The Tractat de prenostication de la vida natural dels hòmens, a late fifteenth-century Catalan incunable, draws on a rich tradition of astrological magic, geomancy, Pythagorean numerology, and Hebrew gematria. This practical manual offers a method of determining the birth sign based on calculations performed on the subject’s name and his or her mother’s name. The critical edition includes a literary, historical, and linguistic study; an English translation; and a Catalan-English glossary. The Tractat reveals Catalan sources for prognostication, a unique expression of medieval syncretism, the mingling of traditions, and the development of new ideas. It is a rare find for Hispanists and others interested in astrology, magic, the history of science, and early print culture.