If we ignore the rather short and uninteresting section on oaths, Crusius ends his account of natural law proper by investigating family life. His idea of families has a strong theological colouring. God, Crusius says, has created humans in such a manner that they seek sexual relations, so that the world would be filled with human beings. Furthermore, Crusius continues, God wants that all children will be raised and taught such important matters as belief in God. This duty of raising and educating children, Crusius thinks, belongs to their parents, and in order that parentage of children were certain, couples should be married, that is, form contracts, which forbid sexual relations with other people and which obligate parents to raise their children.
Crusius notes that although human procreation should be regulated and restricted to married people, not every human being need to have children. Of course, only celibacy is a real alternative for Crusius, since he considers sexual organs should be used only for reproduction. In other words, Crusius think sex is allowed only in the context of marriage.
Although Crusius calls marriage a contract, he notes that it is quite different from an ordinary contract, since it only determines the persons entering the married state, while the duties involved are already determined by our duty to obey God. Thus, although normal contracts can be broken, if the people involved just all want so, marriage contract is meant to last for a whole lifetime, because children need a secure family, so that divorces are acceptable only in very special circumstances, Crusius argues. Although one might think so, Crusius is of the opinion that infertility is no real reason for divorce, since it might well be just temporary. Indeed, the only acceptable reason for divorce, in Crusius’ eyes, is adultery, because it makes the proper end of marriage insecure.
Because the main end of marriage is procreation, Crusius continues, polyandry - marriage of one woman with several men - is in his opinion nonsense, because one woman needs only one man to become pregnant (Crusius does not seem to consider the possibility that a woman might have different children for different men). Polygamy - marriage of one man with several women - Crusius considers at least more possible, but ultimately rejects it. Raising children of many women is more difficult, Crusius says and adds that God must have meant us for monogamy, because the two genders are born in almost same numbers (a few more men are born, Crusius explains, because God has foreseen that men will die in wars).
Although the main purpose of marriage is raising children, Crusius think that it has an important secondary purpose, namely, the communion between husband and wife. In a quite sexistic fashion, Crusius insists that women cannot really give intellectual satisfaction to men, but they can provide companionship and sexual satisfaction - a rather bleak view of what marriage is all about. In a similarly sexistic fashion Crusius says that women have a stronger drive for taking care of children, because God meant them to be more involved with the upbringing of children.
Raising children, Crusius says, involves three different duties. A parent must, firstly, take care that the children live and become strong and healthy, secondly, guide them toward virtuous life, and finally, teach them such skills that they require for becoming useful citizens and for cultivating their skills. Although a parent would die, she is obligated to take care of her children, for instance, through inheritance.
In addition to these duties, Crusius continues, parents have a right - which Crusius calls paternal right - to decide the means by which their children are to be raised. This right involves a right to force children to do something, although only in certain limits: for instance, children are not for sale. Paternal right becomes less and less important, when children grow, although, as one might have suspected, Crusius says that daughters should be ruled by their parents as long as these daughters are not married.
Just like parents, children also have some rights, Crusius notes. Children have a right to demand sustenance and education from their parents, until they get old enough to provide for themselves. Children also have a right to expect that their parents do not restrict their freedom needlessly. Finally, unless their parents have decreed otherwise, the children have a right to inherit their parents.
Crusius also indicates that children must have some duties also. Due to their being minors, children must obey their parents, except if this would mean doing things that are against natural law. In addition, Crusius says, children are obligated to be thankful of what their parents have done for them (of course, he continues, children should love and honour their parents, even if their parents have not raised them at all). An important part of these duties is that children must take care of their parents in their old age. These duties are such that no civil laws can overcome them - for instance, a king should not punish his parents.
