Ryan Ruby at the Poetry Foundation:
The prose poem begins life as a paradox and a provocation. On Christmas Day 1861, Charles Baudelaire sent a letter to Arsène Houssaye, the editor of L’Artiste and Le Presse, journals that had published some of Baudelaire’s short prose pieces. “Who among us has not dreamt, in moments of ambition, of the miracle of a poetic prose, musical without rhythm or rhyme, supple and staccato enough to adapt to the lyrical stirrings of the soul, the undulations of dreams, and sudden leaps of consciousness?” he wrote to Houssaye. The letter was first published as the preface to a posthumous collection of 50 prose pieces titled Petits Poèmes en prose (later, Le Spleen de Paris). Along with the neo-Biblical rhythms of Walt Whitman’s ever-expanding editions of Leaves of Grass, Baudelaire’s “little poems in prose” inspired a generation of French Modernists to undertake experiments in what they called vers libre or free verse: poetry untethered from meter, the feature that had distinguished it as a literary form, in the West at least, since Aristotle’s Rhetoric.