Summa Th. II-II EN Qu.154 a.3

Whether fornication is the most grievous of sins?

Objection: 1. It would seem that fornication is the most grievous of sins. For seemingly a sin is the more grievous according as it proceeds from a greater sensuous pleasure. Now the greatest sensuous pleasure is in fornication, for a gloss on 1Co 7,9 says that the "flame of sensuous pleasure is most fierce in lust." Therefore it seems that fornication is the gravest of sins.
Objection: 1. Further, a sin is the more grievous that is committed against a person more closely united to the sinner: thus he sins more grievously who strikes his father than one who strikes a stranger. Now according to 1Co 6,18, "He that committeth fornication sinneth against his own body," which is most intimately connected with a man. Therefore it seems that fornication is the most grievous of sins.
3. Further, the greater a good is, the graver would seem to be the sin committed against it. Now the sin of fornication is seemingly opposed to the good of the whole human race, as appears from what was said in the foregoing Article. It is also against Christ, according to 1Co 6,15, "Shall I . . . take the members of Christ, and make them the members of a harlot?" Therefore fornication is the most grievous of sins.

On the contrary Gregory says (Moral. xxxiii, 12) that the sins of the flesh are less grievous than spiritual sins.
I answer that The gravity of a sin may be measured in two ways, first with regard to the sin in itself, secondly with regard to some accident. The gravity of a sin is measured with regard to the sin itself, by reason of its species, which is determined according to the good to which that sin is opposed. Now fornication is contrary to the good of the child to be born. Wherefore it is a graver sin, as to its species, than those sins which are contrary to external goods, such as theft and the like; while it is less grievous than those which are directly against God, and sins that are injurious to the life of one already born, such as murder.

Reply to Objection: 1. The sensual pleasure that aggravates a sin is that which is in the inclination of the will. But the sensual pleasure that is in the sensitive appetite, lessens sin, because a sin is the less grievous according as it is committed under the impulse of a greater passion. It is in this way that the greatest sensual pleasure is in fornication. Hence Augustine says (De Agone Christiano [*Serm. ccxciii; ccl de Temp.; see Appendix to St. Augustine's works]) that of all a Christian's conflicts, the most difficult combats are those of chastity; wherein the fight is a daily one, but victory rare: and Isidore declares (De Summo Bono ii, 39) that "mankind is subjected to the devil by carnal lust more than by anything else," because, to wit, the vehemence of this passion is more difficult to overcome.
2. The fornicator is said to sin against his own body, not merely because the pleasure of fornication is consummated in the flesh, which is also the case in gluttony, but also because he acts against the good of his own body by an undue resolution and defilement thereof, and an undue association with another. Nor does it follow from this that fornication is the most grievous sin, because in man reason is of greater value than the body, wherefore if there be a sin more opposed to reason, it will be more grievous.
3. The sin of fornication is contrary to the good of the human race, in so far as it is prejudicial to the individual begetting of the one man that may be born. Now one who is already an actual member of the human species attains to the perfection of the species more than one who is a man potentially, and from this point of view murder is a more grievous sin than fornication and every kind of lust, through being more opposed to the good of the human species. Again, a Divine good is greater than the good of the human race: and therefore those sins also that are against God are more grievous. Moreover, fornication is a sin against God, not directly as though the fornicator intended to offend God, but consequently, in the same way as all mortal sins. And just as the members of our body are Christ's members, so too, our spirit is one with Christ, according to 1Co 6,17, "He who is joined to the Lord is one spirit." Wherefore also spiritual sins are more against Christ than fornication is.

Whether there can be mortal sin in touches and kisses?

