From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
Reason attests that there are objects of the human act which are by their nature “incapable of being ordered” to God, because they radically contradict the good of the person made in his image. These are the acts which, in the Church’s moral tradition, have been termed “intrinsically evil” (intrinsece malum): they are such always and per se, in other words, on account of their very object, and quite apart from the ulterior intentions of the one acting and the circumstances. Consequently, without in the least denying the influence on morality exercised by circumstances and especially by intentions, the Church teaches that “there exist acts which per se and in themselves, independently of circumstances, are always seriously wrong by reason of their object”.131 The Second Vatican Council itself, in discussing the respect due to the human person, gives a number of examples of such acts: “Whatever is hostile to life itself, such as any kind of homicide, genocide, abortion, euthanasia and voluntary suicide; whatever violates the integrity of the human person, such as mutilation, physical and mental torture and attempts to coerce the spirit; whatever is offensive to human dignity, such as subhuman living conditions, arbitrary imprisonment, deportation, slavery, prostitution and trafficking in women and children; degrading conditions of work which treat labourers as mere instruments of profit, and not as free responsible persons: all these and the like are a disgrace, and so long as they infect human civilization they contaminate those who inflict them more than those who suffer injustice, and they are a negation of the honour due to the Creator”.132
With regard to intrinsically evil acts, and in reference to contraceptive practices whereby the conjugal act is intentionally rendered infertile, Pope Paul VI teaches: “Though it is true that sometimes it is lawful to tolerate a lesser moral evil in order to avoid a greater evil or in order to promote a greater good, it is never lawful, even for the gravest reasons, to do evil that good may come of it (cf. Rom 3:8) — in other words, to intend directly something which of its very nature contradicts the moral order, and which must therefore be judged unworthy of man, even though the intention is to protect or promote the welfare of an individual, of a family or of society in general”.133
131 Post-Synodal Apostolic Exhortation Reconciliatio et Pænitentia (December 2, 1984), 17: Acta Apostolicæ Sedis [AAS] 77 (1985), 221; cf. Paul VI, Address to Members of the Congregation of the Most Holy Redeemer, (September 1967): AAS 59 (1967), 962: “Far be it from Christians to be led to embrace another opinion, as if the Council taught that nowadays some things are permitted which the Church had previously declared intrinsically evil. Who does not see in this the rise of a depraved moral relativism, one that clearly endangers the Church’s entire doctrinal heritage?”.
132 Pastoral Constitution on the Church in the Modern World Gaudium et Spes, 27.
133 Encyclical Letter Humanæ Vitæ (July 25, 1968), 14: AAS 60 (1968), 490-491.
— Veritatis Splendor 80