Summa Theologica Suppl, q. 55: Of the Impediment of Affinity
Article 6. Whether affinity is an impediment to marriage?
Objection 1It would seem that affinity is not an impediment to marriage. For nothing is an impediment to marriage except what is contrary thereto. But affinity is not contrary to marriage since it is caused by it. Therefore it is not an impediment to marriage.
Objection 2Further, by marriage the wife becomes a possession of the husband. Now the husband's kindred inherit his possessions after his death. Therefore they can succeed to his wife, although she is affine to them, as shown above (A[5]). Therefore affinity is not an impediment to marriage.
On the contraryIt is written (Lev. 18:8): "Thou shalt not uncover the nakedness of thy father's wife." Now she is only affine. Therefore affinity is an impediment to marriage.
I answer thatAffinity that precedes marriage hinders marriage being contracted and voids the contract, for the same reason as consanguinity. For just as there is a certain need for blood-relations to live together, so is there for those who are connected by affinity: and just as there is a tie of friendship between blood-relations, so is there between those who are affine to one another. If, however, affinity supervene to matrimony, it cannot void the marriage, as stated above (Q[50], A[7]).
Reply to Objection 1Affinity is not contrary to the marriage which causes it, but to a marriage being contracted with an affine, in so far as the latter would hinder the extension of friendship and the curbing of concupiscence, which are sought in marriage.
Reply to Objection 2The husband's possessions do not become one with him as the wife is made one flesh with him. Wherefore just as consanguinity is an impediment to marriage or union with the husband according to the flesh, so is one forbidden to marry the husband's wife.
Article 7. Whether affinity in itself admits of degrees?
Objection 1It would seem that affinity in itself admits of degrees. For any kind of propinquity can itself be the subject of degrees. Now affinity is a kind of propinquity. Therefore it has degrees in itself apart from the degrees of consanguinity by which it is caused.
Objection 2Further, it is stated in the text (Sent. iv, D, 41) that the child of a second marriage could not take a consort from within the degrees of affinity of the first husband. But this would not be the case unless the son of an affine were also affine. Therefore affinity like consanguinity admits itself of degrees.
On the contraryAffinity is caused by consanguinity. Therefore all the degrees of affinity are caused by the degrees of consanguinity: and so it has no degrees of itself.
I answer thatA thing does not of itself admit of being divided except in reference to something belonging to it by reason of its genus: thus animal is divided into rational and irrational and not into white and black. Now carnal procreation has a direct relation to consanguinity, because the tie of consanguinity is immediately contracted through it; whereas it has no relation to affinity except through consanguinity which is the latter's cause. Wherefore since the degrees of relationship are distinguished in reference to carnal procreation, the distinction of degrees is directly and immediately referable to consanguinity, and to affinity through consanguinity. Hence the general rule in seeking the degrees of affinity is that in whatever degree of consanguinity I am related to the husband, in that same degree of affinity I am related to the wife.
Reply to Objection 1The degrees in propinquity of relationship can only be taken in reference to ascent and descent of propagation, to which affinity is compared only through consanguinity. Wherefore affinity has no direct degrees, but derives them according to the degrees of consanguinity.
Reply to Objection 2Formerly it used to be said that the son of my affine by a second marriage was affine to me, not directly but accidentally as it were: wherefore he was forbidden to marry on account of the justice of public honesty rather than affinity. And for this reason this prohibition is now revoked.