The article includes the Spanish translation of a part -the sixth one- of De multiplicatione specierum, originally written in Latin by Roger Bacon, which deals with the idea of natural philosophy. The main feature of this translation is to be comprehensive. Additionally, with educational purposes, the article also includes a philosophical-critical commentary, that is, a brief explanatory paraphrase that facilitates the Hispanic community to approach to the translation. This is the first commented translation to Spanish of a part of Roger Bacon's work (written in Latin and only translated to English until now), who certainly is the forefather of scientific method and true reformer of modern science.