Creativity arises out of the tension between spontaneity and limitations, the latter (like the river banks) forcing the spontaneity into the various forms which are essential to the work of art or poem.
Source: The Courage to Create, 1975.
-- Rollo May, (Apr 21 1909-1994), US psychoanalyst; He provided excellent insights into creativity in The Courage to Create, 1975; also wrote Man's Search for Himself, 1952.