Summa Theologica Suppl, q. 55: Of the Impediment of Affinity
Article 8. Whether the degrees of affinity extend in the same way as the degrees of consanguinity?
Objection 1It would seem that the degrees of affinity do not extend in the same way as the degrees of consanguinity. For the tie of affinity is less strong than the tie of consanguinity, since affinity arises from consanguinity in diversity of species, as from an equivocal cause. Now the stronger the tie the longer it lasts. Therefore the tie of affinity does not last to the same number of degrees as consanguinity.
Objection 2Further, human law should imitate Divine law. Now according to the Divine law certain degrees of consanguinity were forbidden, in which degrees affinity was not an impediment to marriage: as instanced in a brother's wife whom a man could marry although he could not marry her sister. Therefore now too the prohibition of affinity and consanguinity should not extend to the same degrees.
On the contraryA woman is connected with me by affinity from the very fact that she is married to a blood-relation of mine. Therefore in whatever degree her husband is related to me by blood she is related to me in that same degree by affinity: and so the degrees of affinity should be reckoned in the same number as the degrees of consanguinity.
I answer thatSince the degrees of affinity are reckoned according to the degrees of consanguinity, the degrees of affinity must needs be the same in number as those of consanguinity. Nevertheless, affinity being a lesser tie than consanguinity, both formerly and now, a dispensation is more easily granted in the more remote degrees of affinity than in the remote degrees of consanguinity.
Reply to Objection 1The fact that the tie of affinity is less than the tie of consanguinity causes a difference in the kind of relationship but not in the degrees. Hence this argument is not to the point.
Reply to Objection 2A man could not take his deceased brother's wife except, in the case when the latter died without issue, in order to raise up seed to his brother. This was requisite at a time when religious worship was propagated by means of the propagation of the flesh, which is not the case now. Hence it is clear that he did not marry her in his own person as it were, but as supplying the place of his brother.
Article 9. Whether a marriage contracted by persons with the degrees of affinity or consanguinity should always be annulled?
Objection 1It would seem that a marriage contracted by persons within the degrees of affinity or consanguinity ought not always to be annulled by divorce. For "what God hath joined together let no man put asunder" (Mat. 19:6). Since then it is understood that what the Church does God does, and since the Church sometimes through ignorance joins such persons together, it would seem that if subsequently this came to knowledge they ought not to be separated.
Objection 2Further, the tie of marriage is less onerous than the tie of ownership. Now after a long time a man may acquire by prescription the ownership of a thing of which he was not the owner. Therefore by length of time a marriage becomes good in law, although it was not so before.
Objection 3Further, of like things we judge alike. Now if a marriage ought to be annulled on account of consanguinity, in the case when two brothers marry two sisters, if one be separated on account of consanguinity, the other ought to be separated for the same reason. And yet this is not seemly. Therefore a marriage ought not to be annulled on account of affinity or consanguinity.
On the contraryConsanguinity and affinity forbid the contracting of a marriage and void the contract. Therefore if affinity or consanguinity be proved, the parties should be separated even though they have actually contracted marriage.
I answer thatSince all copulation apart from lawful marriage is a mortal sin, which the Church uses all her endeavors to prevent, it belongs to her to separate those between whom there cannot be valid marriage, especially those related by blood or by affinity, who cannot without incest be united in the flesh.
Reply to Objection 1Although the Church is upheld by God's gift and authority, yet in so far as she is an assembly of men there results in her acts something of human frailty which is not Divine. Therefore a union effected in the presence of the Church who is ignorant of an impediment is not indissoluble by Divine authority, but is brought about contrary to Divine authority through man's error, which being an error of fact excuses from sin, as long as it remains. Hence when the impediment comes to the knowledge of the Church, she ought to sever the aforesaid union.
Reply to Objection 2That which cannot be done without sin is not ratified by any prescription, for as Innocent III says (Conc. Later. iv, can. 50: cap. Non debent, De consang. et affinit.), "length of time does not diminish sin but increases it": nor can it in any way legitimize a marriage which could not take place between unlawful persons.
Reply to Objection 3In contentious suits between two persons the verdict does not prejudice a third party, wherefore although the one brother's marriage with the one sister is annulled on account of consanguinity, the Church does not therefore annul the other marriage against which no action is taken. Yet in the tribunal of the conscience the other brother ought not on this account always to be bound to put away his wife, because such accusations frequently proceed from ill-will, and are proved by false witnesses. Hence he is not bound to form his conscience on what has been done about the other marriage: but seemingly one ought to draw a distinction, because either he has certain knowledge of the impediment of his marriage, or he has an opinion about it, or he has neither. In the first case, he can neither seek nor pay the debt, in the second, he must pay, but not ask, in the third he can both pay and ask.