B. Solution of the Question

18. About the second article [n.8] I say that in every law some closeness has been an impediment.

In the law of nature, as was said [n.16], only the closeness in the first degree in the descending line, from the word of Adam [Genesis 2.24], “a man will leave father and mother” (supply: as to conjugal union). Nor is this understood only of the immediate father, but of anyone in the direct line, so that if Adam were alive today, he could not marry another wife [sc. in the direct line]; perhaps too after the multiplication of the human race, if it had stood in its innocence, there would have been prohibition of certain further degrees; but at the beginning it could not be so, because there were no other women than sisters.

19. In the Mosaic Law there was prohibition as to certain further degrees, as is plain in Leviticus 18.8-18.

20. And in the Gospel Law there was at some time prohibition up to the seventh degree, as is plain from the Master in the text [Lombard, Sent. IV d.40 ch.1 n.2], but under Innocent III it was restricted to the fourth degree inclusively, Gregory IX, Decretals IV tit.14 ch.8, ‘Of consanguinity’, “Let the prohibition of conjugal union for the rest not exceed the fourth degree of consanguinity;” and it assigns the reason, “since in the further degrees it cannot now be observed without grave cost;” and afterwards it approves and ratifies it for the future, “Since, therefore,” it says, “prohibition of conjugal union is already extended up to the fourth degree (namely by us here in the Council [Lateran Council IV, 1215],” and later, “thus do we wish it to be perpetual, notwithstanding the Constitutions published some time before this, whether by others or by us.” It also revokes, in the same chapter, all “prohibitions about contracting marriage in the second and third degree of affinity, and about offspring got from second nuptials not being permitted to unite with a relative of the first father - which prohibitions brought in many difficulties and,” as it says, “sometimes the peril of souls.”

21. But whence is it that such or such closeness impedes matrimony simply?

I reply: in the Gospel Law no other prohibition about this from Christ is found beyond the prohibition of the law of nature; nor did Christ explicitly confirm the prohibition about this made in the Mosaic Law. But the Church has sometimes delegitimized persons in remoter degrees, but later in the fourth degree; and the reason for delegitimization [sc. up to the fourth degree and not further] was for preserving peace and friendship in the Church; for up to the fourth degree there does remain friendship by reason of consanguinity, and from then on, as it were, people begin to be extraneous and to weaken in love; and therefore was it fitting to call back the weakening friendship by another bond, namely the conjugal bond.