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We must next consider the impediment of consanguinity. Under this head
there are four points of inquiry:
(1) Whether consanguinity is rightly defined by some?
(2) Whether it is fittingly distinguished by degrees and lines?
(3) Whether certain degrees are by natural law an impediment to marriage?
(4) Whether the impediment degrees can be fixed by the ordinance of the
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Objection 1: It would seem that consanguinity is unsuitably defined by some as
follows: "Consanguinity is the tie contracted between persons descending
from the same common ancestor by carnal procreation." For all men descend
from the same common ancestor, namely Adam, by carnal procreation.
Therefore if the above definition of consanguinity is right, all men
would be related by consanguinity: which is false.
Objection 2: Further, a tie is only between things in accord with one another,
since a tie unites. Now there is not greater accordance between persons
descended from a common ancestor than there is between other men, since
they accord in species but differ in number, just as other men do.
Therefore consanguinity is not a tie.
Objection 3: Further, carnal procreation, according to the Philosopher (De
Gener. Anim. ii, 19), is effected from the surplus food [*Cf. FP, Question ,
Article ]. Now this surplus has more in common with that which is eaten,
since it agrees with it in substance, than with him who eats. Since then
no tie of consanguinity arises between the person born of semen and that
which he eats, neither will there be any tie of kindred between him and
the person of whom he is born by carnal procreation.
Objection 4: Further, Laban said to Jacob (Gn. 29:14): "Thou art my bone and
my flesh," on account of the relationship between them. Therefore such a
kinship should be called flesh-relationship rather than
Objection 5: Further, carnal procreation is common to men and animals. But no
tie of consanguinity is contracted among animals from carnal procreation.
Therefore neither is there among men.
I answer that, According to the Philosopher (Ethic. iii, 11, 12) "all
friendship is based on some kind of fellowship." And since friendship is
a knot or union, it follows that the fellowship which is the cause of
friendship is called "a tie." Wherefore in respect of any kind of a
fellowship certain persons are denominated as though they were tied
together: thus we speak of fellow-citizens who are connected by a common
political life, of fellow-soldiers who are connected by the common
business of soldiering, and in the same way those who are connected by
the fellowship of nature are said to be tied by blood [consanguinei].
Hence in the above definition "tie" is included as being the genus of
consanguinity; the "persons descending from the same common ancestor,"
who are thus tied together are the subject of this tie. while "carnal
procreation" is mentioned as being its origin.
Reply to Objection 1: An active force is not received into an instrument in the
same degree of perfection as it has in the principal agent. And since
every moved mover is an instrument, it follows that the power of the
first mover in a particular genus when drawn out through many mediate
movers fails at length, and reaches something that is moved and not a
mover. But the power of a begetter moves not only as to that which
belongs to the species, but also as to that which belongs to the
individual, by reason of which the child is like the parent even in
accidentals and not only in the specific nature. And yet this individual
power of the father is not so perfect in the son as it was in the father,
and still less so in the grandson, and thus it goes on failing: so that
at length it ceases and can go no further. Since then consanguinity
results from this power being communicated to many through being conveyed
to them from one person by procreation, it destroys itself by little and
little, as Isidore says (Etym. ix). Consequently in defining
consanguinity we must not take a remote common ancestor but the nearest,
whose power still remains in those who are descended from him.
Reply to Objection 2: It is clear from what has been said that blood relations
agree not only in the specific nature but also in that power peculiar to
the individual which is conveyed from one to many: the result being that
sometimes the child is not only like his father, but also his grandfather
or his remote ancestors (De Gener. Anim. iv, 3).
Reply to Objection 3: Likeness depends more on form whereby a thing is actually,
than on matter whereby a thing is potentially: for instance, charcoal has
more in common with fire than with the tree from which the wood was cut.
In like manner food already transformed by the nutritive power into the
substance of the person fed has more in common with the subject nourished
than with that from which the nourishment was taken. The argument however
would hold according to the opinion of those who asserted that the whole
nature of a thing is from its matter and that all forms are accidents:
which is false.
