From the 15th century on, the practitioners of the mechanical arts obtain a new self-confidence and do no longer want to be regarded merely as manual workers. In this context, they frequently underline the mathematical basis of their discipline in order to increase its value from the status of a mechanical to that of a liberal art. In the Spanish golden age period, the military art, amongst others, is conceived as a mathematical discipline. A particularly curious case is the Libro de las grandezas de la espada (1600) whose author, Luis Pacheco de Narváez (1570-1640), propagates the art of swordsmanship, which he names «the true skill», as a science based on the disciplines of the quadrivium. The present article outlines the arguments applied by Pacheco in a preliminary chapter in order to prove the mathematical and scientific status of his discipline.