Why can’t you marry your sister?

Hugo Grotius changes his mind on the sources of the prohibition of consanguineous marriage

Hugo Grotius says in the introductory chapter of De jure belli ac pacis (first published in 1625) that the Natural Law which binds all human beings is knowable to them through their innate reason, with no need for special revelation from God. In fact this Law follows directly from rational human nature, and therefore God, who made human beings what they are, can no more alter the law post factum than he can make two plus two equal five.¹ But the law can nevertheless be said to be divine, because it is only by God’s providence that human beings were created with reason at all:

Natural law is a dictate of right reason that shows that there is either a moral wrongness or moral necessity in a given act, according as it disagrees or agrees with natural reason itself: and therefore that such an act is either forbidden or commanded by God, the author of nature.²

God has also from time to time given special commandments to human beings by direct revelation. These ‘voluntary’ laws (so called because they are given at will by God) cannot be said to form part of the natural law, because they are not discoverable by reason.³ Furthermore they are binding only on those persons to whom God gave them. In particular the laws of Moses must be understood to have applied to the Jews only, not to the whole of mankind:⁴

A law is not binding on those to whom it was not given. Now this law itself tells us to whom it was given: Hear, O Israel. And throughout it says that the covenant was made with them; that they were called to be God’s own people.

However (Grotius goes on), it is evident from the Bible that some voluntary laws were given by God to Adam, or else to Noah and his sons; and by extension to their descendants, who were all of mankind.⁵ The Jews even have a tradition of spelling these laws out: according to them, the universal voluntary laws prohibit things like murder, anarchy, blasphemy, sexual sins, and theft.

But there are two reasons why the universal voluntary law never played a significant role in Grotius’ system. First, he saw no easy way to determine historically what exactly it covered, and what it did not. Of course the Jews had their list of Noahide laws, but that was nothing more than a tradition, extra-biblical and surely corrupt in some respects. Second, the subjects of the Noahide Law (as enumerated by the Jews and as intimated in certain passages of Genesis itself) are mostly crimes so abhorrent that human reason should have been able to derive the prohibitions thereof independently. It seemed to Grotius that God only gave extra laws to Noah as a reinforcement of Natural Law, necessary perhaps after the wickedness which had provoked the Flood.

Therefore, when he set out the Natural Law in De jure belli Grotius mentioned the Law of Noah only rarely, and referred to the Law of Moses only for the same reason as he did the Roman Law; i.e. as a confirmation or an example of what reason itself could work out.

… With one interesting exception. With admirable frankness, Grotius confessed embarrassment on the topic of incestuous marriage.

The question of marriages between those who are related by blood or close relation is a difficult one, and often disputed with great energy. For if anyone tries to come up with certain reasons why such marriages (forbidden by law and custom) should be prohibited by nature, he will discover in the attempt how hard it is: in fact, he will find that there are no such reasons to be found.⁶

That is, it is difficult to say exactly why certain marriages of close kin should be forbidden by the natural law. Marriages along a vertical line of kinship—between a mother and a son, or a father and daughter—are one thing; it is easy to come up with a reason why they are repugnant to human reason. Unions of parents and children are an obvious perversion of the natural family structure that is common to mankind. The natural role of a parent is to be a protector and nurturer of his or her offspring, and a connubial relationship stands in clear conflict with this:

For neither can the husband, who by law is superior in marriage, give that reverence to his mother which is demanded by nature, nor even a daughter to her father; for albeit she is inferior in marriage, marriage itself demands a relationship that is inconsistent with the reverence required from a daughter.

But what about marriages of brothers and sisters, or of first cousins? Here is where Grotius came up short. It was not obvious to him from reason alone why there should be an obstacle to marriage between two people who are equals. There are plenty of reasons why it might be a bad idea to marry your sister, for instance that it’s wise to widen your alliances by marrying into another family: but should that point of prudence make it illegal? Let’s say you contrived a situation in which it were financially or politically advantageous to marry your sister; say, to keep the family’s money from falling into a fortune-hunting suitor’s hands—should it be legal then?