Reply to Objection 4: It is the blood that is proximately changed into the semen,
as proved in De Gener. Anim. i, 18. Hence the tie contracted by carnal
procreation is more fittingly called blood-relationship than
flesh-relationship. That sometimes one relation is called the flesh of
another, is because the blood which is transformed into the man's seed or
into the menstrual fluid is potentially flesh and bone.
Reply to Objection 5: Some say that the reason why the tie of consanguinity is
contracted among men through carnal procreation, and not among other
animals, is because whatever belongs to the truth of human nature in all
men was in our first parent: which does not apply to other animals. But
according to this, matrimonial consanguinity would never come to an end.
However the above theory was disproved in the Second Book (Sent. ii, D,
30: FP, Question , Article ). Wherefore we must reply that the reason for this
is that animals are not united together in the union of friendship
through the begetting of many from one proximate parent, as is the case
with men, as stated above.
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Objection 1: It would seem that consanguinity is unfittingly distinguished by
degrees and lines. For a line of consanguinity is described as "the
ordered series of persons related by blood, and descending from a common
ancestor in various degrees." Now consanguinity is nothing else but a
series of such persons. Therefore a line of consanguinity is the same as
consanguinity. Now a thing ought not to be distinguished by itself.
Therefore consanguinity is not fittingly distinguished into lines.
Objection 2: Further, that by which a common thing is divided should not be
placed in the definition of that common thing. Now descent is placed in
the above definition of consanguinity. Therefore consanguinity cannot be
divided into ascending, descending and collateral lines.
Objection 3: Further, a line is defined as being between two points. But two
points make but one degree. Therefore one line has but one degree, and
for this reason it would seem that consanguinity should not be divided
into lines and degrees.
Objection 4: Further, a degree is defined as "the relation between distant
persons, whereby is known the distance between them." Now since
consanguinity is a kind of propinquity, distance between persons is
opposed to consanguinity rather than a part thereof.
Objection 5: Further, if consanguinity is distinguished and known by its
degrees, those who are in the same degree ought to be equally related.
But this is false since a man's great-uncle and great-nephew are in the
same degree, and yet they are not equally related according to a Decretal
(cap. Porro; cap. Parenteloe, 35, qu. v). Therefore consanguinity is not
rightly divided into degrees.
Objection 6: Further, in ordinary things a different degree results from the
addition of one thing to another, even as every additional unity makes a
different species of number. Yet the addition of one person to another
does not always make a different degree of consanguinity, since father
and uncle are in the same degree of consanguinity, for they are side by
side. Therefore consanguinity is not rightly divided into degrees.
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 (Article ) 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
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
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.
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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
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
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
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
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 (Article , ad 5). Hence the comparison
Reply to Objection 4: It has been shown how consanguinity between married persons
is contrary to the goods of marriage. Hence the Objection proceeds from
Reply to Objection 5: It is not unreasonable for one of two unions to be hindered
by the other, even as where there is identity there is not likeness. In
like manner the tie of consanguinity may hinder the union of marriage.
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Objection 1: It would seem that the degrees of consanguinity that are an
impediment to marriage could not be fixed by the Church so as to reach to
the fourth degree. For it is written (Mt. 19:6): "What God hath joined
together let no man put asunder." But God joined those together who are
married within the fourth degree of consanguinity, since their union is
not forbidden by the Divine law. Therefore they should not be put asunder
by a human law.
Objection 2: Further, matrimony is a sacrament as also is baptism. Now no
ordinance of the Church could prevent one who is baptized from receiving
the baptismal character, if he be capable of receiving it according to
the Divine law. Therefore neither can an ordinance of the Church forbid
marriage between those who are not forbidden to marry by the Divine law.
Objection 3: Further, positive law can neither void nor extend those things
which are natural. Now consanguinity is a natural tie which is in itself
of a nature to impede marriage. Therefore the Church cannot by its
ordinance permit or forbid certain people to marry, any more than she can
make them to be kin or not kin.
