"Dementia" is an ancient term which has been in use since at least the time of Lucretius in 50 B.C.E. where it meant "being out of one's mind". Until the seventeenth century, dementia referred to states of cognitive and behavioural deterioration leading to psychosocial incompetence. This condition could be innate or acquired, and the concept had no reference to a necessarily irreversible condition. It is the concept in this popular notion of psychosocial incapacity that forms the basis for the idea of legal incapacity. By the eighteenth century, at the period when the term entered into European medical discourse, clinical concepts were added to the vernacular understanding such that dementia was now associated with intellectual deficits arising from any cause and at any age. By the end of the nineteenth century, the modern 'cognitive paradigm' of dementia was taking root. This holds that dementia is understood in terms of criteria relating to aetiology, age and course which excludes former members of the family of the demented such as adults with acquired head trauma or children with cognitive deficits. Moreover, it was now understood as an irreversible condition and a particular emphasis was placed on memory loss in regard to the deterioration of intellectual functions.