transmigration of souls or metempsychosis (mətĕm'səkō'sĭs) [Gr.,=change of soul], a belief common to many cultures, in which the soul passes from one body to another, either human, animal, or inanimate. The Australian aborigines believe that an infant is a reincarnation of deceased ancestors and that the soul is continually reborn. Some Indonesian peoples hold that ancestral souls reside in sacred animals, sometimes in preparation for a new incarnation. Similarly, several tribes in western Amazonia avoid eating certain animals, such as deer, because they believe ancestral souls have entered the animals' bodies.
Metempsychosis is a fundamental doctrine of several religions originating in India. In Hinduism, the individual soul enters a new existence after the death of the body. The sum total of past moral conduct, or karma, determines the condition of the soul and the quality of its rebirth. The cycle of rebirth is eternal unless the soul is released by knowledge or arduous effort (see yoga). This release (moksha or mukti) is a form of salvation, and is possible only for the most devout. Buddhist doctrine does not accept the soul or transmigration as such, treating both as illusory. Rather, there is an eternal, undifferentiated stream of being (samsara). Out of this, existences are produced and prolonged according to karma, or past actions. The individual is not a separate entity, but rather a grouping of elements. They revert to the original primal stream when desire, the cause of the transmigratory cycle, ceases. Only devout Buddhists or saints (i.e., those who abandon all desire) are able to realize this oneness.
The Celtic version of metempsychosis does not have the ethical aspect of its Indian counterpart. The Druids of Gaul supposedly taught that after death the soul left one body to enter another, but the second body was not necessarily earthly; little else is known of their beliefs. Examples of metempsychosis in pre-Christian Irish legends indicate that these transmigrations occurred only in the lifetime of heroes. The belief in transmigration was rare in ancient Egypt, although occasional instances occur of a soul uniting with a god, a soul entering an animal for a lifetime, or a voluntary metamorphosis of a person into another form for his own benefit.
The Greek version, an indigenous product, appeared in the Orphic Mysteries, but its best-known proponent was Pythagoras. He believed that souls were reincarnated in various bodily shapes. Empedocles, in his poem Purification, accepted Orphic and Pythagorean beliefs. Plato's views on metempsychosis are derived from these same sources. Plotinus believed that future destiny depended upon the life of the soul in previous incarnations. It is possible that these beliefs were influenced by contact with Indian religion.
Jewish treatment of metempsychosis, as found in the kabbalah, was limited by the need to conform to orthodox scriptures, and the theory of transmigration was tolerated rather than approved. The Jewish theories, derived mainly from Gnostic, Manichaean, and Neoplatonic sources, teach that man has absolute free will, but that his soul is tied and sullied by contact with matter. Demon (imperfect) souls try to prevent the fulfillment of the finite divine plan. To act out this plan, the spotless souls descend from their original abode in heaven and are incarnated. Punishment and atonement for sins is achieved by another incarnation; but before this happens, the now impure soul flits about as a disembodied spirit. If the pious suffer, it is believed to be for sins committed in a previous existence. At the end of the cycles, when all the incarnated souls are once again pure, the Messianic period begins. No theories of transmigration are admitted into Christian religion.
Bibliography
See J. Head, ed., Reincarnation in World Thought (1967); J. Algeo, Reincarnation Explored (1987).

 Metempsychosis
From the Greek meta, "after," and empsychos, "to animate," the belief that after death, the soul passes into another body, either human or animal. In ancient Greece it was roughly equivalent to the idea of reincarnation.
The idea seems to have originated in Egypt but to have first been advocated by Pythagoras around 455 B.C.E. Diogenes Laertius noted that Pythagoras once recognized the soul of a departed friend in a dog that was being beaten. Plato picked up on the idea and expounded it in several of his Dialogues, most notably the Phaedo and Republic. According to the vision of truth that one attains, one will be born in the next life in a body suitable to that attainment, Plato said. The most enlightened will be reborn as a philosopher, musician, artist, or lover. At the lowest level, he placed tyrants. Once a soul has beheld true being, it will pass from animal into human form, he said. Plato also put forth the idea that a person chooses his next life, the very choice being a sign of his character.
The idea of metempsychosis was also held by some of the Gnostics, and it became a source of disagreement between them and the leaders of the Christian church. Irenaeus, the second century bishop of Lyons, wrote at length against the Gnostics in his pacesetting Contra Heresies and singled out metem-psychosis as an idea that was incompatible with Christianity. The church has essentially followed Irenaeus's lead in its consideration of metempsychosis and reincarnation. Origen, a Christian theologian of the third century with a platonic background, tried to defend some aspects of the metempsychosis doctrine, primarily the prior existence of the soul, but soon gave up, having found the idea contrary to the New Testament teachings.
Metempsychosis found its last great philosophical defender in Plotinus (205-270 C.E.), the Neoplatonic philosopher. He saw repeated births of the soul as a means for its education. By being in the body, the soul learns how desirable is the nonphysical existence, Plotinus taught.
The idea of reincarnation lingered in the West, passing through a succession of Gnostic groups, but experienced a re-birth in the twentieth century. It's current spread, however, has a basis in Indian and Oriental ideas of reincarnation, usually attached to the additional notion of karma.
Sources:
Crombie, I. M. Plato: The Midwife's Apprentice. London: Rout-ledge & Kegan Paul, 1964.