This article introduces the justification problem for mathematics, which it explores through the case study of 1820s-1840s rationales for the teaching of mathematics to women in the United States. It argues that, while educators in the 1820s justified women's studies through mental discipline (a common reason for men's study), those of the 1830s-1840s increasingly relied on separate, gendered justifications, tied to emerging ideals of middle-class femininity. This article therefore emphasizes the contingency of the justification problem, which serves to break the present-day cycle of gender stereotypes regarding mathematics.