Fuller, like Mary Wollstonecraft before her, compares the relationship of women and men to that of slave and master. In her Woman in the Nineteenth Century, to me very similar in stylistic and emotional energy to Thoreaus's Walden, she asserts vigorously that humankind is meant to attain the status of "angels," but has fallen short of the goal, in part due to gender inequality. Because humans brought the depravity of Europe to the shores of the "Promised Land," for example, the United States, they mistreated the Native Americans and allowed slavery. They also brought the inequality of women.

Until women are treated as equals to men, Fuller argues, humankind cannot reach its highest spiritual state:

It is the destiny of Man, in the course of the ages, to ascertain and fulfill the law of his being, so that his life shall be seen, as a whole, to be that of an angel or messenger, [but] the action of prejudices and passions which attend, in the day, the growth of the individual, is continually obstructing the holy work that is to make the earth a part of heaven. By Man I mean both man and woman; these are the two halves of one thought. I lay no especial stress on the welfare of either. I believe that the development of the one cannot be effected without that of the other. My highest wish is that this truth should be distinctly and rationally apprehended, and the condition of life and freedom recognized as the same for the daughter and the sons of time.

Much of what Fuller advocated for women, such as the vote and the right to own property after marriage, seems commonplace in our time, but her ideas were considered radical in her era. She did her best to cast her ideas in religious terms in order to make them more agreeable to her audience. 

Fuller characterizes four types of marriage. The lowest is a mere economic arrangement, the second is based on the mutual idolatry of the partners, the third is a primarily an intellectual union based on mutual esteem without love, and the fourth is a spiritual or religious union of two souls who are equal in the eyes of God.

While Fuller treated women as the oppressed class they were, as a good transcendentalist she also focused on men and women as individuals, famously saying that we cannot compare women as a whole to men, but only individual men to individual women. In other words, some women will be superior to some men, and vice versa. But whatever the case, society in her opinion can not progress until the sexes are treated equally.