Commentaria in Summam contra Gentiles, Liber III

I. The third condition of marriage ¹ is that it ought not to take place between relatives. And for this reason, through reasonable causes, it has been ordained by laws that certain persons who are joined according to origin be excluded from marriage.

1. And first it is argued thus. That whereby persons who are one through the same origin have the occasion of loving one another more fervently is reasonably ordained. But this happens from the fact that they are excluded from marriage. Therefore, etc. — The minor is proved. Because, since in marriage there is a joining of diverse persons, while the said persons are excluded from marriage, they recognize that they are not completely diverse, but are one. But unity causes love among persons.

Secondly. Those things which are done between husband and wife possess a certain natural shame. Therefore, from doing these things mutually, those persons ought to have been prohibited to whom, on account of the joining of blood, reverence is due. And this reason is touched upon in the Old Law when it is said: Thou shalt not uncover the nakedness of thy sister. — St. Thomas does not prove the consequence, but leaves it manifest. Because with respect to those things which are done shamefacedly, care is taken lest they be done toward persons to whom reverence is due.

Thirdly. If it were lawful for a man to be joined through intercourse to persons with whom he has the necessity of dwelling, such as sisters and other relatives, an excessive practice of pleasure would follow, because the occasion of intercourse could not be removed from such persons. But it pertains to the corruption of good morals that men should be excessively given to the pleasures of intercourse. Therefore, etc.

Fourthly, and it is a confirmation of the preceding. The pleasure of intercourse would be multiplied. But this multiplication is repugnant to good morals, since such pleasure especially corrupts the good of prudence. Therefore, etc. — The consequence is proved. Because in them love would be multiplied, while the love which is from common origin and being raised together would be joined to the love of concupiscence. But when love is multiplied, it is necessary that the soul be more subjected to pleasures.

But it seems that this reason assumes the contrary of what was said in the first reason. For there it was said that from the fact that related persons are excluded from marriage, they have an occasion of loving one another more fervently. But here it is said that, if there were marriage among them, love would be multiplied. Which things seem to be contrary.

It can be said that in the first reason St. Thomas is speaking of honorable and ordered love. But here he is speaking of dishonorable and disordered love, from which an excessive and disordered practice of pleasure would arise.

Fifthly. Friendship is multiplied among men while strangers are joined through marriages. Therefore it was fitting to be ordained by laws that marriages be contracted with strangers, and not with relatives. — The consequence is proved. Because in human society it is necessary that there be friendship among many.

Sixthly. It is natural that someone be subject to his parents. Therefore it would be unfitting for someone to contract marriage with his parents. — The consequence is proved. Because in marriage there is a kind of social joining.

The conclusion is confirmed by the authority of Levit. XVIII.

Moreover, through this is excluded the custom of those mingling carnally with their relatives.

II. It must be noted that not all these reasons proceed from the same foundation. For since the end of marriage is threefold: primary, namely the good of the offspring; secondary, namely the repression of concupiscence; and per accidens, namely the multiplication of friendship and the alliance of men, as St. Thomas says in the Fourth Book, dist. XI ²: the second reason and the last proceed from the fact that marriage with those who are joined by blood in some way impedes the primary end of marriage, for it is impeded lest they be free for conceiving offspring in a fitting manner, on account of the reverence or subjection of the one to the other; the third and fourth proceed from the fact that the secondary end would be impeded; but the first and fifth proceed from the fact that the end per accidens would be impeded.

2. It must further be observed, as is found in the cited place ³, that in the natural law only the father and mother were barred from marriage, but the remaining degrees were permitted on account of the propagation of the human race. But it is of the divine law that those persons who must live together in the same house be barred from marriage, which are enumerated in Levit. XVIII. But according to human law, many other degrees of consanguinity are prohibited. From which it is clear that consanguinity, as to some persons, impedes marriage by natural law; as to some, by divine law; but as to some, by human law. On this account some of the aforesaid reasons prove only concerning those who are prohibited by natural law, as father and mother; but some concerning those who are prohibited by divine law, namely concerning those whom it is necessary to dwell together; but others also concerning those who are prohibited by human laws, namely concerning all up to the fourth degree inclusively.

III. St. Thomas notes, first, that because, just as natural law is directed to those things which are for the most part, so also positive law is established according to that which happens in the majority of cases; it is not contrary to the aforesaid reasons if in someone it could happen otherwise, for instance that concupiscence would be lessened in someone if he were mated to his sister: because on account of the good of one, the good of many ought not to be set aside.

2. He notes, secondly, that lest a defect which can happen in someone remain without remedy, there resides with legislators, and those like them, namely having like power, the authority to dispense in that which is commonly established, according as is necessary in some particular case: with men indeed, if the law is human; but with God, if the law is divine, just as in the Old Law by dispensation it was granted to have several wives and concubines, and the repudiation of a wife.

It must be noted that by the name concubine is not understood here a woman who is not joined in marriage, as is said in the Fourth Book, d. XXXIII, q. 1, a. 3 ⁴: but one who had something of the character of a wife, in so far, namely, as she was joined in marriage; and had something of the character of a concubine, in so far as the management of the household was not given to her, and there was not with her a communication of the works of life as with a partner. Which pertains to the secondary end of marriage, in which through divine ordination a dispensation was made with some.

3. But a doubt arises. For among those things which had been granted by divine dispensation, the repudiation of a wife is numbered in this place: although nevertheless it was said above ⁵ that St. Thomas inclines toward this opinion, that the Jews could not dismiss a wife without sin.

To this it can be answered in two ways. First, that the repudiation of a wife is said here to have been granted, not because those repudiating a wife did not sin, but because they avoided the penalty of the law, as was said above ⁶. And thus it is not contrary to what was said above.

Secondly, that since there is a twofold opinion concerning this, above St. Thomas spoke according to that opinion which was more pleasing to him; but here he exemplifies also according to the mind of the other opinion, for the reason that it too is probable.