Yeats said that he wrote in form because if he didn’t he wouldn’t know when to stop. Like Samuel Beckett I prefer the word ‘shape’ to ‘form.’ At Trinity [College Dublin] during a course on Aristotle’s Poetics our Greek professor W. B. Stanford told us to come back the following week with our own definition of poetry. Mine was: ‘If prose is a river, then poetry’s a fountain.’ I still feel that’s pretty good because it suggests that ‘form’ (or ‘shape’) is releasing rather than constraining. The fountain is shapely and at the same time free-flowing.
—Michael Longley, “A Jovial Hullabaloo,” One Wide Expanse (The Poet's Chair: Writings from the Ireland Chair of Poetry, University College Dublin Press, 2015)