Duhem capital raises in his true position on what a scientific theory from a philosophical point of view. The physical theory, its purpose and structure, never published until now in Spanish, is critical to fully understand the evolution of ideas in the twentieth century. First published as articles in 1904 and 1905 (Revue de Philosophie), appears then as a first edition book in 1906, in 1914, appears the second, with two articles added: "The Physics of a Believer" and "The value of physical theory. "