Why is incest wrong?
Posted: Tue Dec 30, 2025 8:19 am
Title says all.
Of course I accept the Divine law, I'm only looking for reasons
I tried to search by myself, in the Summa.
https://www.newadvent.org/summa/3154.htm#article9
I can conceive of a man who remains respectful, has many friends, wants to marry a relative who is not living nearby, and would not love each other too ardently - such that his reasons would not apply.
Of course I accept the Divine law, I'm only looking for reasons
I tried to search by myself, in the Summa.
https://www.newadvent.org/summa/3154.htm#article9
While this makes the case that incest is risky or that non-incestuous marriages are better, it does not establish them as absolute evil under the natural law.Now sexual intercourse with women related by consanguinity or affinity is unbecoming to venereal union on three counts.
First, because man naturally owes a certain respect to his parents and therefore to his other blood relations, who are descended in near degree from the same parents: so much so indeed that among the ancients, as Valerius Maximus relates [Dict. Fact. Memor. ii, 1, it was not deemed right for a son to bathe with his father, lest they should see one another naked. Now from what has been said (II-II:142:4; II-II:151:4), it is evident that in venereal acts there is a certain shamefulness inconsistent with respect, wherefore men are ashamed of them. Wherefore it is unseemly that such persons should be united in venereal intercourse. This reason seems to be indicated (Leviticus 18:7) where we read: "She is thy mother, thou shalt not uncover her nakedness," and the same is expressed further on with regard to others.
The second reason is because blood relations must needs live in close touch with one another. Wherefore if they were not debarred from venereal union, opportunities of venereal intercourse would be very frequent and thus men's minds would be enervated by lust. Hence in the Old Law [Leviticus 18] the prohibition was apparently directed specially to those persons who must needs live together.
The third reason is, because this would hinder a man from having many friends: since through a man taking a stranger to wife, all his wife's relations are united to him by a special kind of friendship, as though they were of the same blood as himself. Wherefore Augustine says (De Civ. Dei xv, 16): "The demands of charity are most perfectly satisfied by men uniting together in the bonds that the various ties of friendship require, so that they may live together in a useful and becoming amity; nor should one man have many relationships in one, but each should have one."
Aristotle adds another reason (2 Polit. ii): for since it is natural that a man should have a liking for a woman of his kindred, if to this be added the love that has its origin in venereal intercourse, his love would be too ardent and would become a very great incentive to lust: and this is contrary to chastity. Hence it is evident that incest is a determinate species of lust.
I can conceive of a man who remains respectful, has many friends, wants to marry a relative who is not living nearby, and would not love each other too ardently - such that his reasons would not apply.