Believing as I do that the mind is embodied, I find it difficult to accept that anything of oneself can survive the death of the body. For if the mind is the content of a massively complex system of computations executed in the nervous system and perhaps in other parts of the body, then without this organic “hardware” to act as its medium, how can the mind continue to exist? Saying that the mind could leave the body and continue without a nervous system is like saying that data erased from a hard drive could float around in the air instead of utterly vanishing. There is no law of the conservation of information.
Of course, people say that it is the soul, not the mind, which transmigrates in reincarnation. The soul is said to be made neither of matter nor energy but of something spiritual, that physics knows nothing about. I do, in fact, accept the reality of spiritual experience, but for me, it is something that occurs in the symbolic world we have in our minds and in our networks of social interactions. I have no experience of anything that seems to be a soul, and I don’t see any necessity to assume there is such a thing, particularly with no real evidence. I don’t feel that I have a soul and I don’t feel that I am lacking a soul. So I am very skeptical about reincarnation. But I don’t let my skepticism rob me of the pleasure of thinking about it.
I see some problems with how reincarnation as conventionally imagined works with our evolutionary history, beginning with the earliest humans, hundreds of thousands of years ago. This is assuming that reincarnation occurs only among humans. But I think the same explanations apply (with a little adaptation ) to reincarnation between humans and animals. For instance, the Jataka Tales, which tell of past lives of the Buddha, relate several in which he is in animal form. But for the sake of simplicity, I’ll restrict my speculations to human reincarnation.
Depending on how one defines a human, there was a time very long ago when the first humans lived. These were, of course, very small in number. These earliest humans had the first souls, occurring either naturally or supernaturally. This tiny population, with its presumed evolutionary advantages, began to grow in numbers. I wonder where the souls of their children came from? Only when some of the first humans died would there have been souls available for reincarnation. So from the beginning there must have been a shortage of souls, since there was an ever-increasing number of bodies, with only the souls of those who had lived before to occupy them. Perhaps new souls were created as needed. It seems a little complicated.
Another problem arises when you think about what might occur after a great catastrophe. For instance, during the century of the Black Death, ending around 1400, the world lost perhaps 25% of its population. This means that there would have been millions of souls with no new-born humans to accommodate them. What happened to the extra souls when their usual places to go were not being made? One could imagine a sort of metempsychotic bank which takes up the surplus of souls after a die-off and saves them until there is a time of increase. But such a bank would not have been able to supply extra souls to the increasing population at the beginning of humanity, since there were no previous souls in reserve. The traditional concept of reincarnation does not work unless the population is always the same size, which has certainly not been the case.
This problem arises because of the tacit assumption that when a soul is freed from a dying person, it reincarnates in the immediate future. But there is an alternative hypothesis which disposes of the problem of surpluses or shortages more economically. If a reincarnating soul could jump backward in time and be reborn in its past, or could jump far into its future, then the problem of where surplus souls go after a die-off and where extra souls come from during an expansion simply goes away. After the Black Death, some of the souls would have jumped back in time to be reborn at an earlier time when there was a shortage of souls for all the new humans coming into being. Other souls could have jumped into the future to find places a generation later when the population began to recover, or generations later, into our time when population is increasing even more.
If one were able to remember one’s previous incarnations, some would seem to be in the past and others in the future of whatever time the soul was in. The order could very well be completely non-chronological. It would even be possible for the same soul to inhabit two different bodies at the same time. An older soul could meet a younger version. Assuming that some memories persist, this might explain some experiences of déjà-vu; one really has been there before, just in a different body. Among the six billion humans on this planet, the number of souls within them could be much smaller. This is economical, in a way.
For the purpose of even greater economy, the number of souls jumping back and forth in time could be extremely small. In fact, there need be only one soul for all the humans that now exist, have ever existed, or will exist . This soul zigzags forward and backward throughout time, eventually being incarnated as every being. As it leaves its final body (having been in all others during the span of humanity) the single soul could vanish (into nirvana or oblivion) or return to its first body to repeat the cycle again. This echoes Nietzsche’s idea of eternal recurrence, but in the minds of the reincarnated, not in the physical universe.
Rather than myriad parallel streams, reincarnation seems to be but a single web fashioned from a single strand.
If people became generally aware that they were all either future or previous incarnations of each other, perhaps their feelings, attitudes and actions toward each other would change. One would hope that this change would be for the better. Imagine that one were about to hurt another person in some way and then remembered that this other person was an earlier or later version of oneself. Would the harsh word remain unspoken? Would the clenched fist uncurl?
There is also a dualistic variant, less productive of social benefit. There could be instead two* souls, making their separate yet intertwined journeys back and forth through space and time, forming two interlocked webs. Any individual being would be a stage of either one soul or the other. Perhaps each soul perceives itself as good and its opposite as evil; they could be engaged in an eternal and irresolvable struggle.
*Speculating about three or more souls is less interesting