The Irish poet W. B. Yeats is one of the most important writers in English of the twentieth century, and the system of A Vision is generally recognized as fundamental to the power and achievement of his later poetry. Yet this strange mixture of esoteric geometry, lunar symbolism, and sweeping generalization has proven frustrating to generations of readers, who have found it obscure in both matter and presentation. This book helps readers to approach and understand the origins, structure, and implications of the system. Concentrating on the 1937 revised edition of A Vision, the treatment is divided into major topic areas with several levels: a general introduction to each topic; a fuller and deeper examination of that topic, drawing on A Vision's two versions and the manuscript background, and forming the bulk of each chapter; an examination of how the topic manifests in Yeats's literary work; full notes to explore conceptual and textual problems. The first three chapters examine the background and origins of A Vision; the central seven chapters look at the major elements involved in the system; the following four at the major processes of life and history. The main treatment ends with a summary and conclusion, and is supplemented by a glossary of terms and appendices.