Objection: 1. It would seem that there is no mortal sin in touches and kisses. For the Apostle says (Ep 5,3): "Fornication and all uncleanness, or covetousness, let it not so much as be named among you, as becometh saints," then he adds: "Or obscenity" (which a gloss refers to "kissing and fondling"), "or foolish talking" (as "soft speeches"), "or scurrility" (which "fools call geniality---i.e. jocularity"), and afterwards he continues (Ep 5,5): "For know ye this and understand that no fornicator, or unclean, or covetous person (which is the serving of idols), hath inheritance in the kingdom of Christ and of God," thus making no further mention of obscenity, as neither of foolish talking or scurrility. Therefore these are not mortal sins.
2. Further, fornication is stated to be a mortal sin as being prejudicial to the good of the future child's begetting and upbringing. But these are not affected by kisses and touches or blandishments. Therefore there is no mortal sin in these.
3. Further, things that are mortal sins in themselves can never be good actions. Yet kisses, touches, and the like can be done sometimes without sin. Therefore they are not mortal sins in themselves.

On the contrary A lustful look is less than a touch, a caress or a kiss. But according to Mt 5,28, "Whosoever shall look on a woman to lust after her hath already committed adultery with her in his heart." Much more therefore are lustful kisses and other like things mortal sins.Further, Cyprian says (Ad Pompon, de Virgin., Ep. lxii), "By their very intercourse, their blandishments, their converse, their embraces, those who are associated in a sleep that knows neither honor nor shame, acknowledge their disgrace and crime." Therefore by doing these things a man is guilty of a crime, that is, of mortal sin.
I answer that A thing is said to be a mortal works. /sin in two ways. First, by reason of its species, and in this way a kiss, caress, or touch does not, of its very nature, imply a mortal sin, for it is possible to do such things without lustful pleasure, either as being the custom of one's country, or on account of some obligation or reasonable cause. Secondly, a thing is said to be a mortal sin by reason of its cause: thus he who gives an alms, in order to lead someone into heresy, sins mortally on account of his corrupt intention. Now it has been stated above (I-II 74,8), that it is a mortal sin not only to consent to the act, but also to the delectation of a mortal sin. Wherefore since fornication is a mortal sin, and much more so the other kinds of lust, it follows that in such like sins not only consent to the act but also consent to the pleasure is a mortal sin. Consequently, when these kisses and caresses are done for this delectation, it follows that they are mortal sins, and only in this way are they said to be lustful. Therefore in so far as they are lustful, they are mortal sins.

Reply to Objection: 1. The Apostle makes no further mention of these three because they are not sinful except as directed to those that he had mentioned before.
2. Although kisses and touches do not by their very nature hinder the good of the human offspring, they proceed from lust, which is the source of this hindrance: and on this account they are mortally sinful.
3. This argument proves that such things are not mortal sins in their species.

Whether nocturnal pollution is a mortal sin?

Objection: 1. It would seem that nocturnal pollution is a sin. For the same things are the matter of merit and demerit. Now a man may merit while he sleeps, as was the case with Solomon, who while asleep obtained the gift of wisdom from the Lord (1R 3,2, Par. 1). Therefore a man may demerit while asleep; and thus nocturnal pollution would seem to be a sin.
2. Further, whoever has the use of reason can sin. Now a man has the use of reason while asleep, since in our sleep we frequently discuss matters, choose this rather than that, consenting to one thing, or dissenting to another. Therefore one may sin while asleep, so that nocturnal pollution is not prevented by sleep from being a sin, seeing that it is a sin according to its genus.
3. Further, it is useless to reprove and instruct one who cannot act according to or against reason. Now man, while asleep, is instructed and reproved by God, according to Jb 33,15-16, "By a dream in a vision by night, when deep sleep is wont to lay hold of men [*Vulg.: 'When deep sleep falleth upon men.' St. Thomas is apparently quoting from memory, as the passage is given correctly above, Question [95], Article [6], Objection [1]] . . . Then He openeth the ears of men, and teaching instructeth them in what they are to learn." Therefore a man, while asleep, can act according to or against his reason, and this is to do good or sinful actions, and thus it seems that nocturnal pollution is a sin.

