Reincarnation is literally defined as “to be made flesh again.” The general idea (with a few exceptions) is that when your physical body perishes at the end of your life, a certain essence (called the soul, the spirit, the jiva, the atman, the self, the no-self, the I, and a whole lot of other names depending on where you’re from and what you believe) continues to exist in some sort of intangible, microscopic, or ethereal form, retaining the experiences and lessons learned from the life you’ve just led. This soul takes those experiences, as well as all it has experienced in all of the previous bodies it’s ever inhabited, and transfers them into the next body it inhabits. In many belief systems, this soul continues to learn and evolve, becoming wiser and more enlightened with each incarnation, until eventually it “graduates” from the material world and is able to achieve nirvana, moksha, heaven, oneness with god, or that sort of thing.

There are all sorts of religions that base much of their teachings on a belief in reincarnation. Hinduism is one of the oldest and most widely recognized, first introducing the notion of reincarnation in writing in the Bhagavad Gita, which was written between 2100 and 2600 years ago. Hindu reincarnation incorporates the notion of Karma, which is defined as the sum total of one’s actions throughout the entire existence of that person’s soul. Whether you have built up good or bad karma determines what sort of a body you’re born into. Generally speaking, the more you learn and experience, and the better your karma, the higher level of a physical body you inhabit, beginning with, perhaps, some sort of small plant, and progressing through the animal kingdom until your soul inhabits a beetle, and then a mouse, and then maybe a lion, and then, finally, a human body. After many incarnations, you eventually realize that the material world isn’t all it’s cut out to be, and that the greatest type of existence is a spiritual existence. After lots more spiritual learning and concentration, you eventually leave the material world and, depending on what sort of Hindu school of thought you accept, either become absorbed into an eternal state of peace and happiness, inhabit heaven with god, or become a god yourself.

Buddhists, by contrast, don’t believe in an individual, independent soul that moves from body to body. Instead, they see the cycle of life as an endless continuum, in which we are all part of a universal and all-encompassing energy. While our energy moves from body to body and life to life without maintaining its individuality, the notion of rebirth is still relatively central to Buddhist philosophy; Buddha himself describes his many incarnations in his teachings, and Tibetan Buddhist monks believe that their deceased Lamas are often reborn as new individuals who will resume the role of their predecessors.

There are fringe sects of all major religions, as well as a wide berth of smaller religious groups that refer to reincarnation in some fashion. Plato and Pythagoras spoke of reincarnation in Greek history, Norse mythology mentions it, Ashkenazi and Orthodox Jews have written of it, Native American Inuits accept it as a central tenet of their spirituality, Gnostic and new age Christians believe it was taught in the early days of their faith, and various sufi Muslims interpret certain portions of the Koran as proof of the existence of past lives. Beyond these historic and/or wide-reaching ideologies, there is a host of smaller religious groups that base their convictions on some sort of reincarnation, from Scientologists to Anthroposophists to Theosophists.

Despite such wide-range and historic interest in reincarnation by all sorts of world religions, the western world’s large-scale curiosity in reincarnation is a fairly recent phenomenon. This is due in part to the apparent incompatibility between reincarnation and the dominant religion of the western world, Christianity: most Christians believe in the resurrection of the body of Jesus Christ and the ascent into heaven of all those who accept Christ as the son of god, and the notion of reincarnation might undermine or contradict these beliefs if they are interpreted strictly. Nonetheless, mass media has made significant steps in introducing the idea to western thought, as dozens of major motion pictures and popular books have been made about the subject, and reincarnation has been a major topic of discussion on radio and television talk shows, internet blogs, and the like. In the last fifty years some noteworthy scientific research has been done in an attempt to validate reincarnation, most prominently by Professor Ian Stevenson and Dr. Jim Tucker of the University of Virginia, who, over the course of their respective careers, recorded interviews with thousands of young children who recounted past lives and beginning in 1960 published numerous articles, studies, and books on their findings. There is much skepticism over this body of research in the scientific community, but notable scientists and researchers have at least allowed for the possibility of the existence of the phenomenon. Carl Sagan may have most famously summarized that reincarnation stands with certain ESP (extrasensory perception) phenomena as one of several “examples of contentions that might be true."