Supplementum, Question 65 — Summa Theologiae
Article 1. Whether it is against the natural law to have several wives?
Objection 1. It would seem that it is not against the natural law to have several wives. For custom does not prejudice the law of nature. But “it was not a sin” to have several wives “when this was the custom,” according to Augustine (De Bono Conjug. xv) as quoted in the text (Sent. iv, D, 33). Therefore it is not contrary to the natural law to have several wives.
Objection 2. Further, whoever acts in opposition to the natural law, disobeys a commandment, for the law of nature has its commandments even as the written law has. Now Augustine says (De Bono Conjug. xv; De Civ. Dei xv, 38) that “it was not contrary to a commandment” to have several wives, “because by no law was it forbidden.” Therefore it is not against the natural law to have several wives.
Objection 3. Further, marriage is chiefly directed to the begetting of offspring. But one man may get children of several women, by causing them to be pregnant. Therefore It is not against the natural law to have several wives.
Objection 4. Further, “Natural right is that which nature has taught all animals,” as stated at the beginning of the Digests (1, i, ff. De just. et jure). Now nature has not taught all animals that one male should be united to but one female, since with many animals the one male is united to several females. Therefore it is not against the natural law to have several wives.
Objection 5. Further, according to the Philosopher (De Gener. Animal. i, 20), in the begetting of offspring the male is to the female as agent to patient, and as the craftsman is to his material. But it is not against the order of nature for one agent to act on several patients, or for one craftsman to work in several materials. Therefore neither is it contrary to the law of nature for one husband to have many wives.
Objection 6. On the contrary, That which was instilled into man at the formation of human nature would seem especially to belong to the natural law. Now it was instilled into him at the very formation of human nature that one man should have one wife, according to Genesis 2:24, “They shall be two in one flesh.” Therefore it is of natural law.
Objection 7. Further, it is contrary to the law of nature that man should bind himself to the impossible, and that what is given to one should be given to another. Now when a man contracts with a wife, he gives her the power of his body, so that he is bound to pay her the debt when she asks. Therefore it is against the law of nature that he should afterwards give the power of his body to another, because it would be impossible for him to pay both were both to ask at the same time.
Objection 8. Further, “Do not to another what thou wouldst not were done to thyself” [Cf. Tobit 4:16 is a precept of the natural law. But a husband would by no means be willing for his wife to have another husband. Therefore he would be acting against the law of nature, were he to have another wife in addition.
Objection 9. Further, whatever is against the natural desire is contrary to the natural law. Now a husband’s jealousy of his wife and the wife’s jealousy of her husband are natural, for they are found in all. Therefore, since jealousy is “love impatient of sharing the beloved,” it would seem to be contrary to the natural law that several wives should share one husband.
I answer that, All natural things are imbued with certain principles whereby they are enabled not only to exercise their proper actions, but also to render those actions proportionate to their end, whether such actions belong to a thing by virtue of its generic nature, or by virtue of its specific nature: thus it belongs to a magnet to be borne downwards by virtue of its generic nature, and to attract iron by virtue of its specific nature. Now just as in those things which act from natural necessity the principle of action is the form itself, whence their proper actions proceed proportionately to their end, so in things which are endowed with knowledge the principles of action are knowledge and appetite. Hence in the cognitive power there needs to be a natural concept, and in the appetitive power a natural inclination, whereby the action befitting the genus or species is rendered proportionate to the end. Now since man, of all animals, knows the aspect of the end, and the proportion of the action to the end, it follows that he is imbued with a natural concept, whereby he is directed to act in a befitting manner, and this is called “the natural law” or “the natural right,” but in other animals “the natural instinct.” For brutes are rather impelled by the force of nature to do befitting actions, than guided to act on their own judgment. Therefore the natural law is nothing else than a concept naturally instilled into man, whereby he is guided to act in a befitting manner in his proper actions, whether they are competent to him by virtue of his generic nature, as, for instance, to beget, to eat, and so on, or belong to him by virtue of his specific nature, as, for instance, to reason and so forth. Now whatever renders an action improportionate to the end which nature intends to obtain by a certain work is said to be contrary to the natural law. But an action may be improportionate either to the principal or to the secondary end, and in either case this happens in two ways. First, on account of something which wholly hinders the end; for instance a very great excess or a very great deficiency in eating hinders both the health of the body, which is the principal end of food, and aptitude for conducting business, which is its secondary end. Secondly, on account of something that renders the attainment of the principal or secondary end difficult, or less satisfactory, for instance eating inordinately in respect of undue time. Accordingly if an action be improportionate to the end, through altogether hindering the principal end directly, it is forbidden by the first precepts of the natural law, which hold the same place in practical matters, as the general concepts of the mind in speculative matters. If, however, it be in any way improportionate to the secondary end, or again to the principal end, as rendering its attainment difficult or less satisfactory, it is forbidden, not indeed by the first precepts of the natural law, but by the second which are derived from the first even as conclusions in speculative matters receive our assent by virtue of self-known principles: and thus the act in question is said to be against the law of nature.