This dissertation presents a method for reading William Butler Yeats's A Vision. Establishing parallels between the language of A Vision and that of Jung's Psychological Types both renders A Vision comprehensible at the sentence level and identifies the classical theory of temperaments as a crucial unacknowledged influence on both Yeats and Jung. A reading of Book I of A Vision demonstrates how its cycle of lunar phases functions as a sophisticated psychological typology and reveals the underlying structure of Yeats's system.