This essay focuses on a Cicero codex, now at the National Library in Madrid. It appears to have been realized, as confirmed by the presence of the radiant sun, for a patron close to Gian Galeazzo Visconti, probably his eldest son Giovanni Maria, assigned to succeed his father, according to what was established by imperial diploma of October 13, 1396. This date might be probably taken also as reference for the execution of the manuscript, which opens with a political-philosophical work such as De officiis, dedicated, as known, by Cicero to his son Marco. The chronology, that is also confirmed by the stylistic analysis of the whole ornamental apparatus, presents punctual analogies with the Viscontean codices produced in Pavia in the last decade of the fourteenth century. The result is a book of great preciousness and refinement entrusted to a few illuminators of undoubted fame, connected by a sensitivity and taste capable of combining some of the most ancient styles with the most modern outcomes represented by the activity of Michelino da Besozzo, who cannot be excluded to have personally realized some of the miniatures. The main role, however, was claimed by another illuminator known as the Master of the De natura deorum, whose personality is still elusive today despite his role in the court must have been of the first order as the Cicero codex of Madrid attests.