Hence the institution of marriage which was established in the interest of the woman, and which is the protection of the weaker party. It evidently follows that, on her side, she is held to the same fidelity which she has a right to demand. Conjugal infidelity, on whichever side it occurs, is then a disguised polygamy, and, moreover, an irregular and capricious polygamy, very inferior to the legal; for this recognizes at least certain rules, and establishes with precision the condition of the several wives. But adultery destroys all regular and fixed relations between the married couple; it introduces into marriage the open or clandestine usurpation of sworn rights; it tends to re-establish the primitive and savage state, where the coming together of the sexes depended on chance and caprice. Fidelity is for the married couple a common and reciprocal duty. Each, besides, has peculiar duties. We shall lay particular stress on those of the husband. The first of all, which carries with it all others, is _protection_. "Man, being the head of the family, is its natural protector. He holds his authority from the laws and from usage. Moreover, it results from the very nature of things: for between two persons, even perfectly united, it is difficult, it is impossible, to meet with a constant uniformity of views, sentiments, and wishes. There must be, then, a determining voice; one of the two persons sharing in common domestic authority, must have the privilege of superior authority. Now, what are the titles to this superior authority? These titles are strength and reason. Evidently, power in the family belongs by right to him who is strong enough to defend it and reasonable enough to exercise it. But this authority would only be an insupportable privilege if man pretended to exercise it without doing any thing, without returning to the family in the form of security what it pays him in respect and obedience. _Work_ is the first duty of man as head of the family. This is true of all classes of society, as well of those who live upon their income, as of those who live by their work. For the first have to make themselves worthy of the fortune they have received by noble occupations, or, at least, by preserving it and making it bear fruit through a wise management: and the second have, I do not say, a fortune to acquire, which is an aim rarely attained, but they have a far more pressing object before them, namely, the livelihood of those who live under their protection."[70] No one has better depicted, and in a more delicate and sensible manner, the common duties of husbands and wives than Xenophon, who in this particular is a worthy pupil of Socrates, the one of all the ancient sages who best understood the duties of the family. Socrates relates in the following terms the conversation of Ischomachus and his wife,--a young married pair,--in which the husband instructs his wife in domestic duties.