Mosca is most famous for his works on political theory, Sulla teorica dei governi e sul governo parlamentare (Theory of Governments and Parliamentary Government), published in 1884; Elementi di scienza politica (The Ruling Class), published in 1896; and Storia delle dottrine politiche (History of Political Doctrines), published in 1936. His enduring contribution to political science is the observation that all but the most primitive societies are ruled in fact, if not in theory, by a numerical minority, or elite, which he termed the “political class.” Mosca defined modern elites in terms of their superior organizational skills, which enabled them to gain political power in modern bureaucratic society. He advocated a mixed system of government, partly autocratic, partly liberal, in which “the aristocratic tendency is tempered by a gradual but continuous renewal of the ruling class” by the addition of members of lower social and economic classes who have the desire and the ability to rule.