question" but which covered the complex conflicts arising with modernization: social dislocation, economic suffering, and political turbulence. From the early 19th century, many and various Catholic thinkers responded to the revolutionary tide that the French Revolution and Napoleonic Era inaugurated. But by mid-century a new synthesis of Catholic natural law philosophy, mainly influenced by the writings of St. Thomas Aquinas, combined with the new social sciences of politics and economy, was embraced by the Vatican. It took several decades for this synthesis to become established in Catholic social teaching. Pope Leo XIII in a series of encyclicals spanning 20 years formalized the modern approach to Catholic social teaching, that combines evangelical teachings on the duties to love one another with natural law social scientific arguments on the requirements of human flourishing. These combined principles have been reiterated by subsequent popes, consistently over the subsequent century and more. Rerum novarum The encyclical The publication of Leo XIII's encyclical Rerum novarum on 15 May 1891 marked the beginning of the development of a recognizable body of social teaching in the Catholic Church. It was written at a time when the previously agrarian population of Italy and western Europe were undergoing rapid urbanisation in the newly industrialized cities with many living in conditions of squalor and poverty. Similar trends took place in the Americas. Leo's predecessor, Pius IX, had seen the end of the church's control of the lands of the Papal States and had become isolated in the Vatican. Pius had railed against the unification of Italy during the Risorgimento and this cause had consumed his time in the last years of his pontificate; he had lost the faith of the Romans leading to them voting to incorporate into the newly integrated Italy in 1870. Writers have commented that Leo, upon taking up office as pope and shorn of the role as temporal ruler of three million mainly rural subjects, saw that the newly created industrial working class was the responsibility of the church and that Rerum novarum was a response to competition of ideas in the communist analysis of the social conditions facing the poor of industrialization through such books as Das Kapital and The Communist Manifesto. The opening phrases of Rerum novarum state "that some opportune remedy must be found quickly for the misery and wretchedness pressing so unjustly on the majority of the working class... so that a small