“Impairment” is often used as a synonym for “disability,” as when a person is described as “hearing impaired.” In this context, “impairment” is a euphemism, deemed more appropriate than terms like “handicapped” or “deformed,” which are now largely defunct. Yet the status of “impairment” as a substitute for different conceptions of debility is complicated by the fact that, both within disability studies and in medical conceptions of the body, “impairment” is frequently distinguished from “disability.” Within the British “social model” of disability, “impairment” signifies physical or biological lack (a missing arm, the experience of blindness), while “disability” refers to the process that converts a perceived deficiency into an obstacle. As the British activist group the Union of the Physically Impaired against Segregation (UPIAS) put it in 1975: “Disability is something imposed on top of our impairments, by the way we are unnecessarily isolated and excluded from full participation in society” (14). As a result, clinicians, scholars, activists, and the lay public alike often construe impairment as more scientific—as somehow more reliable and precise—than disability (Oliver 1996; Oppenheim 2009, 474–475; McRuer 2006, 34). Yet this concept of impairment has its roots not in the medical profession or in disability activism but...