Inalienable possessions (or immovable property) are things such as land or objects that are symbolically identified with the groups that own them and so cannot be permanently severed from them. Landed estates in the Middle Ages, for example, had to remain intact and even if sold, they could be reclaimed by blood kin. As a legal classification, inalienable possessions date back to Roman times. According to Barbara Mills, "Inalienable possessions are objects made to be kept (not exchanged), have symbolic and economic power that cannot be transferred, and are often used to authenticate the ritual authority of corporate groups".