On the contrary Augustine says (Gn ad lit. xii, 15): "When the same image that comes into the mind of a speaker presents itself to the mind of the sleeper, so that the latter is unable to distinguish the imaginary from the real union of bodies, the flesh is at once moved, with the result that usually follows such motions; and yet there is as little sin in this as there is in speaking and therefore thinking about such things while one is awake."
I answer that Nocturnal pollution may be considered in two ways. First, in itself; and thus it has not the character of a sin. For every sin depends on the judgment of reason, since even the first movement of the sensuality has nothing sinful in it, except in so far as it can be suppressed by reason; wherefore in the absence of reason's judgment, there is no sin in it. Now during sleep reason has not a free judgment. For there is no one who while sleeping does not regard some of the images formed by his imagination as though they were real, as stated above in the I 84,8, ad 2. Wherefore what a man does while he sleeps and is deprived of reason's judgment, is not imputed to him as a sin, as neither are the actions of a maniac or an imbecile.Secondly, nocturnal pollution may be considered with reference to its cause. This may be threefold. One is a bodily cause. For when there is excess of seminal humor in the body, or when the humor is disintegrated either through overheating of the body or some other disturbance, the sleeper dreams things that are connected with the discharge of this excessive or disintegrated humor: the same thing happens when nature is cumbered with other superfluities, so that phantasms relating to the discharge of those superfluities are formed in the imagination. Accordingly if this excess of humor be due to a sinful cause (for instance excessive eating or drinking), nocturnal pollution has the character of sin from its cause: whereas if the excess or disintegration of these superfluities be not due to a sinful cause, nocturnal pollution is not sinful, neither in itself nor in its cause.A second cause of nocturnal pollution is on the part of the soul and the inner man: for instance when it happens to the sleeper on account of some previous thought. For the thought which preceded while he was awake, is sometimes purely speculative, for instance when one thinks about the sins of the flesh for the purpose of discussion; while sometimes it is accompanied by a certain emotion either of concupiscence or of abhorrence. Now nocturnal pollution is more apt to arise from thinking about carnal sins with concupiscence for such pleasures, because this leaves its trace and inclination in the soul, so that the sleeper is more easily led in his imagination to consent to acts productive of pollution. In this sense the Philosopher says (Ethic. i, 13) that "in so far as certain movements in some degree pass" from the waking state to the state of sleep, "the dreams of good men are better than those of any other people": and Augustine says (Gn ad lit. xii, 15) that "even during sleep, the soul may have conspicuous merit on account of its good disposition." Thus it is evident that nocturnal pollution may be sinful on the part of its cause. on the other hand, it may happen that nocturnal pollution ensues after thoughts about carnal acts, though they were speculative, or accompanied by abhorrence, and then it is not sinful, neither in itself nor in its cause.The third cause is spiritual and external; for instance when by the work of a devil the sleeper's phantasms are disturbed so as to induce the aforesaid result. Sometimes this is associated with a previous sin, namely the neglect to guard against the wiles of the devil. Hence the words of the hymn at even: "Our enemy repress, that so our bodies no uncleanness know" [*Translation W. K. Blount].On the other hand, this may occur without any fault on man's part, and through the wickedness of the devil alone. Thus we read in the Collationes Patrum (Coll. xxii, 6) of a man who was ever wont to suffer from nocturnal pollution on festivals, and that the devil brought this about in order to prevent him from receiving Holy Communion. Hence it is manifest that nocturnal pollution is never a sin, but is sometimes the result of a previous sin.

Reply to Objection: 1. Solomon did not merit to receive wisdom from God while he was asleep. He received it in token of his previous desire. It is for this reason that his petition is stated to have been pleasing to God (1R 3,10), as Augustine observes (Gn ad lit. xii, 15).
2. The use of reason is more or less hindered in sleep, according as the inner sensitive powers are more or less overcome by sleep, on account of the violence or attenuation of the evaporations. Nevertheless it is always hindered somewhat, so as to be unable to elicit a judgment altogether free, as stated in the I 84,8, ad 2. Therefore what it does then is not imputed to it as a sin.
3. Reason's apprehension is not hindered during sleep to the same extent as its judgment, for this is accomplished by reason turning to sensible objects, which are the first principles of human thought. Hence nothing hinders man's reason during sleep from apprehending anew something arising out of the traces left by his previous thoughts and phantasms presented to him, or again through Divine revelation, or the interference of a good or bad angel.

Whether seduction should be reckoned a species of lust?

