"The Tractat de prenostication de la vida natural dels homens", a late 15th-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; and 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.