9 The impact of this remarkable encyclical is still with us even today, so many years after it was written. It is discernible in the writings of the Popes who succeeded Pope Leo. In their social and economic teaching they have frequent recourse to the Leonine Encyclical, either to draw inspiration from it and clarify its application, or to find in it a stimulus to Catholic action. It is discernible too in the subsequent legislation of a number of States. What further proof need we of the permanent validity of the solidly grounded principles, practical directives and fatherly appeals contained in this masterly encyclical? It also suggests new and vital criteria by which men can judge the magnitude of the social question as it presents itself today, and decide on the course of action they must take.
Source: Mater et Magistra (Vatican.va)