Objection: 1. It would seem that seduction should not be reckoned a species of lust. For seduction denotes the unlawful violation of a virgin, according to the Decretals (XXXVI, qu. 1) [*Append. Grat. ad can. Lex illa]. But this may occur between an unmarried man and an unmarried woman, which pertains to fornication. Therefore seduction should not be reckoned a species of lust, distinct from fornication.
2. Further, Ambrose says (De Patriarch. [*De Abraham i, 4]): "Let no man be deluded by human laws: all seduction is adultery." Now a species is not contained under another that is differentiated in opposition to it. Therefore since adultery is a species of lust, it seems that seduction should not be reckoned a species of lust.
3. Further, to do a person an injury would seem to pertain to injustice rather than to lust. Now the seducer does an injury to another, namely the violated maiden's father, who "can take the injury as personal to himself" [*Gratian, ad can. Lex illa], and sue the seducer for damages. Therefore seduction should not be reckoned a species of lust.

On the contrary Seduction consists properly in the venereal act whereby a virgin is violated. Therefore, since lust is properly about venereal actions, it would seem that seduction is a species of lust.
I answer that When the matter of a vice has a special deformity, we must reckon it to be a determinate species of that vice. Now lust is a sin concerned with venereal matter, as stated above (Question [153], Article [1]). And a special deformity attaches to the violation of a virgin who is under her father's care: both on the part of the maid, who through being violated without any previous compact of marriage is both hindered from contracting a lawful marriage and is put on the road to a wanton life from which she was withheld lest she should lose the seal of virginity: and on the part of the father, who is her guardian, according to Si 42,11, "Keep a sure watch over a shameless daughter, lest at any time she make thee become a laughing-stock to thy enemies." Therefore it is evident that seduction which denotes the unlawful violation of a virgin, while still under the guardianship of her parents, is a determinate species of lust.

Reply to Objection: 1. Although a virgin is free from the bond of marriage, she is not free from her father's power. Moreover, the seal of virginity is a special obstacle to the intercourse of fornication, in that it should be removed by marriage only. Hence seduction is not simple fornication, since the latter is intercourse with harlots, women, namely, who are no longer virgins, as a gloss observes on 2Co 12,, "And have not done penance for the uncleanness and fornication," etc.
2. Ambrose here takes seduction in another sense, as applicable in a general way to any sin of lust. Wherefore seduction, in the words quoted, signifies the intercourse between a married man and any woman other than his wife. This is clear from his adding: "Nor is it lawful for the husband to do what the wife may not." In this sense, too, we are to understand the words of Nb 5,13: "If [Vulg.: 'But'] the adultery is secret, and cannot be provided by witnesses, because she was not found in adultery [stupro]."
3. Nothing prevents a sin from having a greater deformity through being united to another sin. Now the sin of lust obtains a greater deformity from the sin of injustice, because the concupiscence would seem to be more inordinate, seeing that it refrains not from the pleasurable object so that it may avoid an injustice. In fact a twofold injustice attaches to it. One is on the part of the virgin, who, though not violated by force, is nevertheless seduced, and thus the seducer is bound to compensation. Hence it is written (Ex 22,16-17): "If a man seduce a virgin not yet espoused, and lie with her, he shall endow her and have her to wife. If the maid's father will not give her to him, he shall give money according to the dowry, which virgins are wont to receive." The other injury is done to the maid's father: wherefore the seducer is bound by the Law to a penalty in his regard. For it is written (Dt 22,28-29): "If a man find a damsel that is a virgin, who is not espoused, and taking her, lie with her, and the matter come to judgment: he that lay with her shall give to the father of the maid fifty sicles of silver, and shall have her to wife, and because he hath humbled her, he may not put her away all the days of his life": and this, lest he should prove to have married her in mockery, as Augustine observes. [*Questions. in Dt., qu. xxxiv.]

Whether rape is a species of lust, distinct from seduction?

