Reincarnation

Reincarnation (literally, "becoming flesh again") is the belief that the soul or spirit of a person, which exists separately from their body, does not die when the body dies, but is/can be born again into another body. It is common to many religions and New Age beliefs.

Walt Whitman, Ralph Waldo Emerson, and other transcendentalists generally accepted the idea of reincarnation and past lives.

In religion
Reincarnation is most prominent in Hinduism and Buddhism, both of which believe that the soul moves through a life-death-rebirth cycle (known as samsara). In many Hindu traditions, the soul can be animal, human, god or demon. The exact form and status in which a person is reborn is decided by his or her actions in the previous life, a concept called karma.

Buddhism believes in reincarnation of all beings, until and unless a state of soul-perfection (nirvana) is reached.

Daoism in philosophy also believes in reincarnation of all beings, but there will be no real end for it and souls do not play a central part. To quote (or Chuang-Tsu, 庄周): Birth is not a beginning; death is not an end. There is existence without limitation; there is continuity without a starting-point. Existence without limitation is Space. Continuity without a starting point is Time. There is birth, there is death, there is issuing forth, there is entering in. Modern Chinese folk-belief usually overlooks the philosophy and only recognizes a Buddhism-like reincarnation of the soul.

Other modern religions that formally accept reincarnation include Sikhism, Jainism, African vodun (and the various voodoo offshoots), and several Native American religions, specifically the Inuit.

Reincarnation is also found in Celtic religion, although there is debate among cultural historians whether souls alternate between an otherworld and a regular world.

Judaism
Belief in reincarnation is present in Judaism, particularly in Kabbalah, and it is known as "gilgul neshamot", which translates to "cycle of souls". Most commonly, Chasidic texts tend to accept the doctrine of reincarnation. Reincarnation is frequently cited to justify scriptural claims that all Jewish souls were present at Sinai or to claim that converts to Judaism are reincarnated former Jews, that great teachers are reincarnated because they are not done teaching, or that non-Jewish souls can be reincarnated so that they have an opportunity to become Jewish.

Christianity
Christianity is not generally thought of as having "reincarnation" as part of its doctrine, with the exception of Rosicrucian beliefs. However, there is some Biblical text available if people want to try to shoehorn reincarnation into it. Some of these passages are quite tedious and often require either stretching the imagination or taking the concept of "born again" a little too literally. Another theory is that reincarnation was part of the initial doctrine of Christianity but was abolished sometime in the 4th or 5th century. Basically, if you're going to try and control people and have them do what you say in this life, then it's probably a bad idea to claim that they get a second chance by being able to reincarnate.

Past lives and memories
Why is it whenever people talk about reincarnation they're always a king or a general or a famous person? How come they're never a serf or a rock or a bug?

Since a soul is understood to be the human identity, and human identity relies on memories and genetic makeup, true reincarnation would involve total (or near-total) recall of the previous life in addition to identical wiring of the neurons thereby preserving the individual's unique personality. Plato's philosophy includes the theory of or recovering knowledge gained in past existences, by skilled questioning. Religions teaching reincarnation accept that entities are reincarnated or reborn in a different form and do not maintain the previous identity.

Some hypnotherapists claim to be able to "regress" patients into past lives, essentially claiming to unlock these unconscious memories. However, no one has objectively demonstrated memories of a previous life, such as knowing a closely-kept secret known only to Napoleon's family, for example (even though thousands of people were Napoleon despite the short timeframe of 200 years and less than fifteen generations). The result is that even if reincarnation was true, it would be pointless, because it would be the same as a person suffering total amnesia and starting their life over.

Interestingly, those who have discovered their previous lives have demonstrated that there were far more Egyptian pharaohs and queens than mud encrusted plebs who served under them. This extends to other royal personages, samurai, high priestesses, and other interesting job descriptions. On the other hand, if you were, say, Ronald Reagan looking back over your past life, is it more likely that you'd remember yourself as a film star or President than as a swimming pool lifeguard? And that's just one life. Or, imagine looking back over your own life, this time around. Do you remember the times in which exciting, interesting events happens or the 200 uneventful days that you spent in 2nd grade?

Otherkin and Therians claim to have been mythical creatures or animals in a past life.

Whatever the case may be for the rest of lifeforms, it is a well known fact that cats are reincarnated eight times before going not gently into that good night.