Il Fermo Proposito §13

¹³ Our Predecessor, Leo XIII, of blessed memory, has pointed out, especially in that memorable encyclical "Rerum Novarum" and in later documents, the object to which Catholic Action should be particularly devoted, namely, "the practical solution of the social question according to Christian principles." Following these wise rules, We Ourselves in Our motu proprio of December 18, 1903, concerning Popular Christian Action-which in itself embraces the whole Catholic social movement - We Ourselves have laid down fundamental principles which should serve as a practical rule of action as well as a bond of harmony and charity. On these documents, therefore and within their most holy and necessary scope, Catholic Action, although varied and multiple in form while directed toward the same social good, must be regulated and united.