The belief that not only men but also all women can master clarity of thought is an important element in the most reactionary of Astell's writings, Some Reflections upon Marriage, Occasion'd by the Duke and Duchess of Mazarine's Case, published in 1700. Written in response to witnessing the divorce of a friend of Lady Catharine Jones, this work argues that a sound education is a requirement for any woman wishing to enter a healthy marriage. In addition to criticizing men who marry for money, power, or out of the vain desire to display an attractive wife, Astell paints marriage as an unhealthy state for most women, and therefore a state sought only by the irrational: "A Woman has no mighty Obligations to the Man who makes Love to her; she has no Reason to be fond of being a Wife, or to reckon it a Piece of Preferment when she is taken to be a Man's Upper-Servant; it is no Advantage to her in this World; if rightly managed it may prove one as to the next." While economic necessity and social constraints might force a woman into such an injurious institution as marriage, according to Astell a sound education would arm her with the skills necessary to turn the situation to her favor.
In 1706 Astell released a third edition of Some Reflections upon Marriage, responding to critics of her work and urging England's womenfolk to strive for a marriage based on true friendship rather than necessity or pride. "Let us learn to pride ourselves in something more excellent than the invention of a Fashion," she counsels readers, "and not entertain such a degrading thought of our own worth as to imagine that … the best improvement we can make of these is to attract the Eyes of men." In the Appendix of this work is her most-quoted line among feminists: "If all men are born free, how is it that women are born slaves? as they must be if the being subjected to the inconstant, uncertain, unknown, arbitrary Will of Men, be the perfect Condition of Slavery?"
Perhaps because it was not overtly defiant of male authority, A Serious Proposal to the Ladies was immensely popular among women readers, and through its wide circulation Astell won many fans. Perhaps not surprisingly, it also won its share of detractors. In June and again in September of 1709 the popular Tatler included essays by writers Jonathan Swift and Richard Steele that attacked Astell's idea of a women's school. Dubbing Astell "Madonella," the essays satirized her so-called "Order of Platonics" by imagining this order of reclusive, fragile nuns hiding while their nunnery is rudely entered by a group of rough gentlemen. Flattering Madonella by praising her writing skill, the men gain mastery over the situation; in short, they hold these educated women to their "inconstant, uncertain, unknown, arbitrary Will."
The proposal for a quasi-religious college for women that Astell first outlined in A Serious Proposal to the Ladies was revived in The Christian Religion as Profess'd by a Daughter of the Church of England, a plea for furthering women's education that was addressed to England's Queen Ann, who had taken the throne in 1702. Although because of this work the school was reported to have been at least considered by Anne, it never came to fruition due to rumors by Anne's Protestant advisors that it would result in the reestablishment of Catholic nunneries.
After 1709, perhaps partially in response to the ridiculing she received in the Tatler, Astell ceased writing. Her last published book was a revised edition of Bart'lemy Fair; or, an Enquiry after Wit; In Which Due Respect Is Had to a Letter concerning Enthusiasm, which appeared in 1722. Now in middle age, Astell refocused her attention toward opening a charity school. With the help of her patrons, she succeeded, and a school for girls was established at London's Chelsea Hospital that remained operational until the late 1800s. Ultimately succumbing to breast cancer, Astell died on May 9, 1731, at the age of sixty-four in Chelsea, England.