too short, they don't seem long enough to carry you through ten minutes of music, so you have to start searching about for other ideas; contrasting ones that seem to fit with the original ones.

Aaron Copland: the Life and Work of an Uncommon Man, ISBN 0805049096.