[W]hat is disability? Who are people with disabilities? And conversely, what does it mean to be nondisabled? When the US Supreme Court struggled to define obscenity in 1964, Justice Potter Stewart threw up his hands in frustration and wrote, “I know it when I see it." It’s temptingly easy to do the same about disability. We generally assume that disability is a clearly defined category, unchanging and concrete. Closer inspection, however, reveals that disability is often elusive and changing. Not only do people with disabilities have a history, but the concept of disability has a history as well.... Disability can be contextual, and its meanings have changed over time."
from A Disability History of the United States by Kim E. Nielsen, pp. xiv, xv.