Objection 4: Further, an ordinance of positive law should have some reasonable
cause, since it is for this reasonable cause that it proceeds from the
natural law. But the causes that are assigned for the number of degrees
seem altogether unreasonable, since they bear no relation to their
effect; for instance, that consanguinity be an impediment as far as the
fourth degree on account of the four elements as far as the sixth degree
on account of the six ages of the world, as far as the seventh degree on
account of the seven days of which all time is comprised. Therefore
seemingly this prohibition is of no force.
Objection 5: Further, where the cause is the same there should be the same
effect. Now the causes for which consanguinity is an impediment to
marriage are the good of the offspring, the curbing of concupiscence, and
the extension of friendship, as stated above (Article ), which are equally
necessary for all time. Therefore the degrees of consanguinity should
have equally impeded marriage at all times: yet this is not true since
consanguinity is now an impediment to marriage as far as the fourth
degree, whereas formerly it was an impediment as far as the seventh.
Objection 6: Further, one and the same union cannot be a kind of sacrament and
a kind of incest. But this would be the case if the Church had the power
of fixing a different number in the degrees which are an impediment to
marriage. Thus if certain parties related in the fifth degree were
married when that degree was an impediment, their union would be
incestuous, and yet this same union would be a marriage afterwards when
the Church withdrew her prohibition. And the reverse might happen if
certain degrees which were not an impediment were subsequently to be
forbidden by the Church. Therefore seemingly the power of the Church does
not extend to this.
Objection 7: Further, human law should copy the Divine law. Now according to
the Divine law which is contained in the Old Law, the prohibition of
degrees does not apply equally in the ascending and descending lines:
since in the Old Law a man was forbidden to marry his father's sister but
not his brother's daughter. Therefore neither should there remain now a
prohibition in respect of nephews and uncles.
On the contrary, Our Lord said to His disciples (Lk. 10:16): "He that
heareth you heareth Me." Therefore a commandment of the Church has the
same force as a commandment of God. Now the Church sometimes has
forbidden and sometimes allowed certain degrees which the Old Law did not
forbid. Therefore those degrees are an impediment to marriage.
Further, even as of old the marriages of pagans were controlled by the
civil law, so now is marriage controlled by the laws of the Church. Now
formerly the civil law decided which degrees of consanguinity impede
marriage, and which do not. Therefore this can be done now by a
commandment of the Church.
I answer that, The degrees within which consanguinity has been an
impediment to marriage have varied according to various times. For at the
beginning of the human race father and mother alone were debarred from
marrying their children, because then mankind were few in number, and
then it was necessary for the propagation of the human race to be ensured
with very great care, and consequently only such persons were to be
debarred as were unfitted for marriage even in respect of its principal
end which is the good of the offspring, as stated above (Article ).
