Reply to Objection 3: Further, a relation results sometimes from a movement in each extreme, for instance fatherhood and sonship, and a relation of this kind is really in both extremes. Sometimes it results from the movement of one only, and this happens in two ways. In one way when a relation results from the movement of one extreme without any movement previous or concomitant of the other extreme; as in the Creator and the creature, the sensible and the sense, knowledge and the knowable object: and then the relation is in one extreme really and in the other logically only. In another way when the relation results from the movement of one extreme without any concomitant movement, but not without a previous movement of the other; thus there results equality between two men by the increase of one, without the other either increasing or decreasing then, although previously he reached his actual quantity by some movement or change, so that this relation is founded really in both extremes. It is the same with consanguinity and affinity, because the relation of brotherhood which results in a grown child on the birth of a boy, is caused without any movement of the former's at the time, but by virtue of that previous movement of his wherein he was begotten; wherefore at the time it happens that there results in him the aforesaid relation through the movement of another. Likewise because this man descends through his own birth from the same stock as the husband, there results in him affinity with the latter's wife, without any new change in him.
Whether affinity remains after the death of husband or wife?
Objection 1: It would seem that affinity does not remain after the death of husband or wife, between the blood-relations of husband and wife or "vice versa." Because if the cause cease the effect ceases. Now the cause of affinity was the marriage, which ceases after the husband's death, since then "the woman . . . is loosed from the law of the husband" ( Rom. 7:2 ). Therefore the aforesaid affinity ceases also.
Objection 2: Further, consanguinity is the cause of affinity. Now the consanguinity of the husband with his blood-relations ceases at his death. Therefore, the wife's affinity with them ceases also.
On the contrary, Affinity is caused by consanguinity. Now consanguinity binds persons together for all time as long as they live. Therefore affinity does so also: and consequently affinity (between two persons) is not dissolved through the dissolution of the marriage by the death of a third person.
I answer that, A relation ceases in two ways: in one way through the corruption of its subject, in another way by the removal of its cause; thus likeness ceases when one of the like subjects dies, or when the quality that caused the likeness is removed. Now there are certain relations which have for their cause an action, or a passion or movement (Metaph. v, 20): and some of these are caused by movement, through something being moved actually; such is the relation between mover and moved: some of them are caused through something being adapted to movement, for instance the relations between the motive power and the movable, or between master and servant; and some of them result from something, having been moved previously, such as the relation between father and son, for the relation between them is caused not by (the con) being begotten now, but by his having been begotten. Now aptitude for movement and for being moved is transitory; whereas the fact of having been moved is everlasting, since what has been never ceases having been. Consequently fatherhood and sonship are never dissolved through the removal of the cause, but only through the corruption of the subject, that is of one of the subjects. The same applies to affinity, for this is caused by certain persons having been joined together not by their being actually joined. Wherefore it is not done away, as long as the persons between whom affinity has been contracted survive, although the person die through whom it was contracted.
Reply to Objection 1: The marriage tie causes affinity not only by reason of actual union, but also by reason of the union having been effected in the past.
Reply to Objection 2: Consanguinity is not the chief cause of affinity, but union with a blood-relation, not only because that union is now, but because it has been. Hence the argument does not prove.
Whether unlawful intercourse causes affinity?
Objection 1: It would seem that unlawful intercourse does not cause affinity. For affinity is an honorable thing. Now honorable things do not result from that which is dishonorable. Therefore affinity cannot be caused by a dishonorable intercourse.
Objection 2: Further, where there is consanguinity there cannot be affinity; since affinity is a relationship between persons that results from carnal intercourse and is altogether void of blood-relationship. Now if unlawful intercourse were a cause of affinity, it would sometimes happen that a man would contract affinity with his blood-relations and with himself: for instance when a man is guilty of incest with a blood-relation. Therefore affinity is not caused by unlawful intercourse.
