Yet the distinction between ages slowly became established, and was a certainty in the eighteenth century: Dementia is a sort of inability to form judgements and to reason in a healthy manner: it has been given different names, according to the age during which it becomes apparent: in childhood it is known as silliness or foolishness, but when it affects a person who is of an age to reason it is known as imbecility; when it comes on at an advanced age, it is known as senility, dotage or a return to childhood.