『친절하고 고요한 달』의 첫 번째 에세이인 『인간령』에서 예이츠는 『비전』으로 정리되는 그의 중심사상인 마스크의 이론을 가장 명확하게 정리해서 제시하고 있다. 이 에세이에서 예이츠는 마스크로 제시되는 서로 상반되는 이미지들의 갈등이 예술의 창작행위에 가장 중요한 에너지를 공급하며 이러한 대칭이 역설적으로 조화를 이룬다는 것을 주장하고 있다.

Per Amica Silentia Lunae (Yeats translated it into Through the Friendly Silences of Moon) was written between January and May of 1917, and consists of a Prologue and an Epilogue for Iseult Gonne, "Ego Dominus Tuss," Anima Hominis and Anima Mundi. In Anima Mundi Yeats said, "I have always sought to bring my mind close to the mind of Indian and Japanese poets, old women in Connacht, mediums in Soho, lay brothers whom I imagine dreaming in some mediaeval monastery the dreams of their village, learned authors who refer all to antiquity: to immerse it in the general mind where that mind is scarce separable from what we have began to call 'the subconscious'... ." In the background of his theory of anti-self there are Indian and Japanese Religious thought, Celtic folklore, Spiritualism, the Order of Golden Dawn, and the great poet Dante. "Ego Dominus Tuss" is a dialogue between two men, Hic and Ille, who discuss poetry and creative process. Ille, like Yeats, is a daimonic poet walking in the moonlight. For Yeats, lunar and subjective were always the antitheses to solar and objective. Poesis called for complete subjectivity, for entry into the friendly silence of the moon. The moon is always associated with feminine divinity. With the help of a mask, Ille is calling to the opposite of his daily self, his anti-self. In the end of the poem, Ille found his anti-self. In Anima Hominis Yeats said that the saint like Christ and Buddha, and the poets like Dante and Keats attained the anti-self. The anti-self is a egoless self, the higher self. Saint, hero, and poet are all inspired. Yeats said, "Saint or hero works in his own flesh and blood and not in paper or parchment, have more deliberate understanding of that other flesh and blood." Only when Yeats became the anti-self could he become a totally subjective mind, overcome the illusion of duality, and find a "revelation of realty." Yeats could receive daimonic inspiration only during visionary experiences. Finally Yeats found the anti-self he felt ecstasy. According to his theory, the production of art was an expression of the artist's longing for "Unity of Being." In Per Amica silentia Lunae Yeats said "the poet, because he may not stand within the sacred house but lives amid the whirlwinds that beset its threshold, may find his pardon."