Afterwards however, the human race having multiplied, more persons were
excluded by the law of Moses, for they already began to curb
concupiscence. Wherefore as Rabbi Moses says (Doc. Perp. iii, 49) all
those persons were debarred from marrying one another who are wont to
live together in one household, because if a lawful carnal intercourse
were possible between them, this would prove a very great incentive to
lust. Yet the Old Law permitted other degrees of consanguinity, in fact
to a certain extent it commanded them; to wit that each man should take a
wife from his kindred, in order to avoid confusion of inheritances:
because at that time the Divine worship was handed down as the
inheritance of the race. But afterwards more degrees were forbidden by
the New Law which is the law of the spirit and of love, because the
worship of God is no longer handed down and spread abroad by a carnal
birth but by a spiritual grace: wherefore it was necessary that men
should be yet more withdrawn from carnal things by devoting themselves to
things spiritual, and that love should have a yet wider play. Hence in
olden times marriage was forbidden even within the more remote degrees of
consanguinity, in order that consanguinity and affinity might be the
sources of a wider natural friendship; and this was reasonably extended
to the seventh degree, both because beyond this it was difficult to have
any recollection of the common stock, and because this was in keeping
with the sevenfold grace of the Holy Ghost. Afterwards, however, towards
these latter times the prohibition of the Church has been restricted to
the fourth degree, because it became useless and dangerous to extend the
prohibition to more remote degrees of consanguinity. Useless, because
charity waxed cold in many hearts so that they had scarcely a greater
bond of friendship with their more remote kindred than with strangers:
and it was dangerous because through the prevalence of concupiscence and
neglect men took no account of so numerous a kindred, and thus the
prohibition of the more remote degrees became for many a snare leading to
damnation. Moreover there is a certain fittingness in the restriction of
the above prohibition to the fourth degree. First because men are wont to
live until the fourth generation, so that consanguinity cannot lapse into
oblivion, wherefore God threatened (Ex. 20:5) to visit the parent's sins
on their children to the third and fourth generation. Secondly, because
in each generation the blood, the identity of which causes consanguinity,
receives a further addition of new blood, and the more another blood is
added the less there is of the old. And because there are four elements,
each of which is the more easily mixed with another, according as it is
more rarefied it follows that at the first admixture the identity of
blood disappears as regards the first element which is most subtle; at
the second admixture, as regards the second element; at the third, as to
the third element; at the fourth, as to the fourth element. Thus after
the fourth generation it is fitting for the carnal union to be repeated.
Reply to Objection 1: Even as God does not join together those who are joined
together against the Divine command, so does He not join together those
who are joined together against the commandment of the Church, which has
the same binding force as a commandment of God.
Reply to Objection 2: Matrimony is not only a sacrament but also fulfills an
office; wherefore it is more subject to the control of the Church's
ministers than baptism which is a sacrament only: because just as human
contracts and offices are controlled by human laws, so are spiritual
contracts and offices controlled by the law of the Church.
Reply to Objection 3: Although the tie of consanguinity is natural, it is not
natural that consanguinity forbid carnal intercourse, except as regards
certain degrees, as stated above (Article ). Wherefore the Church's
commandment does not cause certain people to be kin or not kin, because
they remain equally kin at all times: but it makes carnal intercourse to
be lawful or unlawful at different times for different degrees of
Reply to Objection 4: The reasons assigned are given as indicating aptness and
congruousness rather than causality and necessity.
Reply to Objection 5: The reason for the impediment of consanguinity is not the
same at different times: wherefore that which it was useful to allow at
one time, it was beneficial to forbid at another.
Reply to Objection 6: A commandment does not affect the past but the future.
Wherefore if the fifth degree which is now allowed were to be forbidden
at any time, those in the fifth degree who are married would not have to
separate, because no impediment supervening to marriage can annul it; and
consequently a union which was a marriage from the first would not be
made incestuous by a commandment of the Church. In like manner, if a
degree which is now forbidden were to be allowed, such a union would not
become a marriage on account of the Church's commandment by reason of the
former contract, because they could separate if they wished.
Nevertheless, they could contract anew, and this would be a new union.
Reply to Objection 7: In prohibiting the degrees of consanguinity the Church
considers chiefly the point of view of affection. And since the reason
for affection towards one's brother's son is not less but even greater
than the reasons for affection towards one's father's brother, inasmuch
as the son is more akin to the father than the father to the son (Ethic.
viii, 12), therefore did the Church equally prohibit the degrees of
consanguinity in uncles and nephews. On the other hand the Old Law in
debarring certain persons looked chiefly to the danger of concupiscence
arising from cohabitation; and debarred those persons who were in closer
intimacy with one another on account of their living together. Now it is
more usual for a niece to live with her uncle than an aunt with her
nephew: because a daughter is more identified with her father, being part
of him, whereas a sister is not in this way identified with her brother,
for she is not part of him but is born of the same parent. Hence there
was not the same reason for debarring a niece and an aunt.