Status Updates From Richard Wagner: The Sorcere...

"By looking at three of the late, great works - the Ring, Tristan und Isolde and Parsifal - we can see that the initial creative spark was not so much a single musical idea as a conceptual nucleus of some kind. That might have included a musical idea, but it was also likely to have embraed a creatove cell that might be mythological, philosophical or existential in nature" (p. 192)