28 In it the Supreme Pontiff confirmed the right and duty of the Catholic Church to work for an equitable solution of the many pressing problems weighing upon human society and calling for a joint effort by all the people. He reiterated the principles of the Leonine encyclical and stressed those directives which were applicable to modern conditions. In addition, he took the opportunity not only to clarify certain points of this teaching which had given rise to difficulties even in the minds of Catholics, but also to reformulate Christian social thought in the light of changed conditions.
Source: Mater et Magistra (Vatican.va)