10 Another important aspect, which has many applications to our own day, is the concept of the relationship between the State and its citizens. Rerum novarum criticizes two social and economic systems: socialism and liberalism. The opening section, in which the right to private property is reaffirmed, is devoted to socialism. Liberalism is not the subject of a special section, but it is worth noting that criticisms of it are raised in the treatment of the duties of the State. The State cannot limit itself to "favouring one portion of the citizens", namely the rich and prosperous, nor can it "neglect the other", which clearly represents the majority of society. Otherwise, there would be a violation of that law of justice which ordains that every person should receive his due. "When there is question of defending the rights of individuals, the defenceless and the poor have a claim to special consideration. The richer class has many ways of shielding itself, and stands less in need of help from the State; whereas the mass of the poor have no resources of their own to fall back on, and must chiefly depend on the assistance of the State. It is for this reason that wage-earners, since they mostly belong to the latter class, should be specially cared for and protected by the Government". These passages are relevant today, especially in the face of the new forms of poverty in the world, and also because they are affirmations which do not depend on a specific notion of the State or on a particular political theory. Leo XIII is repeating an elementary principle of sound political organization, namely, the more that individuals are defenceless within a given society, the more they require the care and concern of others, and in particular the intervention of governmental authority. In this way what we nowadays call the principle of solidarity, the validity of which both in the internal order of each nation and in the international order I have discussed in the Encyclical Sollicitudo rei socialis, is clearly seen to be one of the fundamental principles of the Christian view of social and political organization. This principle is frequently stated by Pope Leo XIII, who uses the term "friendship", a concept already found in Greek philosophy. Pope Pius XI refers to it with the equally meaningful term "social charity". Pope Paul VI, expanding the concept to cover the many modern aspects of the social question, speaks of a "civilization of love".
Source: Centesimus Annus (Vatican.va)