Objection: 1. It would seem that rape is not a species of lust, distinct from seduction. For Isidore says (Etym. v, 26) that "seduction [stuprum], or rape, properly speaking, is unlawful intercourse, and takes its name from its causing corruption: wherefore he that is guilty of rape is a seducer." Therefore it seems that rape should not be reckoned a species of lust distinct from seduction.
2. Further, rape, apparently, implies violence. For it is stated in the Decretals (XXXVI, qu. 1 [*Append. Grat. ad can. Lex illa]) that "rape is committed when a maid is taken away by force from her father's house that after being violated she may be taken to wife." But the employment of force is accidental to lust, for this essentially regards the pleasure of intercourse. Therefore it seems that rape should not be reckoned a determinate species of lust.
3. Further, the sin of lust is curbed by marriage: for it is written (1Co 7,2): "For fear of fornication, let every man have his own wife." Now rape is an obstacle to subsequent marriage, for it was enacted in the council of Meaux: "We decree that those who are guilty of rape, or of abducting or seducing women, should not have those women in marriage, although they should have subsequently married them with the consent of their parents." Therefore rape is not a determinate species of lust distinct from seduction.
4. Further, a man may have knowledge of his newly married wife without committing a sin of lust. Yet he may commit rape if he take her away by force from her parents' house, and have carnal knowledge of her. Therefore rape should not be reckoned a determinate species of lust.

On the contrary Rape is unlawful sexual intercourse, as Isidore states (Etym. v, 26). But this pertains to the sin of lust. Therefore rape is a species of lust.
I answer that Rape, in the sense in which we speak of it now, is a species of lust: and sometimes it coincides with seduction; sometimes there is rape without seduction, and sometimes seduction without rape.They coincide when a man employs force in order unlawfully to violate a virgin. This force is employed sometimes both towards the virgin and towards her father; and sometimes towards the father and not to the virgin, for instance if she allows herself to be taken away by force from her father's house. Again, the force employed in rape differs in another way, because sometimes a maid is taken away by force from her parents' house, and is forcibly violated: while sometimes, though taken away by force, she is not forcibly violated, but of her own consent, whether by act of fornication or by the act of marriage: for the conditions of rape remain no matter how force is employed. There is rape without seduction if a man abduct a widow or one who is not a virgin. Hence Pope Symmachus says [*Ep. v ad Caesarium; Cf. can. Raptores xxxvi, qu. 2], "We abhor abductors whether of widows or of virgins on account of the heinousness of their crime."There is seduction without rape when a man, without employing force, violates a virgin unlawfully.

Objection: 1. Since rape frequently coincides with seduction, the one is sometimes used to signify the other.
2. The employment of force would seem to arise from the greatness of concupiscence, the result being that a man does not fear to endanger himself by offering violence.
3. The rape of a maiden who is promised in marriage is to be judged differently from that of one who is not so promised. For one who is promised in marriage must be restored to her betrothed, who has a right to her in virtue of their betrothal: whereas one that is not promised to another must first of all be restored to her father's care, and then the abductor may lawfully marry her with her parents' consent. Otherwise the marriage is unlawful, since whosoever steals a thing he is bound to restore it. Nevertheless rape does not dissolve a marriage already contracted, although it is an impediment to its being contracted. As to the decree of the council in question, it was made in abhorrence of this crime, and has been abrogated. Wherefore Jerome [*The quotation is from Can. Tria. xxxvi, qu. 2] declares the contrary: "Three kinds of lawful marriage," says he, "are mentioned in Holy Writ. The first is that of a chaste maiden given away lawfully in her maidenhood to a man. The second is when a man finds a maiden in the city, and by force has carnal knowledge of her. If the father be willing, the man shall endow her according to the father's estimate, and shall pay the price of her purity [*Cf. Dt 22,23-29]. The third is, when the maiden is taken away from such a man, and is given to another at the father's will."We may also take this decree to refer to those who are promised to others in marriage, especially if the betrothal be expressed by words in the present tense.
4. The man who is just married has, in virtue of the betrothal, a certain right in her: wherefore, although he sins by using violence, he is not guilty of the crime of rape. Hence Pope Gelasius says [*Can. Lex illa, xxvii, qu. 2; xxxvi, qu. 1]: "This law of bygone rulers stated that rape was committed when a maiden, with regard to whose marriage nothing had so far been decided, was taken away by force."

Whether adultery is determinate species of lust, distinct from the other species?

