Thomas Aquinas

“ 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. ”

“ 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 [4975] (A [3] ) . ”

“ 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. ”

“ 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 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and spread abroad by a carnal birth but by a spiritual grace: wherefore it was necessary that men should be yet more withdrawn from carnal things by devoting themselves to things spiritual, and that love should have a yet wider play.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 ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ Therefore such a kinship should be called flesh-relationship rather than blood-relationship [consanguinitas] .
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." ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”