On Liberty by John Stuart Mill Chapter 5 Page 35

the affairs of another are his own affairs. The State, while it respects the liberty of each in what specially regards himself, is bound to maintain a vigilant control over his exercise of any power which it allows him to possess over others. This obligation is almost entirely disregarded in the case of the family relations, a case, in its direct influence on human happiness, more important than all the others taken together. The almost despotic power of husbands over wives needs not be enlarged upon here, because nothing more is needed for the complete removal of the evil, than that wives should have the same rights, and should receive the protection of law in the same manner, as all other persons; and because, on this subject, the defenders of established injustice do not avail themselves of the plea of liberty, but stand forth openly as the champions of power.