Veritatis Splendor, paragraph 80, in which the late Pope John Paul II condemns slavery as an “intrinsically evil” act.

“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’ [Reconciliatio et Paenitentia (December 2, 1984), 17: AAS 77 (1985), 221; cf. Paul VI, Address to Members of the Congregation of the Most Holy Redeemer, (September 1967): AAS 59 (1967), 962].

  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’

[Gaudium et Spes, 27].”  http://www.vatican.va/edocs/ENG0222/__P8.HTM (accessed February 25, 2009).