This book is a translation of a key commentary on the Book of Changes, or Yijing (I Ching), perhaps the most broadly influential text of classical China. The Yijing first appeared as a divination text in Zhou-dynasty China (ca. 1045-256 bce) and later became a work of cosmology, philosophy, and political theory as commentators supplied it with new meanings. While many English translations of the Yijing itself exist, none are paired with a historical commentary as thorough and methodical as that written by the Confucian scholar Cheng Yi, who turned the original text into a coherent work of political theory.