William Wordsworth said this, in his Preface to Lyrical Ballads. Essentially, he is speaking against the notion that poetry is completely arbitrary or that "good" poetry can be created without an intensity of thoughts and feelings about its subject for a long period of time. Further, he is arguing that poetry is more emotionally driven than intellectually driven.
He makes the case that all of hispoetry has a distinct purpose, but that the purpose evolves out of a deep sense of emotion which is often meditated on before writing can take place. He admits that at the outset, the purpose of a specific poem may not be apparent, but that it always comes through because of the intensity of his feelings about the subject on which he is writing.