Witold Lutoslawski once said (though he was speaking in polish and I was never positive that i fully understood him) that every piece of music has a "key idea". And he claimed that it COULD be but NEED NOT be a fragment of musical material, a melody or set of chords or rhythm. But that (presumably for good, successful works only) the whole composition had to conform to this idea for it to be successful. I think he felt both his Livres and his 'Cello concerto succeeded in this way.
(sorry Polish speakers, I can't do the funny L that his name requires.)
(sorry Polish speakers, I can't do the funny L that his name requires.)