Not yet become a Buddha,
this ancient pine tree,
Issa was a great poet, my favorite haiku master, he's very "human." But while the image here is beautiful, I don't like the philosophy underneath it. Okay, so: the Hindus believed that one must be born in the Brahmin caste in order to attain Moksha, enlightenment. Buddhism arose out of that milleau, asked, "why wait?" and said Nirvana is there for all, always. But it was said that all other beings, from gods to demons, angels to the grasses and insects, were stuck in the wheel of Samsara, waiting to be human; for only humans could attain Nirvana. Now, this seems pleasant, for a moment-- even the gods envy our auspicious position; good for us, we're in the right sphere of life. But there are implications (aren't there always?).
I look out my window at the oak growing up through the lawn, and wonder what this tree could ever do to produce the karma, good or bad, that is required to keep it in this wheel of samsara, the round of suffering. It's just a tree! It grows exactly as it does, perfectly. No ulterior motives, no nothing. Likewise the birds in its branches; they are simply themselves. Even if they do have glimmers of recognizable consciousness, they act on instinct, they live in the moment, as the Buddhas instruct. Karma is translated as "action," understood as acting with expectation; trees and birds don't act that way. Crossing into Taoism, the central idea is to attain a state of wu-wei, or "non-doing," in the sense of just going with the flow of the situations, of the Tao; it is the mark of a sage. And yet this is exactly what the tree is doing out there.
So one must wonder: if a tree or a bird or any other living thing dies, after its life of pure existence, is it reborn as a human again? It there, between one human life and another, a one-life interlude as some other creature or plant? It leads to a ridiculous chain of logic: imagine how many insects die every day, how many rodents, or any of the smaller creatures. A seed, an acorn, is alive, it is a baby tree; but most do not live to adulthood, most don't make it out of the seed. What about individual cells? In my body, untold numbers of cells die every day, both my own cells and the bacteria. They didn't ring up any karma, surely. Point is, there aren't enough humans to be born to offset the supposed progression from plant to animal to human.
And on the other hand, if a man is bad in this life, accumulates much karma, he is often imagined to be reborn as a worm or a bean plant or whatever. This is seen as bad. I say, hey, great! To live as anything other than a human is to live in peace. Ever seen an irritated tree? Or a spiteful worm? A bird miserable about his lot in life? Animals, and of course plants, don't worry, don't plot, don't stress, don't agonize over decisions, don't act out of hatred (or out of compassion for that matter). They simply do their thing, purely. Wu-wei.
The difference is the difference between pain and suffering. The former is physical, but the latter is purely mental. It's the build-up of all suffering into one moment, through memory and the miserable, self-centered "why me?" thoughts. Animals feel pain as pain, but they don't worry about hurting; they simply hurt. Only humans suffer. We're the ones out of sync; the trees are doing just fine. And if bitter old grandpa was reborn as a dung beetle, good for him! He's escaped the wheel.
All kidding aside, it's kind of a natural progression of thought, I think. From class oriented salvation, to human oriented salvation, to Buddhahood for all that lives, and for the rocks as well. Ever farther from the heirarchies we go. To leave behind the human centered concepts has been a theme of the past few centuries, thanks to telescopes and microscopes and science. It's good; it lets us shed such confusions and contradictons, it leads to a more honest spiritual system, a more reasonable philosophy.