THE DUALISM OF SOUL AND BODY which runs through Yeats' poetry is almost as plainly marked in his work for the theatre. At the root of his lifelong scorn of naturalism was the dislike of a religious mind for an art that denies the primacy of the supernatural. His problem, therefore, as a dramatist was to find a form that would admit this primacy without rejecting the life of the passions and the senses. This problem could emerge only gradually, as Yeats' awareness of the natural world grew to a conviction of its reality and value. In his plays this conviction is reached in On Baile's Strand. The problem, however, remains unsolved—its difficulties, indeed, only now become apparent: how to give passion dramatic expression without trapping the soul in character, and how to affirm. a world opposed at once to the supernatural and, by its naturalness, to the formality of art. Not until Purgatory are these difficulties resolved.