The dementia concept has been reformulated through its history and the 19th century was remarkable in the construction of this concept as we understand it today. Like other syndromes, much of the history of the dementia concept comes from the attempt to separate it from other nosological conditions, giving it a unique identity. The fundamental elements for the arising of the dementia modern concept were: a) correlation of the observed syndrome with organic-cerebral lesions; b) understanding of the irreversibility of the dementia evolution; c) its relation with human ageing; and d) the choice of the cognitive dysfunction as a clinical marker of the dementia concept.