W. B. Yeats’s poetry showcases an extraordinary range of themes, reflecting his core philosophy of the Unity of Being. He integrates seemingly contradictory ideas, drawing from Ireland’s Druid traditions, Christianity, Hinduism, Buddhism, and spiritualism. His works align ideologically with Buddhist teachings and Indian Upanishadic philosophy. Yeats develops his anti-self theory, explores in Per Amica Silentia Lunae. This theory links self-discipline, artistic inspiration, and the mystical summoning of the Daimon. Yeats viewed the anti-self as a counter-force to his everyday self, accessed through meditation, as exemplified in “Ego Dominus Tuus.” Yeats’s fascination with mysticism extends to Indian and Japanese philosophies. His studies of Japanese Noh drama deepen his understanding of Zen Buddhism, while his connections with Indian figures like Rabindranath Tagore and Swami Purohit shape his engagement with the Upanishads. Notably, Yeats co-translates The Ten Principal Upanishads, exploring concepts like Atman—the eternal self central to Hindu thought. This study compares the Upanishadic notion of an unchanging Atman with Buddhism’s dynamic non-self doctrine, reflecting Yeats’s philosophical explorations. This study also analyzes how his anti-self theory bridges these opposing views, linking his quest for the Unity of Being to his poetry’s meditative and spiritual dimensions. Following Yeats’s anti-self theory, it can be interpreted that Yeats sought to embody Avalokitesvara, appearing at the Celtic Twilight—a transitional space akin to Avalokitesvara’s symbolic role at the threshold of worlds. By synthesizing Upanishadic and Zen Buddhist perspectives, this study illuminates Yeats’s poetic investigation of human nature and his artistic evolution.