Objection 7: Further, if two persons be akin to one another there is always the same measure of kinship between them, since the distance from one extreme to the other is the same either way. Yet the degrees of consanguinity are not always the same on either side, since sometimes one relative is in the third and the other in the fourth degree. Therefore the measure of consanguinity cannot be sufficiently known by its degrees.
I answer that, Consanguinity as stated (A[1]) is a certain propinquity based on the natural communication by the act of procreation whereby nature is propagated. Wherefore according to the Philosopher (Ethic. viii, 12) this communication is threefold. one corresponds to the relationship between cause and effect, and this is the consanguinity of father to son, wherefore he says that "parents love their children as being a part of themselves." Another corresponds to the relation of effect to cause, and this is the consanguinity of son to father, wherefore he says that "children love their parents as being themselves something which owes its existence to them." The third corresponds to the mutual relation between things that come from the same cause, as brothers, "who are born of the same parents," as he again says (Ethic. viii, 12). And since the movement of a point makes a line, and since a father by procreation may be said to descend to his son, hence it is that corresponding to these three relationships there are three lines of consanguinity, namely the "descending" line corresponding to the first relationship, the "ascending" line corresponding to the second, and the "collateral" line corresponding to the third. Since however the movement of propagation does not rest in one term but continues beyond, the result is that one can point to the father's father and to the son's son, and so on, and according to the various steps we take we find various degrees in one line. And seeing that the degrees of a thing are parts of that thing, there cannot be degrees of propinquity where there is no propinquity. Consequently identity and too great a distance do away with degrees of consanguinity; since no man is kin to himself any more than he is like himself: for which reason there is no degree of consanguinity where there is but one person, but only when one person is compared to another.
Nevertheless there are different ways of counting the degrees in various lines. For the degree of consanguinity in the ascending and descending line is contracted from the fact that one of the parties whose consanguinity is in question, is descended from the other. Wherefore according to the canonical as well as the legal reckoning, the person who occupies the first place, whether in the ascending or in the descending line, is distant from a certain one, say Peter, in the first degree---for instance father and son; while the one who occupies the second place in either direction is distant in the second degree, for instance grandfather, grandson and so on. But the consanguinity that exists between persons who are in collateral lines is contracted not through one being descended from the other, but through both being descended from one: wherefore the degrees of consanguinity in this line must be reckoned in relation to the one principle whence it arises. Here, however, the canonical and legal reckonings differ: for the legal reckoning takes into account the descent from the common stock on both sides, whereas the canonical reckoning takes into account only one, that namely on which the greater number of degrees are found. Hence according to the legal reckoning brother and sister, or two brothers, are related in the second degree, because each is separated from the common stock by one degree; and in like manner the children of two brothers are distant from one another in the fourth degree. But according to the canonical reckoning, two brothers are related in the first degree, since neither is distant more than one degree from the common stock: but the children of one brother are distant in the second degree from the other brother, because they are at that distance from the common stock. Hence, according to the canonical reckoning, by whatever degree a person is distant from some higher degree, by so much and never by less is he distant from each person descending from that degree, because "the cause of a thing being so is yet more so." Wherefore although the other descendants from the common stock be related to some person on account of his being descended from the common stock, these descendants of the other branch cannot be more nearly related to him than he is to the common stock. Sometimes, however, a person is more distantly related to a descendant from the common stock, than he himself is to the common stock, because this other person may be more distantly related to the common stock than he is: and consanguinity must be reckoned according to the more distant degree.
Reply to Objection 1: This objection is based on a false premise: for consanguinity is not the series but a mutual relationship existing between certain persons, the series of whom forms a line of consanguinity.
Reply to Objection 2: Descent taken in a general sense attaches to every line of consanguinity, because carnal procreation whence the tie of consanguinity arises is a kind of descent: but it is a particular kind of descent, namely from the person whose consanguinity is in question, that makes the descending line.
Reply to Objection 3: A line may be taken in two ways. Sometimes it is taken properly for the dimension itself that is the first species of continuous quantity: and thus a straight line contains actually but two points which terminate it, but infinite points potentially, any one of which being actually designated, the line is divided, and becomes two lines. But sometimes a line designates things which are arranged in a line, and thus we have line and figure in numbers, in so far as unity added to unity involves number. Thus every unity added makes a degree in a particular line: and it is the same with the line of consanguinity: wherefore one line contains several degrees.
Reply to Objection 4: Even as there cannot be likeness without a difference, so there is no propinquity without distance. Hence not every distance is opposed to consanguinity, but such as excludes the propinquity of blood-relationship.
Reply to Objection 5: Even as whiteness is said to be greater in two ways, in one way through intensity of the quality itself, in another way through the quantity of the surface, so consanguinity is said to be greater or lesser in two ways. First, intensively by reason of the very nature of consanguinity: secondly, extensively as it were, and thus the degree of consanguinity is measured by the persons between whom there is the propagation of a common blood, and in this way the degrees of consanguinity are distinguished. Wherefore it happens that of two persons related to one person in the same degree of consanguinity, one is more akin to him than the other, if we consider the quantity of consanguinity in the first way: thus a man's father and brother are related to him in the first degree of consanguinity, because in neither case does any person come in between; and yet from the point of view of intensity a man's father is more closely related to him than his brother, since his brother is related to him only because he is of the same father. Hence the nearer a person is to the common ancestor from whom the consanguinity descends, the greater is his consanguinity although he be not in a nearer degree. In this way a man's great-uncle is more closely related to him than his great-nephew, although they are in the same degree.
Reply to Objection 6: Although a man's father and uncle are in the same degree in respect of the root of consanguinity, since both are separated by one degree from the grandfather, nevertheless in respect of the person whose consanguinity is in question, they are not in the same degree, since the father is in the first degree, whereas the uncle cannot be nearer than the second degree, wherein the grandfather stands.
