From my Ives site:
Many years after publishing the work, Ives remarked that the "Concord" Sonata was his one work that never seemed finished; it was a perpetual work in motion, a continual improvisation: "I don't know as I shall ever write [my improvisations] out, as it may take away the daily pleasure of playing this music and seeing it grow and feeling that it is not finished... I may always have the pleasure of not finishing it..."
. . .
I love that quote, that way of thinking about music. It's a jazz-like way of relating to music rather than a stance one normally associates with "classical" music.