From The Nation:
It is said that the tradition of English poetry began with Caedmon–an illiterate seventh-century lay brother who, ashamed of his inability to versify when the harp was passed around at a feast, fell asleep in his stable among the animals and dreamed of an angel. This angel, too, bade him sing, and again Caedmon protested that he did not know any songs; but then, inexplicably, he found himself obeying the angel's dictum: “Sing the beginning of the creatures!” Immediately on waking he wrote down the eulogy to the world and its maker that had been transmitted to him in his dream; today the nine-line Anglo-Saxon “Caedmon's Hymn” is the earliest known English poem–a product of what poets now often call “dictation.” The gods (or God), the muses (or the Muse); afflatus, ecstasy, poetic madness: the lore of poetry worldwide attests to the claim that poetry at its best emerges from somewhere “other”–a source beyond the poet's ego and conscious mind. Sometimes the poem appears in dreams, as with Caedmon; sometimes during autohypnosis, as with William Butler Yeats. James Merrill's medium of choice was his Ouija board; Jack Spicer's, his orphic radio. A key interchange in the transition from angels to radios is the visionary poetry of Rainer Maria Rilke.
More here.