the difference between composition and improvisation is that in composition you have all the time you want to think about what to say in 15 seconds, while in improvisation you have only 15 seconds.
The quote is taken from Derek Bailey’s Improvisation: Its Nature and Practice in Music, 141. Bailey refers the quote to a brochure named Listen to Lacy, published by Wiener Musik Galerie in 1990 to accompany a concert series