Objection: 1. It would seem that adultery is not a determinate species of lust, distinct from the other species. For adultery takes its name from a man having intercourse "with a woman who is not his own [ad alteram]," according to a gloss [*St. Augustine: Serm. li, 13 de Divers. lxiii] on Ex 20,14. Now a woman who is not one's own may be of various conditions, namely either a virgin, or under her father's care, or a harlot, or of any other description. Therefore it seems that adultery is not a species of lust distinct from the others.
2. Further, Jerome says [*Contra Jovin. i]: "It matters not for what reason a man behaves as one demented. Hence Sixtus the Pythagorean says in his Maxims: He that is insatiable of his wife is an adulterer," and in like manner one who is over enamored of any woman. Now every kind of lust includes a too ardent love. Therefore adultery is in every kind of lust: and consequently it should not be reckoned a species of lust.
3. Further, where there is the same kind of deformity, there would seem to be the same species of sin. Now, apparently, there is the same kind of deformity in seduction and adultery: since in either case a woman is violated who is under another person's authority. Therefore adultery is not a determinate species of lust, distinct from the others.

On the contrary Pope Leo [*St. Augustine, De Bono Conjug. iv; Cf. Append. Grat. ad can. Ille autem. xxxii, qu. 5] says that "adultery is sexual intercourse with another man or woman in contravention of the marriage compact, whether through the impulse of one's own lust, or with the consent of the other party." Now this implies a special deformity of lust. Therefore adultery is a determinate species of lust.
I answer that Adultery, as its name implies, "is access to another's marriage-bed [ad alienum torum]" [*Cf. Append. Gratian, ad can. Ille autem. xxxii, qu. 1]. By so doing a man is guilty of a twofold offense against chastity and the good of human procreation. First, by accession to a woman who is not joined to him in marriage, which is contrary to the good of the upbringing of his own children. Secondly, by accession to a woman who is united to another in marriage, and thus he hinders the good of another's children. The same applies to the married woman who is corrupted by adultery. Wherefore it is written (Si 23,32-33): "Every woman . . . that leaveth her husband . . . shall be guilty of sin. For first she hath been unfaithful to the law of the Most High" (since there it is commanded: "Thou shalt not commit adultery"); "and secondly, she hath offended against her husband," by making it uncertain that the children are his: "thirdly, she hath fornicated in adultery, and hath gotten children of another man," which is contrary to the good of her offspring. The first of these, however, is common to all mortal sins, while the two others belong especially to the deformity of adultery. Hence it is manifest that adultery is a determinate species of lust, through having a special deformity in venereal acts.

Reply to Objection: 1. If a married man has intercourse with another woman, his sin may be denominated either with regard to him, and thus it is always adultery, since his action is contrary to the fidelity of marriage, or with regard to the woman with whom he has intercourse; and thus sometimes it is adultery, as when a married man has intercourse with another's wife; and sometimes it has the character of seduction, or of some other sin, according to various conditions affecting the woman with whom he has intercourse: and it has been stated above (Article [1]) that the species of lust correspond to the various conditions of women.
2. Matrimony is specially ordained for the good of human offspring, as stated above (Article [2]). But adultery is specially opposed to matrimony, in the point of breaking the marriage faith which is due between husband and wife. And since the man who is too ardent a lover of his wife acts counter to the good of marriage if he use her indecently, although he be not unfaithful, he may in a sense be called an adulterer; and even more so than he that is too ardent a lover of another woman.
3. The wife is under her husband's authority, as united to him in marriage: whereas the maid is under her father's authority, as one who is to be married by that authority. Hence the sin of adultery is contrary to the good of marriage in one way, and the sin of seduction in another; wherefore they are reckoned to differ specifically. Of other matters concerning adultery we shall speak in the Third Part [*XP, Question [59], Article [3]; XP, Questions [60],62], when we treat of matrimony.

Whether incest is a determinate species of lust?

