Ibbur (Hebrew: עיבור, "pregnancy" or "impregnation" or "incubation"), is one of the transmigration forms of the soul and has similarities with Gilgul neshamot. Ibbur is always good or positive, while dybbuk (Yiddish: דיבוק), is negative. Ibbur is the most positive form of possession, and the most complicated. It happens when a righteous soul decides to occupy a living person's body for a time, and joins, or spiritually "impregnates" the existing soul. Ibbur is always temporary, and the living person may or may not know that it has taken place. Often the living person has graciously given consent for the Ibbur. The reason for Ibbur is always benevolent—the departed soul wishes to complete an important task, to fulfil a promise, or to perform a Mitzva (a religious duty) that can only be accomplished in the flesh. In Lurianic Kabbalah, ibbur occurs when an incomplete soul which cannot achieve tikun is completed by the addition of the soul of a tzadik, or spiritual master. Luria believed this to be possible even whilst the possessor was still alive.