I can't remember where I read this, but it doesn't really matter because it's not true: It was something along the lines of it being necessary to have at least two musicians to play jazz. The writer was probably simply repeating something heard in another context, about jazz being conversational in spirit or something. And it is, to a considerable degree, but it can be played solo at the highest levels as well. The listening and responding aspects are still there, but it's one's own statements (along with the structural elements of the piece) that have to be used in the process. In fact, it could be argued that some musicians reach their creative peaks on their own. Keith Jarrett is one, in my opinion, and Joe Pass, the great guitarist, another. Here's a good video of Pass doing the Ellington classic, "Do Nothin' Til You Hear from Me": (http://www.youtube.com/watch?v=gZfQUYOEzpg).