According to WHO (World Health Organization) disability "is an umbrella term, covering impairments, activity limitations, and participation restrictions. An impairment is a problem in body function or structure; an activity limitation is a difficulty encountered by an individual in executing a task or action; while a participation restriction is a problem experienced by an individual in involvement in life situations. Thus disability is a complex phenomenon, reflecting an interaction between features of a person’s body and features of the society in which he or she lives".

The definition does not say the social burden that word brings to people who have a deficiency, a limitation or restriction. A nearsighted person has a limitation because without glasses have trouble seeing, but being a standard limitation is not considered disabled.

The object of this investigation is to develop a theory that allows us to understand what causes that certain deficiencies are labelled or qualified as disability, with its consequent social burden.