1. Property is a means to the preservation of human life. And because natural life cannot be preserved in one and the same person of the father living on for all time, nature arranges for its preservation by the son succeeding his father in likeness of species: wherefore it is appropriate that the son should succeed his father in his property. It is natural therefore that the father's interest in his son should continue to the end of his life, and that father and mother should dwell together to the end.*
2. Woman is taken into partnership with man for the need of childbearing: therefore when the fertility and beauty of woman ceases, there is a bar against her being taken up by another man. If then a man, taking a woman to wife in the time of her youth, when beauty and fertility wait upon her, could send her away when she was advanced in years, he would do the woman harm, contrary to natural equity.
3. It is manifestly absurd for the woman to be able to send away the man, seeing that woman is naturally subject to the rule of man, and it is not in the power of a subject to run away from control. It being then against the order of nature for the woman to be allowed to desert the man, if the man were allowed to desert the woman, the partnership of man and woman would not be on fair terms, but would be a sort of slavery on the woman's side.
4. Men show a natural anxiety to be sure of their own offspring; and whatever stands in the way of that assurance runs counter to the natural instinct of the race. But if the man could send away the woman, or the woman the man, and form a connexion with another, certainty as to parentage would be difficult, when a woman had intercourse first with one man and then with another.
5. The greater the love, the more need for it to be firm and lasting. But the love of man and woman is counted strongest of all; seeing that they are united, not only in the union of the sexes, which even among beasts makes a sweet partnership, but also for the sharing in common of all domestic life, as a sign whereof a man leaves even father and mother for the sake of his wife (Gen. ii, 24). It is fitting therefore for marriage to be quite indissoluble.
6. Of natural acts, generation alone is directed to the good of (the specific) nature: for eating and the separation from the body of other excretions concern the individual, but generation has to do with the preservation of the species. Hence, as law is instituted for the common good, the function of procreation ought to be regulated by laws divine and human. Now the laws laid down ought to proceed on the basis of the dictate of nature (ex naturali instinctu), if they are human laws, as in the exact sciences every human discovery takes its origin from principles naturally known: but if they are divine laws, they not only develop the dictate of nature, but also make up the deficiency of what nature dictates, as dogmas divinely revealed surpass the capacity of natural reason. Since then there is in the human species a natural exigency for the union of male and female to be one and indivisible, such unity and indissolubility must needs be ordained by human law. To that ordinance the divine law adds a supernatural reason, derived from the significancy of marriage as a type of the inseparable union of Christ with His Church, which is one as He is one.* Thus then irregularities in the act of generation are not only contrary to the dictate of nature, but are also transgressions of laws divine and human:* hence on this account any irregular behaviour in this matter is even a greater sin than in the matter of taking food or the like. But since all other factors in human life should be subordinate to that which is the best thing in man, it follows that the union of male and female must be regulated by law, not from the mere point of view of procreation, as in other animals, but also with an eye to good manners, or manners conformable to right reason, as well for man as an individual, as also for man as a member of a household or family, or again as a member of civil society. Thus understood, good manners involve the indissolubility of the union of male and female: for they will love one another with greater fidelity, when they know that they are indissolubly united: each partner will take greater care of the things of the house, reflecting that they are to remain permanently in possession of the same things: occasions of quarrels are removed, that might otherwise arise between the husband and the wife's relations, if the husband were to divorce his wife; and thus affinity becomes a firmer bond of amity: also occasions of adultery are cut off, occasions which would readily offer themselves, if husband could divorce his wife, or wife her husband.*
Hence it is said: But I say to you that whoever putteth away his wife, except for fornication, and marrieth another, committeth adultery; and he that marrieth her that is put away, committeth adultery (Matt. xix, 9): But to them that are united in marriage, it is not I that give commandment, but the Lord, that the wife depart not from her husband (1 Cor. vii, 10).
Divorce was reckoned an impropriety also among the ancient Romans, of whom Valerius Maximus (De memor. dictis, II, 1) relates that they believed that the marriage tie ought not to be broken off even for barrenness.*
Hereby the custom is banned of putting away wives, which however in the Old Law was permitted to the Jews for their hardness of heart, because they were prone to the killing of their wives: so the less evil was permitted to keep out the greater.
3.122 : Of the reason for which Simple Fornication is a Sin by Divine Law, and of the Natural Institution of Marriage
3.124 : That Marriage ought to be between one Man and one Woman