2. The only things which are immutable are those which stem from God. Everything which is the work of Man is subject to change. The laws of Nature are the same at all times and in all countries. Human laws change according to the times, places and intellectual progress. In marriage, what is of a Divine Order is the union of the sexes, so that the substitution of those who die may be put into effect. But the conditions which regulate this union are so human in design that in the whole world there are not, even in Christendom, two countries where these laws are exactly identical, and none where in the course of time they do not suffer changes. The result of this is that according to civil law, what is legitimate in one country at a certain time is considered to be adultery in another country and at another time. This is due to the fact that civil law has as its objective the regulation of the interests of families, interests which vary according to customs and local necessities. In this manner for example, in certain countries a religious marriage is the only legitimate one; in others it is also necessary to have a civil marriage and finally there are yet other places where a civil marriage is sufficient on its own.