Charles Ives set himself an impossible problem [in his 114 Songs of 1922]. He wanted to use pitch distance to represent the fact that God is infinitely close to man. But what is an infinitesimally close pitch distance? In the end Ives gave up and left it to the singer to decide. Maybe what Ives wanted was a smallest perceptible pitch difference. There is no standard notation for this.

— Wilfrid Hodges, “The Geometry of Music,” in John Fauvel, ed., Music and Mathematics, 2006