The author gives an interpretation of mutatio ipso iure, i.e. endurance of proprietary right (usucapio). Subject to the fulfilment of conditions of endurance, tenure (possessio) will be transformed into the ownership (dominium). It is a transformation ipso iure resulting from the objective right, without human intervention. Therefore the author regards endurance as a kind of, (necessary change). The legal basis of mutatio is the will of the legislator to tolerate the transformation of tenure to ownership and it concerns each legal order, which has officially introduced the institute of endurance. The transformation of tenure to ownership eliminates the chaos in ownership relations and represents the effort at a successful legal expression.