7 In close connection with the right to private property, Pope Leo XIII's Encyclical also affirms other rights as inalienable and proper to the human person. Prominent among these, because of the space which the Pope devotes to it and the importance which he attaches to it, is the "natural human right" to form private associations. This means above all the right to establish professional associations of employers and workers, or of workers alone. Here we find the reason for the Church's defence and approval of the establishment of what are commonly called trade unions: certainly not because of ideological prejudices or in order to surrender to a class mentality, but because the right of association is a natural right of the human being, which therefore precedes his or her incorporation into political society. Indeed, the formation of unions "cannot ... be prohibited by the State", because "the State is bound to protect natural rights, not to destroy them; and if it forbids its citizens to form associations, it contradicts the very principle of its own existence". Together with this right, which — it must be stressed — the Pope explicitly acknowledges as belonging to workers, or, using his own language, to "the working class", the Encyclical affirms just as clearly the right to the "limitation of working hours", the right to legitimate rest and the right of children and women to be treated differently with regard to the type and duration of work. If we keep in mind what history tells us about the practices permitted or at least not excluded by law regarding the way in which workers were employed, without any guarantees as to working hours or the hygienic conditions of the work-place, or even regarding the age and sex of apprentices, we can appreciate the Pope's severe statement: "It is neither just nor human so to grind men down with excessive labour as to stupefy their minds and wear out their bodies". And referring to the "contract" aimed at putting into effect "labour relations" of this sort, he affirms with greater precision, that "in all agreements between employers and workers there is always the condition expressed or understood" that proper rest be allowed, proportionate to "the wear and tear of one's strength". He then concludes: "To agree in any other sense would be against what is right and just".
Source: Centesimus Annus (Vatican.va)