Objection 3: Further, unlawful intercourse is according to nature or against nature. Now affinity is not caused by unnatural unlawful intercourse as decided by law (can. Extraordinaria, xxxv, qu. 2,3). Therefore it is not caused only by unlawful intercourse according to nature.
On the contrary, He who is joined to a harlot is made one body ( 1 Cor. 6:16 ). Now this is the reason why marriage caused affinity. Therefore unlawful intercourse does so for the same reason.
Further, carnal intercourse is the cause of affinity, as shown by the definition of affinity, which definition is as follows: Affinity is the relationship of persons which results from carnal intercourse and is altogether void of blood-relationship. But there is carnal copulation even in unlawful intercourse. Therefore unlawful intercourse causes affinity.
I answer that, According to the Philosopher (Ethic. viii, 12) the union of husband and wife is said to be natural chiefly on account of the procreation of offspring, and secondly on account of the community of works: the former of which belongs to marriage by reason of carnal copulation, and the latter, in so far as marriage is a partnership directed to a common life. Now the former is to be found in every carnal union where there is a mingling of seeds, since such a union may be productive of offspring, but the latter may be wanting. Consequently since marriage caused affinity, in so far as it was a carnal mingling, it follows that also an unlawful intercourse causes affinity in so far as it has something of natural copulation.
Reply to Objection 1: In an unlawful intercourse there is something natural which is common to fornication and marriage, and in this respect it causes affinity. There is also something which is inordinate whereby it differs from marriage, and in this respect it does not cause affinity. Hence affinity remains honorable, although its cause is in a way dishonorable.
Reply to Objection 2: There is no reason why diverse relations should not be in the same subject by reason of different things. Consequently there can be affinity and consanguinity between two persons, not only on account of unlawful but also on account of lawful intercourse: for instance if a blood-relation of mine on my father's side marries a blood-relation of mine on my mother's side. Hence in the above definition the words "which is altogether void of blood-relationship" apply to affinity as such. Nor does it follow that a man by having intercourse with his blood-relation contracts affinity with himself, since affinity, like consanguinity, requires diversity of subjects, as likeness does.
Reply to Objection 3: In unnatural copulation there is no mingling of seeds that makes generation possible: wherefore a like intercourse does not cause affinity.
Whether affinity is caused by betrothal?
Objection 1: It would seem that affinity cannot be caused by betrothal. For affinity is a lasting tie: whereas a betrothal is sometimes broken off. Therefore it cannot cause affinity.
Objection 2: Further if the hymen be penetrated without the deed being consummated, affinity is not contracted. Yet this is much more akin to carnal intercourse than a betrothal. Therefore betrothal does not cause affinity.
Objection 3: Further, betrothal is nothing but a promise of future marriage. Now sometimes there is a promise of future marriage without affinity being contracted, for instance if it take place before the age of seven years; or if a man having a perpetual impediment of impotence promise a woman future marriage; or if a like promise be made between persons to whom marriage is rendered unlawful by a vow; or in any other way whatever. Therefore betrothal cannot cause affinity.
On the contrary, Pope Alexander (cap. Ad audiendem, De spons. et matrim.) forbade a certain woman to marry a certain man, because she had been betrothed to his brother. Now this would not be the case unless affinity were contracted by betrothal. Therefore, etc.
I answer that, Just as a betrothal has not the conditions of a perfect marriage, but is a preparation for marriage, so betrothal causes not affinity as marriage does, but something like affinity. This is called "the justice of public honesty," which is an impediment to marriage even as affinity and consanguinity are, and according to the same degrees, and is defined thus: "The justice of public honesty is a relationship arising out of betrothal, and derives its force from ecclesiastical institution by reason of its honesty." This indicates the reason of its name as well as its cause, namely that this relationship was instituted by the Church on account of its honesty.
Reply to Objection 1: Betrothal, by reason not of itself but of the end to which it is directed, causes this kind of affinity known as "the justice of public honesty": wherefore just as marriage is a lasting tie, so is the aforesaid kind of affinity.