I'd like to offer the premise that life can be defined according to a carbon-based biological form's ability to willfully move about in the world. The associated premise: that illness and dying is largely the process of losing or conceding the capacity for willful motion. Creatures who are alive are those that act in the world with something resembling volition or will. Unliving things or dead creatures do not have this capacity.
Now, there are people who would argue that the planet itself is a living entity, but I doubt that many people would say the motion of glaciers moving from mountains to the sea, or volcanoes spitting lava, or earthquaking splitting open the crust are deliberate acts of will. We generally believe these to be actions caused by laws of physics, not choice.
Something resembling volition seems to begin to appear with simple plant life forms. If not willful motion, then the responses seem to be more directly and visibly a reaction to stimuli. In watching a field of sunflowers rotate their heads following the sun, for example, it's not a huge leap to imagine something like the beginnings of willful motion.
In the most simple animals, like plankton or jelly-fish, the line between plant and animal begins to blur, and the responses and capacity for action become more sophisticated. Movement seems to be based on the rudiments of choice. And in animals that are more evolved, it does seem that something like willful choosing of action occurs. In a dog that lovingly licks your face, or a caged gorilla that stares at you through the glass with something that genuinely looks like interest or curiosity, you do see something like the will to choose. (In likelihood there is probably more programmed instinct in these things than we imagine. The eagle soaring on the breeze isn't likely really enjoying the sensation of leisurely flight, but is constantly searching for things on the ground to eat, using the most efficient means of locomotion. We tend to anthropomorphize many animal behaviors.)
We of course like to imagine that the human species represents the pinnacle of biological evolution, and that every movement and action is done out of free will, of our own volition. In reality, I think, much of our behavior is still conditioned and instinctual. But nonetheless, there is considerable freedom in being human; and that freedom is about our volition, our ability to move in the world out of simple will.
And it's not a hard reach to think that in a few million years some descendent of the current human species will evolve a far more sophisticated capacity for movement. The ability to telekinetically move objects, or even move ourselves physically using just the power of the mind, could conceivably be an evolutionary stage yet to come. (No, I don't believe this kind of thing exists yet among yogis or New Age messiahs).
Having now watched my father and some other older people get sick and die—and seeing other aging folks who are beginning to get ill—I've come to the conclusion that the death process is largely about conceding that ability for motion in the physical world. In my father's case, the will to travel extensively around the whole world became restricted to living and vacationing within one or two states, then to his home town, then to his home and yard, then to only one floor of his home, then to a nursing home, then simply to his one room in the nursing home. In the final weeks, that movement became confined to his bed, and then finally even the ability for the mind itself to move about willfully in imagination was conceded. And then he was dead. It was almost as though it was the dwindling of motion that was the culprit.
Even when otherwise healthy people become temporarily sick with ailments like a simple cold or flu, we generally find our willful movement is curtailed, and we remain at home or even in bed for the duration. We know we are returning to health at the moment we venture out to the house to do yard work or when we return to going to restaurants or movies or parties. When I'm sick, it's very hard for me to stay in the house or in bed constantly; sometimes against the advice of my wife or doctors, I go for a walk around the block when sick, simply because I don't feel so sick when I'm able and willing to move. The distinction between cause and effect isn't clearcut. Sometimes I think it is lack of motion that creates illness, not the other way around.
I think, in fact, that you can judge the relative health of a person simply by watching how, and how much, they move. A person who moves little is almost certainly unhealthy and perhaps even dying, while somebody who moves often and in sophisticated, willful ways, is almost certainly at the peak of health.
Beginning to recover from one of those nasty summer colds I've had for the last few days, I went for a long walk tonight. I must say, the movement felt really good.