The Talmud refers to the spirits of the dead and to exorcism, but again the concept of transmigration is not stressed. The earliest versions are traced to various non-Jewish sources, including Greek, Indian, Gnostic, Christian, and the Islamic Mutazila sect. The concept entered Judaism in earnest only during the 8th century.
Medieval Jewish scholars objected to it, believing (as many continue to believe today) that any type of mysticism is extremely dangerous, and can influence and contaminate not only one's pure religious faith, but his or her very life. There is some truth to that; cults, with their brain-washing techniques, are a good example of how mysticism can deteriorate. Nevertheless the concept of transmigration developed and found serious followers, and by the 12th century it became an established part of the Kabbalah. The 16th century schools of mysticism embraced it, including the Safed circle headed by Isaac Luria. When Hasidism developed, the belief took final hold. There is a vast body of Jewish literature that dwells on the transmigration of souls, and it spans the centuries mentioned above. In this entire body of myth and legend, which includes books, folktales, and plays, the souls described can be roughly divided into three forms, depending on each soul's origin and intent.
The first form is the Gilgul, which is the Hebrew word for "rolling," but means, in this context, the transmigration of the soul. Generally, it is represented as a natural sequence in the life of the soul, who must occupy various bodies to learn the many lessons it needs before it can be free to reunite with God. The soul simply enters the body at birth (not at conception), just as the infant is about to leave the mother's body, and prepares to live whatever normal life span has been allotted to it.
Special situations require a different approach to transmigration. The second form of transmigration is the Dybbuk, a disembodied spirit possessing a living body that belongs to another soul. There are various origins attributed to these spirits. The earliest description usually hinted that they may be nonhuman demons. Later it was assumed they were the spirits of persons who have died. The dybbuk may be the soul of a sinner, who wishes to escape the just punishment meted to it by the angels of the grave (see the article Afterlife) who seek to beat them, or to avoid another form of soul punishment, which is wandering the earth. A dybbuk may seek revenge for some evil that was done to it while it lived. Alternatively, it may be lost, and will enter a body simply to seek a rabbi who would be able to help it and send it on its way. The living person may or may not know that a dybbuk is occupying his or her body, or it may be tormented by it. This depends on the intent of the possessing soul.
The third form is the Ibbur. The literal translation of the word from Hebrew means "impregnation." 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.