A. Descriptions of Certain Names

9. As to the first, let this be the first description: consanguinity is a tie between persons descending by propagation from the same physical person.

10. Second: a person from whom several descend by physical propagation is called a stem.

11. Third: a line of consanguinity is an ordered collection of persons joined by consanguinity.

12. Fourth: this line is divided into ascending and descending and transverse [lateral] lines. A descending line goes from the propagating person to the propagated persons; an ascending line conversely goes from a propagated person to those from whom he descended. And although there is the same line for ascenders as for descenders, like the road from Athens to Thebes and conversely [Ord. d.27 n.4; d.13 n.68], yet they are different in ascending, as father, grandfather, great grandfather, etc., and different in descending, as son, grandson etc. Transverse is when both persons descend from the same person but neither from the other.

13. Fifth: a degree is a determinate closeness of person to person or a closeness coming from physical propagation. And a degree properly is found in an ascending and descending line, because there properly is there superior and inferior as to position; but it is less proper in a transverse line. However, by taking degree generally, although he who is close to another in transverse line is not properly inferior to him (for he does not descend from him), yet he is under someone else who is of the same position or degree as he; and so in a transverse line degrees are spoken of insofar as this one is under someone who is of the same position as himself, namely when in a like degree this one is in this line in some sort of degree and the other in that one.

14. First rule: in a straight line there are as many degrees as there are persons, minus one. The proof of this is that there are as many degrees as there are propagations, since a degree is a relation or closeness between person and person; and the persons are one more than the propagations, because the person presupposed to the whole collection is not propagated in it. Hence if Enoch was the seventh from Adam [Genesis 5.1-18], there are only six generations from Adam to Enoch, because Adam is set down as ungenerated.

15. Second rule: in a transverse line the degrees are computed according to greater remoteness from the stem, Gregory IX, Decretals IV tit.14 ch.9, ‘On consanguinity’, “According to the approved rule, by what number of degrees a man who is from the stem is more remote in distance from the stem, by that number is he also remote from anyone else in another descending line.” And the reason for this is that persons who are in a transverse line do not have closeness with each other save because they are in the stem or back to the stem; and therefore they cannot be more closely joined to each other than the more remote of them may be united to the stem.

16. As to the issue at hand there is also a rule that a degree in a descending line is stronger than in a transverse line. And the natural reason is that offspring are more joined to the parent than offspring to offspring; and therefore it is more against the law of nature to be conjoined in the first degree in a direct line than in a transverse line; for that conjunction was always against matrimony, and more the conjunction of son with mother than of father with daughter, because there is a greater irreverence.

17. Hence in History of Animals 9.47.631a1-7 Aristotle says that, after a blindfolded horse knew its mother and later discovered the fact, it threw itself down headlong; and it is read elsewhere about an elephant [ibid. 630b31-631a1, cited in Richard of Middleton, Sent. IV d.40 q.2], that was engineered into knowing its mother, and afterwards, when it perceived the fact, killed the one who engineered it. From which it is apparent that this is also against the law of nature, as it belongs to the brutes; hence a degree descending by propagation, and most especially of mother to son, impedes matrimony most of all.