My dissertation explores the relationship between interest in ideal architectural geometries and radical reform movements in the United States during the first half of the nineteenth century. Focusing on the example of octagon buildings, I argue that numerous antebellum Americans, from Thomas Jefferson to Orson Fowler, saw domestic-scale, geometrically distinctive architecture as a tool to cultivate new kinds of private “selves”—stronger, healthier, more rational subjectivities capable of negotiating and transforming an emergent capitalist and democratic society.