Mysticism was Yeats's lifelong concern and became the center of his work. He believed that mysticism can explain the truth of death, rebirth, the cycle of the universe. By founding his playwriting principles on mysticism, he revived Druidic belief, which had been formed mostly from Irish legends, lore, Celtic mysteries, his reading on Druidism ever since early childhood in Sligo. Yeats thought that a poet must be a mystic and that symbolism is the mystic's aptitude. To treat mystical themes, he needed to make things remote from reality by using symbols, but because of symbolic and mystical content some critics insist he founded his drama based on Noh plays. Even though he thought Noh is an ideal form of drama, it does not mean his drama is an extension of Noh plays. Symbols and the supernatural content were his solid principles long before he saw Noh plays. Furthermore, the similar themes in both Noh plays and Yeats's are something he had already learned from Celtic mysticism. The Shadowy Waters is an extremely mysterious story showing visionary experiences of Druidic otherworldliness. In this play, Forgael is following certain birds which are the souls of the dead in search of love. The ghost lovers in The Dreaming of the Bones have been dreaming of fulfilling their love since they died seven hundred years ago, but they can not achieve their dream because of the sin committed during life. This connotes something about Yeats's belief in purgation. He believed the dead have to live anew until the purgation is finished. He shows the same belief in Purgatory, where the dead can not free themselves from purgation due to the consequences of transgressions. In all of the three plays Yeats demonstrates Druidic doctrine that the dead exist and dream in the eternal cycle of birth.