Vague ideas about what happens, if anything, after we die have always circulated within every human civilization. Some were particularly vague, such as that the afterlife was simply a world of shadows and despair. But the traditions of yoga in ancient India, and the spiritual experiences generated by different forms of meditation, led to enough shared knowledge to inspire the philosophical ruminations recorded in the Upanishads. Then a mathematician in ancient Greece, totally out of context of that culture, began to analyze those possibilities rationally, exposing European cultures to Pythagorean concepts about reincarnation.
That focus on philosophical rationality returned to India when Vyasa wrote the Bhagavad Gita, in which Arjuna remains rationally unconvinced in spite of the many fascinating explanations given by Krishna. And now, with those ancient traditions supporting it, the 21st century has become inundated with Near Death Experiences and reliable evidence from children’s memories about their last lifetimes, opening up very different perspectives on who we really are. The most important question, though, is whether we will keep becoming more rational about our lives or continue to fantasize instead.