The Subjection of Women is a sustained argument that the legal and social subordination of women to men is both unjust in principle and harmful in practice. Mill announces his thesis in the opening paragraph: the existing arrangement is wrong in itself and ought to be replaced by a principle of perfect equality. He then spends the four chapters of the essay defending that claim against the objections that were standard in his time.
The first chapter addresses the weight of custom and feeling that makes the subjection of women appear natural. Mill observes that when an opinion rests on feeling rather than reason, disproofs of the arguments in its favor do not shake it; adherents simply conclude that the feeling must have some deeper ground the arguments cannot reach. He argues that the mere antiquity or universality of an arrangement proves nothing about its justice, and that the subordination of women, far from being natural, is the one form of domination that has never been seriously tested or consented to.
The second chapter examines marriage law in close detail. By the law of the time, a wife vowed lifelong obedience, could own no property independently, and had no legal recourse against a husband's violence or demands. Mill concludes that no slave, except one immediately attached to a master's person, is a slave in so complete a sense as a wife under common law. Marriage is the only actual bondage remaining in the legal order.
The third chapter turns to the question of women's intellectual and professional capacities. Mill argues that these cannot be determined under present conditions, because what appears as women's nature is in fact the product of an unnatural system of cultivation. He refuses any assertion about fixed differences between the sexes as long as the evidence is corrupted by the very inequality under examination, and insists that women who have been allowed to compete have proved themselves capable of virtually everything men have done.
The fourth chapter asks what good would result from change. Mill argues that the most universal of human relations being governed by justice rather than force would by itself improve the moral character of society. In addition, the practical talent of half the human race would be freed for public and private use. He closes by noting that every restraint on any human being's freedom dries up, to that extent, the principal fountain of human happiness.