 As in early philosophical debates about competing models, the most controversial issue in defining disability is the relationship between these two features. At one extreme are definitions that imply, or are read to imply, that biological impairments are the sole causes of limitation. The definitions in the World Health Organization’s 1980 International Classification of Impairment, Disability, and Handicap, and the Disability Discrimination Act (UK) have been interpreted this way. At the other extreme are definitions that restrict the limitations faced by disabled people (as such) solely to “contemporary social organization”, such as the definition given by the Union of the Physically Impaired Against Segregation (UPIAS 1976). In between are definitions which assert that individual impairment and the social environment are jointly sufficient causes of limitation. Perhaps the best-known example is the WHO’s International Classification of Functioning, Disability and Health (World Health Organization 2001, referred to as ICF), which emphasizes that disability is a “dynamic interaction between health conditions and environmental and personal factors”.