Summa Theologiae by St Thomas Aquinas
SS: Treatise On Fortitude And Temperance
Q154 Of The Parts Of Lust
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A2 Whether simple fornication is a mortal sin?

[a] Objection 1:
It would seem that simple fornication is not a mortal sin. For things that come under the same head would seem to be on a par with one another. Now fornication comes under the same head as things that are not mortal sins: for it is written (Acts 15:29): "That you abstain from things sacrificed to idols, and from blood, and from things strangled, and from fornication." But there is not mortal sin in these observances, according to 1 Tim. 4:4, "Nothing is rejected that is received with thanksgiving." Therefore fornication is not a mortal sin.

[b] Objection 2:
Further, no mortal sin is the matter of a Divine precept. But the Lord commanded (Osee 1:2): "Go take thee a wife of fornications, and have of her children of fornications." Therefore fornication is not a mortal sin.

[c] Objection 3:
Further, no mortal sin is mentioned in Holy Writ without disapprobation. Yet simple fornication is mentioned without disapprobation by Holy Writ in connection with the patriarchs. Thus we read (Gn. 16:4) that Abraham went in to his handmaid Agar; and further on (Gn. 30:5, 9) that Jacob went in to Bala and Zelpha the handmaids of his wives; and again (Gn. 38:18) that Juda was with Thamar whom he thought to be a harlot. Therefore simple fornication is not a mortal sin.

[d] Objection 4:
Further, every mortal sin is contrary to charity. But simple fornication is not contrary to charity, neither as regards the love of God, since it is not a sin directly against. God, nor as regards the love of our neighbor, since thereby no one is injured. Therefore simple fornication is not a mortal sin.

[e] Objection 5:
Further, every mortal sin leads to eternal perdition. But simple fornication has not this result: because a gloss of Ambrose [* The quotation is from the Gloss of Peter Lombard, who refers it to St. Ambrose: whereas it is from Hilary the deacon] on 1 Tim. 4:8, "Godliness is profitable to all things," says: "The whole of Christian teaching is summed up in mercy and godliness: if a man conforms to this, even though he gives way to the inconstancy of the flesh, doubtless he will be punished, but he will not perish." Therefore simple fornication is not a mortal sin.

[f] Objection 6:
Further, Augustine says (De Bono Conjug. xvi) that "what food is to the well-being of the body, such is sexual intercourse to the welfare of the human race." But inordinate use of food is not always a mortal sin. Therefore neither is all inordinate sexual intercourse; and this would seem to apply especially to simple fornication, which is the least grievous of the aforesaid species.

[g] On the contrary,
It is written (Tob. 4:13): "Take heed to keep thyself... from all fornication, and beside thy wife never endure to know a crime." Now crime denotes a mortal sin. Therefore fornication and all intercourse with other than one's wife is a mortal sin.

[h] Further, nothing but mortal sin debars a man from God's kingdom. But fornication debars him, as shown by the words of the Apostle (Gal. 5:21), who after mentioning fornication and certain other vices, adds: "They who do such things shall not obtain the kingdom of God." Therefore simple fornication is a mortal sin.

[i] Further, it is written in the Decretals (XXII, qu. i, can. Praedicandum): "They should know that the same penance is to be enjoined for perjury as for adultery, fornication, and wilful murder and other criminal offenses." Therefore simple fornication is a criminal or mortal sin.

[j] I answer that,
Without any doubt we must hold simple fornication to be a mortal sin, notwithstanding that a gloss [* St. Augustine, QQ. in Deut., qu. 37] on Dt. 23:17, says: "This is a prohibition against going with whores, whose vileness is venial." For instead of "venial" it should be "venal," since such is the wanton's trade. In order to make this evident, we must take note that every sin committed directly against human life is a mortal sin. Now simple fornication implies an inordinateness that tends to injure the life of the offspring to be born of this union. For we find in all animals where the upbringing of the offspring needs care of both male and female, that these come together not indeterminately, but the male with a certain female, whether one or several; such is the case with all birds: while, on the other hand, among those animals, where the female alone suffices for the offspring's upbringing, the union is indeterminate, as in the case of dogs and like animals. Now it is evident that the upbringing of a human child requires not only the mother's care for his nourishment, but much more the care of his father as guide and guardian, and under whom he progresses in goods both internal and external. Hence human nature rebels against an indeterminate union of the sexes and demands that a man should be united to a determinate woman and should abide with her a long time or even for a whole lifetime. Hence it is that in the human race the male has a natural solicitude for the certainty of offspring, because on him devolves the upbringing of the child: and this certainly would cease if the union of sexes were indeterminate.

