The mummies of ancient Egypt, modern day crypts and coffins, even keeping someone's ashes in an urn or collumbarium are all representative of wanting to keep the person separated from the "rest" of the world, refusing to relinquish your self even in death. Says a lot about a person's or culture's mindset, doesn't it? What is so gruesome about natural processes returning your body to the Earth? It's going to happen eventually anyway. (Perhaps this goes hand in hand with the mindset that refuses the notion that humankind belongs to the Earth. But that's a musing for another day.) It's not just a lesson in impermanence, it's a lesson in letting go of the "self." Arguably, the two lessons are one and the same.
What exactly is the "self"? Is there really a part of you or I that can be guarded so jealously from the rest of the universe?
The tendency is to think that one's skin is the ultimate barrier between "self" and "other." But is not the body made up of proteins, lipids, water, and minerals which had to be obtained from fellow citizens in the web of life? Are we not hosts to millions of microorganisms (e.g. intestinal bacteria) with whom we share symbiotic relationships? Do we not breathe in oxygen which has passed through the lungs and bodies of countless others before us? Are we not made up of the same atoms and quarks and neutrinos that make up the stars, planets, whales, trees, diatoms, rocks, and rivers? The body is constantly repairing and essentially remaking itself; I think it's something like every seven years, you are effectively a new person because the vast majority of your cells have been replaced. How can one's "self" be defined by a body that is inextricably linked with the rest of the universe, and ever in flux?
What about Descartes and "I think, therefore I am?" Are we that abstract phantom called the mind? If we are our thoughts, then surely we are still everchanging, because the mind is ever changing, even moreso than the body. We assume that the existence of thought presupposes the existence of a thinker (i.e. "self"), but as Alan Watts points out, this is a rule of grammar, not of nature. Would I cease to be a "self" if I stopped all thought? This seems to be a belief in many Eastern schools of philosophy, and an interesting point brought up in Mark Epstein's aptly named Thoughts Without a Thinker. Am I, perhaps, only a "self" because of thinking of myself as a self? Is cogitating the existence of "self" the ultimate delusion? (It's true what they say, thinking about your mind is like trying to bite your own teeth!)
Then perhaps we can be defined entirely by our souls. Or can we? Is believing in a self-ish soul that is yours and yours alone an extension of believing in a self-ish body? Is what we call spirit or soul just a small spark from a divine flame --- a fire of life, if you will --- borrowed within our bodies for a short time, to be relinquished back to the general flame upon death, so that the fire can live on in another? Now, I'm not a physics buff by any means, but I seem to remember Einstein proving to us that matter is energy, and energy is matter. By extension, then, perhaps body is soul, and soul is body. So perhaps the theory of transmigration of souls is another way of expressing the aformentioned cycle wherein the atoms that make up our bodies are recycled throughout all of creation. If this is the case, then the soul doesn't "belong" to us alone, either, does it?
I stand by a previous assertion that any human description of the universe will always be incomplete, because they are still words for what is known to us trying to describe the infinite unknown. Nonetheless I believe the metaphor of a "web of life" or a "tree of life" to be a valid one. Our minds can draw arbitrary lines between this strand and that strand, this branch and that branch. But perhaps in the end, since everything is connected, there are no real lines to be drawn.
"Because each existence is in constant change, there is no abiding self...the self-nature of each existence is nothing but change itself, the self nature of all existence. There is no special, separate self-nature for each existence." Shunryu Suzuki
