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.
Now marriage has for its principal end the begetting and rearing of children, and this end is competent to man according to his generic nature, wherefore it is common to other animals (Ethic. viii, 12), and thus it is that the "offspring" is assigned as a marriage good. But for its secondary end, as the Philosopher says (Ethic. viii, 12), it has, among men alone, the community of works that are a necessity of life, as stated above (Question [41], Article [1]). And in reference to this they owe one another "fidelity" which is one of the goods of marriage. Furthermore it has another end, as regards marriage between believers, namely the signification of Christ and the Church: and thus the "sacrament" is said to be a marriage good. Wherefore the first end corresponds to the marriage of man inasmuch as he is an animal: the second, inasmuch as he is a man; the third, inasmuch as he is a believer. Accordingly plurality of wives neither wholly destroys nor in any way hinders the first end of marriage, since one man is sufficient to get children of several wives, and to rear the children born of them. But though it does not wholly destroy the second end, it hinders it considerably for there cannot easily be peace in a family where several wives are joined to one husband, since one husband cannot suffice to satisfy the requisitions of several wives, and again because the sharing of several in one occupation is a cause of strife: thus "potters quarrel with one another" [*Aristotle, Rhet. ii, 4], and in like manner the several wives of one husband. The third end, it removes altogether, because as Christ is one, so also is the Church one. It is therefore evident from what has been said that plurality of wives is in a way against the law of nature, and in a way not against it.