4.4. Natural right and positive right
91. Positive right should make an effort of carrying out the requirements of natural right. It does this in the form of conclusions (natural right prohibits murder, positive right prohibits abortion), or through determinations (the natural right prescribes that the guilty ought to be punished, criminal positive law determines the punishment to be applied to all classes of crimes).(82) Inasmuch as they derive truly from natural rights and therefore from the eternal law, human positive laws obligate in conscience. In the contrary case, they do not so obligate. “If the law is not just, it is not even a law.”(83) The positive law is able, or rather ought to, change to remain faithful to its proper calling. In fact, in a certain way, there exists a progress of human reason which, little by little, grasps a better consciousness of what is more suitable for the good of the community. On the other hand, the historical conditions of the life of society change themselves (for good or evil), and the law must adapt to itself to those.(84) So the legislator should determine what is just in concrete historical situations.(85)
92. Natural rights are measures of human relations prior to the will of the legislator. They are given because men live in society. Natural right is what is naturally just before any legal formulation. It is expressed particularly in the subjective rights of the person, like the right with respect to one’s life, to the integrity of the person, to religious freedom, to freedom of thought; the right to form a family and to educate children according to one’s own convictions; the right to associate with others, and to participate the life of the community. . . . These rights, to which contemporary thought attaches great importance, have their source, not in the fluctuating desires of individuals, but in the very structure of human beings and in their humanizing relations. The rights of the human person emerge, therefore, from the just order that should reign in the connections between the men. To recognize these natural rights of man means to recognize the objective order of human relations founded upon the natural law.
(82) Cf. St. Thomas Aquinas, Summa theologiae, Ia-IIae, q. 95, art. 2.
(83) St. Augustine. De libero arbitrio, I, V, 11 [Corpus christianorum, series latina, 29, 217]: "In fact, it seems to me not to be a law, that which is not right"; St. Thomas Aquinas, Summa theologiae, Ia-IIae, q. 93, art 3, ad 2: "Human law has the nature of law in so far as it partakes of right reason; and it is clear that, in this respect, it is derived from the eternal law. But in so far as it deviates from reason, it is called an unjust law, and has the nature, not of law, but of violence. (Lex humana intantum habet rationem legis, inquantum est secundum rationem rectam, et secundum hoc manifestum est quod a lege aeterna derivatur. Inquantum vero a ratione recedit, sic dicitur lex iniqua, et sic non habet rationem legis, sed magis violentiae cuiusdam)”; Ia-IIae, q. 95, art. 2: "Consequently every human law has just so much of the nature of law, as it is derived from the law of nature. But if in any point it deflects from the law of nature, it is no longer a law but a perversion of law. (Unde omnis lex humanitus posita intantum habet de ratione legis, inquantum a lege naturae derivatur. Si vero in aliquo a lege naturali discordet, iam non erit lex sed legis corruptio).”
(84) Cf. St. Thomas Aquinas, Summa theologiae, Ia-IIae, q. 97, art. 1.
(85) According to St. Augustine, the legislator, to do a good work, should consult the eternal law; cf. St. Augustine, De vera religione, XXXI, 58 [Corpus christianorum, series latina, 32, 225] : "The temporal lawgiver, if he is wise and good, consults the eternal law, that no man can judge, so that according to its unchanging norms he is able to recognize what at that moment it is fitting to command or to prohibit. (Conditor tamen legum temporalium, si vir bonus est et sapiens, illam ipsam consulit aeternam, de qua nulli animae iudicare datum est; ut secundum eius immutabiles regulas, quid sit pro tempore iubendum vetandumque discernat)". In a seculariezed society, in which not all recognize the signs of this eternal law, the search, the defense, and the expression of the natural right by means of the positive law guarantees its legitimacy.