Faced with this problem, Grotius fell back on something which he was otherwise reluctant to invoke:

As for horizontal degrees of affinity (especially the ones mentioned in Leviticus 18), though we may grant that these prohibitions do not come from the mere law of nature, it can be seen that they were established by an act of God’s will.

That is: the rational law of nature does not prohibit sibling-marriage, but the universal voluntary law does.

Here below (in the JPS translation) is the eighteenth chapter of Leviticus which Grotius cites on this point. It contains laws given to the Children of Israel in the desert, before they go to the Promised Land, which is occupied by Canaanites and other foreign peoples.

And the Lord spoke unto Moses, saying: Speak unto the children of Israel, and say unto them: I am the Lord your God. After the doings of the land of Egypt, wherein ye dwelt, shall ye not do; and after the doings of the land of Canaan, whither I bring you, shall ye not do; neither shall ye walk in their statutes. Mine ordinances shall ye do, and My statutes shall ye keep, to walk therein: I am the Lord your God. Ye shall therefore keep My statutes, and Mine ordinances, which if a man do, he shall live by them: I am the Lord. None of you shall approach to any that is near of kin to him, to uncover their nakedness. I am the Lord. The nakedness of thy father, and the nakedness of thy mother, shalt thou not uncover: she is thy mother; thou shalt not uncover her nakedness. The nakedness of thy father’s wife shalt thou not uncover: it is thy father’s nakedness. The nakedness of thy sister, the daughter of thy father, or the daughter of thy mother, whether born at home, or born abroad, even their nakedness thou shalt not uncover. …

Defile not ye yourselves in any of these things; for in all these the nations are defiled, which I cast out from before you. And the land was defiled, therefore I did visit the iniquity thereof upon it, and the land vomited out her inhabitants. Ye therefore shall keep My statutes and Mine ordinances, and shall not do any of these abominations; neither the home-born, nor the stranger that sojourneth among you—for all these abominations have the men of the land done, that were before you, and the land is defiled— that the land vomit not you out also, when ye defile it, as it vomited out the nation that was before you. For whosoever shall do any of these abominations, even the souls that do them shall be cut off from among their people. Therefore shall ye keep My charge, that ye do not any of these abominable customs, which were done before you, and that ye defile not yourselves therein: I am the Lord your God.

Grotius reasons as follows about these laws:

That these laws did not bind the Jews only, but rather all mankind, may be gathered from the words of God as written by Moses: Defile not ye yourselves in any of these things: for in all these the nations are defiled which I cast out before you: And the land is defiled. And: For all these abominations have the men of the land done, which were before you, and the land is defiled. For if the Canaanites and their neighbours sinned by doing such things, it follows that they were bound by as some pre-existing law. And because this law was not a merely natural law, the only remaining possibility is that it was given by God; whether to the Canaanites in particular (quite unlikely, and unsupported by the text) or else to the human race at their first creation, or else in their restoration after the Flood.

That is, we may infer a special divine prohibition of these sins from the very fact that the Canaanites who committed them were punished with exile from their land to make way for the conquering Children of Israel. That the prohibition was universal may be inferred from the fact that God, by all accounts, did not reveal it specially to the Canaanites at any time since the Flood. They were just supposed to know about it, even though there is no way they could have discovered it by reason. It may be deduced from all this that, although the Natural Law says nothing about horizontal consanguinity, the prohibition thereof is no less binding on all of humanity.

I’m reluctant to get into the gratuitous and easily solvable question of whether Grotius was somehow a “secular” thinker (by which is apparently meant that he sought to divorce the Bible from his legal thought).⁷ But let this unusual twist in the argument of De jure belli serve as a reminder, as if any were needed, that the world for Grotius was still ‘full of God’. And the Bible was nothing other than a true record of God’s dealings with mankind, from the Creation down to Jesus Christ. Just because he did not accept the Old Testament or the Gospel as straightforward sources of Natural or International Law (for reasons which he took care to explain as clearly as possible), Grotius did not deem these texts to be in any respect less divine, or less expressive of God’s demands on human beings. Finally, the Natural Law and Law of Nations that are outlined systematically in De jure belli ac pacis do not exhaust the laws that Grotius considered to be binding on humankind, whether on nations or individuals. They exist alongside other laws of equal divinity and importance.

