Throughout Yeats's life, his occultism absorbed much of his time and energy. Yeats's occultism supported and enriched his poetry and plays, providing him with themes, the symbols, a philosophy that affirmed recurring self-renewal. Through the development of Yeats's occult thinking, from the Golden Dawn, through Per Amica Silentia Lunae, to A Vision, a continuous, coherent direction can be traced. Books II and III of A Vision, deal with the nature of the human soul, its different principles, and its progression after death. In "The Completed Symbol," Yeats elaborates on the Four Principles of the soul ― the Husk, the Passionate Body, the Spirit, and the Celestial Body. The Principles find their Unity in the Celestial Body, man's archetype in Heaven. In "The Soul in Judgement", examining the six after-death states, death, in general, is also presented as a transfer of consciousness from the physical plane to a higher one. During the first three states, or until Beatitude, the Spirit passes each time into a higher state of consciousness; after Beatitude, following the circular pattern of "The Great Wheel," the Spirit lapses slowly into relative unconsciousness. These six states, like the twenty-eight phases, affect each other, and in each one the Spirit has to act under certain laws. The soul has to pass through all of these states in order to progress and to prepare for its reentry into the physical world. This belief in the six after-death states stems from the occult sources mainly Theosophy which also teaches that the soul passes through six planes of consciousness after death -- the divine, the monadic, the spiritual, the intuitional, the mental, and the astral plane or plane of passions and emotions. Yeats uses the lunar cycle to explain the soul's journey between lives. The concept of the Thirteenth Sphere is important because in the occult traditions, the number thirteen is also symbolic of unity and perfection. In A Vision the Thirteenth Sphere represents Unity since in it all antinomies are resolved. Yeats's view of the soul is directly related to his belief in a universal duality ― the existence of opposite but equal forces that dominate a cycle alternately.

The purpose of this research is to study the Yeat's view on the good and evil in human nature. Throughout his life, Yeats has made a spiritual, mystical and mythological world in which he tries to portray the eternally dichotomized nature of human consciousness. Yet, he attempts to harmonize the antinomies, the contraries that highlight human nature. Yeats's life and art is full of such attempts to unify harmoniously opposite forces: body and soul, good and evil, light and darkness, the sun and the moon, the antithetical and the primary, etc.. In this almost impossible unison of conflicting forces, Yeats hopes to find the unity of the two. For Yeats, instinct without spirituality, intellect without emotion, wisdom without action, and good without evil can only express a part of human nature, and he refused to deny one side of human being. He did not want to separates his soul from matter, good from evil but to find the perfect balance and attain the assertion of the 'Unity of Being.'