[k] This union with a certain definite woman is called matrimony; which for the above reason is said to belong to the natural law. Since, however, the union of the sexes is directed to the common good of the whole human race, and common goods depend on the law for their determination, as stated above ([3534] FS, Q [90], A [2]), it follows that this union of man and woman, which is called matrimony, is determined by some law. What this determination is for us will be stated in the Third Part of this work (XP, Q [50], seqq.), where we shall treat of the sacrament of matrimony. Wherefore, since fornication is an indeterminate union of the sexes, as something incompatible with matrimony, it is opposed to the good of the child's upbringing, and consequently it is a mortal sin.

[l] Nor does it matter if a man having knowledge of a woman by fornication, make sufficient provision for the upbringing of the child: because a matter that comes under the determination of the law is judged according to what happens in general, and not according to what may happen in a particular case.

[m] Reply to Objection 1:
Fornication is reckoned in conjunction with these things, not as being on a par with them in sinfulness, but because the matters mentioned there were equally liable to cause dispute between Jews and Gentiles, and thus prevent them from agreeing unanimously. For among the Gentiles, fornication was not deemed unlawful, on account of the corruption of natural reason: whereas the Jews, taught by the Divine law, considered it to be unlawful. The other things mentioned were loathsome to the Jews through custom introduced by the law into their daily life. Hence the Apostles forbade these things to the Gentiles, not as though they were unlawful in themselves, but because they were loathsome to the Jews, as stated above ([3535] FS, Q [103], A [4], ad 3).

[n] Reply to Objection 2:
Fornication is said to be a sin, because it is contrary to right reason. Now man's reason is right, in so far as it is ruled by the Divine Will, the first and supreme rule. Wherefore that which a man does by God's will and in obedience to His command, is not contrary to right reason, though it may seem contrary to the general order of reason: even so, that which is done miraculously by the Divine power is not contrary to nature, though it be contrary to the usual course of nature. Therefore just as Abraham did not sin in being willing to slay his innocent son, because he obeyed God, although considered in itself it was contrary to right human reason in general, so, too, Osee sinned not in committing fornication by God's command. Nor should such a copulation be strictly called fornication, though it be so called in reference to the general course of things. Hence Augustine says (Confess. iii, 8): "When God commands a thing to be done against the customs or agreement of any people, though it were never done by them heretofore, it is to be done"; and afterwards he adds: "For as among the powers of human society, the greater authority is obeyed in preference to the lesser, so must God in preference to all."

[o] Reply to Objection 3:
Abraham and Jacob went in to their handmaidens with no purpose of fornication, as we shall show further on when we treat of matrimony ([3536] XP, Q [65], A [5], ad 2). As to Juda there is no need to excuse him, for he also caused Joseph to be sold.

[p] Reply to Objection 4:
Simple fornication is contrary to the love of our neighbor, because it is opposed to the good of the child to be born, as we have shown, since it is an act of generation accomplished in a manner disadvantageous to the future child.

[q] Reply to Objection 5:
A person, who, while given to works of piety, yields to the inconstancy of the flesh, is freed from eternal loss, in so far as these works dispose him to receive the grace to repent, and because by such works he makes satisfaction for his past inconstancy; but not so as to be freed by pious works, if he persist in carnal inconstancy impenitent until death.

[r] Reply to Objection 6:
One copulation may result in the begetting of a man, wherefore inordinate copulation, which hinders the good of the future child, is a mortal sin as to the very genus of the act, and not only as to the inordinateness of concupiscence. On the other hand, one meal does not hinder the good of a man's whole life, wherefore the act of gluttony is not a mortal sin by reason of its genus. It would, however, be a mortal sin, if a man were knowingly to partake of a food which would alter the whole condition of his life, as was the case with Adam.

[s] Nor is it true that fornication is the least of the sins comprised under lust, for the marriage act that is done out of sensuous pleasure is a lesser sin.