Objection: 1. It would seem that incest is not a determinate species of lust. For incest [*'Incestus' is equivalent to 'in-castus = 'unchaste'] takes its name from being a privation of chastity. But all kinds of lust are opposed to chastity. Therefore it seems that incest is not a species of lust, but is lust itself in general.
2. Further, it is stated in the Decretals (XXXVI, qu. 1 [*Cf. Append. Grat. ad can. Lex illa]) that "incest is intercourse between a man and a woman related by consanguinity or affinity." Now affinity differs from consanguinity. Therefore it is not one but several species of lust.
3. Further, that which does not, of itself, imply a deformity, does not constitute a determinate species of vice. But intercourse between those who are related by consanguinity or affinity does not, of itself, contain any deformity, else it would never have been lawful. Therefore incest is not a determinate species of lust.

On the contrary The species of lust are distinguished according to the various conditions of women with whom a man has unlawful intercourse. Now incest implies a special condition on the part of the woman, because it is unlawful intercourse with a woman related by consanguinity or affinity as stated (Objection [2]). Therefore incest is a determinate species of lust.
I answer that As stated above (Articles [1],6) wherever we find something incompatible with the right use of venereal actions, there must needs be a determinate species of lust. Now sexual intercourse with women related by consanguinity or affinity is unbecoming to venereal union on three counts. First, because man naturally owes a certain respect to his parents and therefore to his other blood relations, who are descended in near degree from the same parents: so much so indeed that among the ancients, as Valerius Maximus relates [*Dict. Fact. Memor. ii, 1], it was not deemed right for a son to bathe with his father, lest they should see one another naked. Now from what has been said (Question [142], Article [4]: Question [151], Article [4]), it is evident that in venereal acts there is a certain shamefulness inconsistent with respect, wherefore men are ashamed of them. Wherefore it is unseemly that such persons should be united in venereal intercourse. This reason seems to be indicated (Lv 18,7) where we read: "She is thy mother, thou shalt not uncover her nakedness," and the same is expressed further on with regard to others.The second reason is because blood relations must needs live in close touch with one another. Wherefore if they were not debarred from venereal union, opportunities of venereal intercourse would be very frequent and thus men's minds would be enervated by lust. Hence in the Old Law [*Lv 18] the prohibition was apparently directed specially to those persons who must needs live together.The third reason is, because this would hinder a man from having many friends: since through a man taking a stranger to wife, all his wife's relations are united to him by a special kind of friendship, as though they were of the same blood as himself. Wherefore Augustine says (De Civ. Dei xv, 16): "The demands of charity are most perfectly satisfied by men uniting together in the bonds that the various ties of friendship require, so that they may live together in a useful and becoming amity; nor should one man have many relationships in one, but each should have one."Aristotle adds another reason (2 Polit. ii): for since it is natural that a man should have a liking for a woman of his kindred, if to this be added the love that has its origin in venereal intercourse, his love would be too ardent and would become a very great incentive to lust: and this is contrary to chastity. Hence it is evident that incest is a determinate species of lust.

Reply to Objection: 1. Unlawful intercourse between persons related to one another would be most prejudicial to chastity, both on account of the opportunities it affords, and because of the excessive ardor of love, as stated in the Article. Wherefore the unlawful intercourse between such persons is called "incest" antonomastically.
2. Persons are related by affinity through one who is related by consanguinity: and therefore since the one depends on the other, consanguinity and affinity entail the same kind of unbecomingness.
3. There is something essentially unbecoming and contrary to natural reason in sexual intercourse between persons related by blood, for instance between parents and children who are directly and immediately related to one another, since children naturally owe their parents honor. Hence the Philosopher instances a horse (De Animal. ix, 47) which covered its own mother by mistake and threw itself over a precipice as though horrified at what it had done, because some animals even have a natural respect for those that have begotten them. There is not the same essential unbecomingness attaching to other persons who are related to one another not directly but through their parents: and, as to this, becomingness or unbecomingness varies according to custom, and human or Divine law: because, as stated above (Article [2]), sexual intercourse, being directed to the common good, is subject to law. Wherefore, as Augustine says (De Civ. Dei xv, 16), whereas the union of brothers and sisters goes back to olden times, it became all the more worthy of condemnation when religion forbade it.

Summa Th. II-II EN Qu.154 a.3