Reply to Objection 7: Two persons are always related in the same degree to one another, although they are not always distant in the same number of degrees from the common ancestor, as explained above.
Whether consanguinity is an impediment to marriage by virtue of the natural law?
Objection 1: It would seem that consanguinity is not by natural law an impediment to marriage. For no woman can be more akin to a man than Eve was to Adam, since of her did he say ( Gn. 2:23 ): "This now is bone of my bones and flesh of my flesh." Yet Eve was joined in marriage to Adam. Therefore as regards the natural law no consanguinity is an impediment to marriage.
Objection 2: Further, the natural law is the same for all. Now among the uncivilized nations no person is debarred from marriage by reason of consanguinity. Therefore, as regards the law of nature, consanguinity is no impediment to marriage.
Objection 3: Further, the natural law is what "nature has taught all animals," as stated at the beginning of the Digests (i, ff. De just. et jure). Now brute animals copulate even with their mother. Therefore it is not of natural law that certain persons are debarred from marriage on account of consanguinity.
Objection 4: Further, nothing that is not contrary to one of the goods of matrimony is an impediment to marriage. But consanguinity is not contrary to any of the goods of marriage. Therefore it is not an impediment thereto.
Objection 5: Further, things which are more akin and more similar to one another are better and more firmly united together. Now matrimony is a kind of union. Since then consanguinity is a kind of kinship, it does not hinder marriage but rather strengthens the union.
On the contrary, According to the natural law whatever is an obstacle to the good of the offspring is an impediment to marriage. Now consanguinity hinders the good of the offspring, because in the words of Gregory (Regist., epis. xxxi) quoted in the text (Sent. iv, D, 40): "We have learnt by experience that the children of such a union cannot thrive." Therefore according to the law of nature consanguinity is an impediment to matrimony.
Further, that which belongs to human nature when it was first created is of natural law. Now it belonged to human nature from when it was first created that one should be debarred from marrying one's father or mother: in proof of which it was said ( Gn. 2:24 ): "Wherefore a man shall leave father and mother": which cannot be understood of cohabitation, and consequently must refer to the union of marriage. Therefore consanguinity is an impediment to marriage according to the natural law.
I answer that, In relation to marriage a thing is said to be contrary to the natural law if it prevents marriage from reaching the end for which it was instituted. Now the essential and primary end of marriage is the good of the offspring. and this is hindered by a certain consanguinity, namely that which is between father and daughter, or son and mother. It is not that the good of the offspring is utterly destroyed, since a daughter can have a child of her father's semen and with the father rear and teach that child in which things the good of the offspring consists, but that it is not effected in a becoming way. For it is out of order that a daughter be mated to her father in marriage for the purpose of begetting and rearing children, since in all things she ought to be subject to her father as proceeding from him. Hence by natural law a father and mother are debarred from marrying their children; and the mother still more than the father, since it is more derogatory to the reverence due to parents if the son marry his mother than if the father marry his daughter; since the wife should be to a certain extent subject to her husband. The secondary essential end of marriage is the curbing of concupiscence; and this end would be forfeit if a man could marry any blood-relation, since a wide scope would be afforded to concupiscence if those who have to live together in the same house were not forbidden to be mated in the flesh. Wherefore the Divine law debars from marriage not only father and mother, but also other kinsfolk who have to live in close intimacy with one another and ought to safeguard one another's modesty. The Divine law assigns this reason ( Lev. 18:10 ): "Thou shalt not uncover the nakedness" of such and such a one, "because it is thy own nakedness."
But the accidental end of marriage is the binding together of mankind and the extension of friendship: for a husband regards his wife's kindred as his own. Hence it would be prejudicial to this extension of friendship if a man could take a woman of his kindred to wife since no new friendship would accrue to anyone from such a marriage. Wherefore, according to human law and the ordinances of the Church, several degrees of consanguinity are debarred from marriage.
Accordingly it is clear from what has been said that consanguinity is by natural law an impediment to marriage in regard to certain persons, by Divine law in respect of some, and by human law in respect of others.
Reply to Objection 1: Although Eve was formed from Adam she was not Adam's daughter, because she was not formed from him after the manner in which it is natural for a man to beget his like in species, but by the Divine operation, since from Adam's rib a horse might have been formed in the same way as Eve was. Hence the natural connection between Eve and Adam was not so great as between daughter and father, nor was Adam the natural principle of Eve as a father is of his daughter.
Reply to Objection 2: That certain barbarians are united carnally to their parents does not come from the natural law but from the passion of concupiscence which has clouded the natural law in them.
Reply to Objection 3: Union of male and female is said to be of natural law, because nature has taught this to animals: yet she has taught this union to various animals in various ways according to their various conditions. But carnal copulation with parents is derogatory to the reverence due to them. For just as nature has instilled into parents solicitude in providing for their offspring, so has it instilled into the offspring reverence towards their parents: yet to no kind of animal save man has she instilled a lasting solicitude for his children or reverence for parents; but to other animals more or less, according as the offspring is more or less necessary to its parents, or the parents to their offspring. Hence as the Philosopher attests (De Animal. ix, 47) concerning the camel and the horse, among certain animals the son abhors copulation with its mother as long as he retains knowledge of her and a certain reverence for her. And since all honest customs of animals are united together in man naturally, and more perfectly than in other animals, it follows that man naturally abhors carnal knowledge not only of his mother, but also of his daughter, which is, however, less against nature, as stated above.
Moreover consanguinity does not result from carnal procreation in other animals as in man, as stated above (A[1], ad 5). Hence the comparison fails.