With this publication of Book 21, we at last have a definitive English-language translation of the crown jewel of Astrologia Gallica, Jean-Baptiste Morin de Villefranche's 26-volume magnum opus. In this one slim volume, Morin, a 17th c. French astrologer, physician, and mathematician, sets out the rules for a rational approach to horoscope interpretation that he summarizes in two compact phrases that encapsulate his entire tradition: "Of the Active Determination of the Celestial Bodies & the Passive Determination of Sublunary Things." Beginning in 1950, Zoltan Mason made Morin's approach available to English-speaking students for many years through the classes he held in his New York City bookstore. Three abridged English-language paraphrases of Book 21 were published in the early to mid-1970s. But not until this translation from Penelope Sitter has a full and reliable rendering of the centrally important volume of Morin's masterwork been available in English. This translation of Book 21 brings Morin's voice to life, and provides comprehensive endnotes, a detailed subject index, and an index of persons. It will be the authoritative and complete text in English for contemporary students and a foundation for future university level programs on our perennial art.