The Porfirian era (''el Porfiriato''), in the last quarter of the 19th century and the first decade of the 20th century, was marked by economic progress and peace. After four decades of civil unrest and war, Mexico saw the development of philosophy and the arts, promoted by President Díaz himself. Since that time, as accentuated during the Mexican Revolution, cultural identity has had its foundation in the ''mestizaje'', of which the indigenous (i.e. Amerindian) element is the core. In light of the various ethnicities that formed the Mexican people, José Vasconcelos in ''La Raza Cósmica'' (The Cosmic Race) (1925) defined Mexico to be the melting pot of all races (thus extending the definition of the ''mestizo'') not only biologically but culturally as well. Other Mexican intellectuals grappled with the idea of ''Lo Mexicano'', which seeks "to discover the national ethos of Mexican culture." Nobel laureate Octavio Paz explores the notion of a Mexican national character in  ''The Labyrinth of Solitude''.