Ruskin is referring to an airy sprite in Shakespeare 's The Tempest whom the witch Sycorax has imprisoned in a cloven pine and whom Prospero releases by his magic, employing him to give effect to his designs; this character, is generally identified with the poetic imagination, with Ruskin assigning him the role of opposing Caliban in the play: 'Ariel and Caliban are respectively the spirits of faithful and imaginative labour, opposed to rebellious, hurtful, and slavish labour.' ( Works, 17.258.)