A captivating study of translation, adaptation, and intellectual cross-pollination that situates the Castilian Hermes in the center of medieval Mediterranean cultural exchange Hermes Trismegistus, a Hellenistic conflation of the Greek Hermes (god of interpretative wisdom) and the Egyptian Thoth (god of wisdom) was considered by many in the medieval world as the father of culture. Between c. 300 BCE - c. 1200 CE various treatises were attributed to the legendary sage, becoming known as the Hermetica - a combination of diverse philosophical and spiritual systems, addressing subjects such as alchemy, magic, and astrology. The Hermetica circulated widely, with premodern translations in Latin, Hebrew, Syriac, Persian, Arabic, and other Eastern languages. Whilst these iterations have been thoroughly researched, little attention has been paid to the Castilian Hermes, the first rendition of the wisdom traditions of Hermes Trismegistus in a Romance language. This book follows the ways in which Hermetic knowledge was brought to the Iberian Peninsula, showing how Hermes became the philosophical and spiritual inspiration for Christian, Arabic, and Jewish scholars there. Udaondo Alegre unveils the pivotal role of King Alfonso X ("the Learned") of Castile (1252-84) in creating this Spanish Hermes. Through the meticulous tracing of source texts and literary influences, the author explores the myriad ways in which Hermes crossed religious and linguistic boundaries to embody a composite intellectual identity, emblematic of medieval Spain's multicultural ethos. Alfonso's court is revealed as the site for a unique convergence of translation and interpretation that shaped a distinctly "Hispanic" Hermes.