A more interesting point is that Grotius’s resort to the voluntary law of God was the result of a recent innovation in his thought on the matter. In an earlier minor treatise of ca. 1613, Grotius had solved the problem of horizontal consanguinity in a different and more rudimentary way, by simply declaring no reasons for the prohibition to be necessary. In what seems to be an earlier draft of one of the paragraphs quoted above, he wrote:⁸

Nothing prevents the principle that a reason cannot be given for everything which is settled on from being extended to certain precepts of the Natural Law, especially if a reason is sought such that the precepts can be deduced from it by necessary consequence. Take for example the sexual union of persons who are related by blood or affinity: even though we do not have a written law of God declaring such a relation to be illicit, still we must know that it is so; for the Roman jurists say that there is such a thing as incest by natural law, and the Apostle Paul says that such a crime is unheard of among the nations. [1 Cor. 5.1] But if anyone wishes to supply a reason for this precept, he will not easily find one to which nothing can be opposed, or one which is equally certain and evident as the precept itself. But indeed what need is there to look closely into these causes, inasmuch as they are to be referred to the judgement of God? [Rom. 2.15.]⁹

Evidently by 1625, when De jure belli was published, Grotius was no longer satisfied by this hand-waving approach to the problem. Instead of declaring the prohibition of consanguinity to be a part of the Natural Law, for reasons ineffable; he removed the precept altogether from the province of reason, and gave it a new home among the primordial laws that were given directly by God to the children of Noah.

1. §1.1.10. Here and below I give my own translations from the Latin original.

2. §1.1.10.

3. §1.1.10, 15.

4. §1.1.16.

5. §1.2.5.

6. §2.5.12.

7. Cf. chapter V of that amazing work of Mark Somos, Secularisation and the Leiden Circle (Leiden / Boston: Brill, 2011), pp. 383–437. He proves his case by suggesting that Grotius’s numerous invocations of the Bible in his works are so perverse and badly argued that they are not to be taken at face value, and are instead evidence for Grotius’ disdain for Biblical Law as applied to the Law of Nations. We read on p. 436: ‘In conclusion, let me restate once more that I do not argue that Grotius did not believe in God, or that he set out in IPC to undermine the Bible. Instead, he presented an unbroken string of forced interpretations with shocking implications for just war theory. He did this in order to show that the Bible should not be used in international law at all.’

8. Hugonis Grotii Defensio Capitis quinti Maris liberi oppugnati a Guillelmo Welwodo Iuris Civilis professore XXVII ejus libri scripti Anglico sermone cui titulum fecit Compendium legum Maritimarum. Printed as Appendix C of Samuel Muller, Mare clausum. Bijdrage tot de geschiedenis der rivaliteit van Engeland en Nederland in de zeventiende eeuw (Amsterdam: Frederik Muller, 1872).

9. P. 347: ‘Nam quod dicitur non omnium quae constituta sunt rationem reddi posse, id ad quaedam etiam iuris gentium praecepta quominus extendatur, quid obstet non video, praesertim si ratio quaeratur talis ex qua per necessarium consecutionem praecepta emanent. Exemplum sit in concubitu personarum quae sanguine vel affinitate proxime jungantur; esse talem conjunctionem illicitam etiamsi legem Dei scriptam non haberemus, ignorare tamen minime liceret; cum Romani iurisconsulti aliquod esse dicant incestum iure gentium [L. ult. ff. de Ritu nupt. (l. 69. D. XXIII. 2.)] et Paulus Apostolus tale facinus dicat „οὐδὲ ἐν τοῖς ἔθνεσιν ὀνομάζεσθαι“. [1 Cor. V. vs. 1] Rationem autem praecepti huius si quis velit reddere, haud facile talem inveniet cui nihil posset opponi, aut aeque certam et evidentem atque est ipsum praeceptum. Et certe quid causas scrutari opus est, cum referenda haec sint „πρὸς τὰ δικαιώματα τοῦ θεοῦ“. [Rom. 1. (vs. 32.)]’