Yeats's poetry and writings were a display of his passion for mysticism and the occult. This view on Yeats has been largely expressed in various publications. Many of Yeats's critics, including Ellmann, agree that the roots of Yeats's system are in Theosophy. The roots of Yeats's philosophy are in Theosophy, being a comprehensive, unifying systems of all occult tradition, and the first metaphysical system that Yeats encountered. Being faced with the dilemma between faith and disbelief, Yeats contacted numerous texts on the subject occultism and met Blavatsky, the founder of Theosophy society, claimed to have the ability to offer a "synthesis" of religion, science, and philosophy. After many metaphysical conversations with her and many hours of long thought on the issue, Yeats took one of his first steps on his path of occult wisdom. Yeats's fascination with occultism and mysticism was so profound, and his need to create a unifying mythology so great, that he decided to develop an esoteric system of his own. Thus, between 1917 and 1925, Yeats had written A Vision, an elaborate, complicated system that is of importance in understanding Yeats's works. The first version, published in 1925, was later revised, and final version was published in 1937. In Book IV and V of A Vision Yeats had expounded the notion that history moves in great two-thousand-year cycles. This circle represents the moon and the twenty-eight phases of the moon which are closely related to the progression of time and world history. Yeats suggested all things are subject to a cycle of changes, which can be regarded as bi-polar, passing from a state of objectivity to one of subjectivity before returning to objectivity again. In this view he was strongly influenced by the Theosophists, especially Blavatsky and the Kabbalists, who saw the law of periodicity as one of the fundamental and absolute laws of the universe. Yeats believed that history was cyclic and that every 2,000 years a new cycle begins, which is the opposite of the cycle that has preceded it. In his poem "The Second Coming," the birth of Christ begins one cycle, which ends, as the poem ends, with a "rough beast," mysterious and menacing, who "slouches towards Bethlehem to be born." Yeats's theory of the historical cycle is directly related to his belief in a universal duality -- the existence of opposite but equal forces that dominate a cycle alternately. This view is in accordance with the occult traditions which teach that the First Cause exhibits periodically different aspects of itself. Yeats believed that kingdoms rise movement of history is an hour within the day of a large movement, and that all these cycles are caught within one all-inclusive "Great Year" which has a cosmic purpose. The Kabbalah says the alternation between judgement and mercy must be on equal terms. The germ finally goes back to its root principle, the Unity out of which everything proceeds.