I have a hunch that the quality and complexity,and maybe even the value of life, may be judged on the nature of volition and mobility present in the creature we're studying.
To judge an object as "alive" there must be something that we can call will, and also the capability of willful and deliberate movement. The sophistication of a life form can be judged based on the complexity of that volition to motion.
To certain followers of paganism, the earth itself is regarding as living, and on some level this might be true. A rock, for example, when enlivened with the energy of gravity or tectonic upheaval, has a "will" to move. This is a pretty rudimentary will, though, which is why most people would regard a rock as an non-living object.
A touch more sophistication, though, and we enter the world of plants, creations which show some basic volition as their leaves reach and grope for sunlight, as their roots stretch for water and soil. And plants have the capability of motion. Sunflowers reach east in the morning, west in the evening. Seeds travel on the wind. In my own garden, a single cranesbill geranium has moved itself a full eight feet from where I planted it ten years ago. Some form of will and volition is clearly present here.
A plant becomes an animal, I suspect, when its volition and mobility become faster and more sophisticated. Animals can run or fight or migrate, a capacity that raises them about the world of plants.
In more sophisticated animals, including humans, the capacity for motion becomes ever more refined, until it becomes evident through abstract creation, in art, or in the extremely subtle motion of thought and imagination. If we were to define what it means to be human, we might suggest that it depends on the capacity for symbolic thought—the most subtle of motion.
As I watch the process of life and death in humans, in animals, in plants, even in rocks, it occurs to me that the process of death is largely a matter of gradually relinquishing these various forms of volition and motion.
In some older people, for example, it is physical motion that is first surrendered or taken away through illness. Life most certainly continues, though, and in pretty fine fashion, so long as there is motion and volition of thought, of imagination. This may well explain why older people often find themselves with an increasing instinct to spirituality and imagination, since these things offer an outlet for the instincts to volition and movement. I know some people confined to wheelchairs who are far freer than world-class athletes, so nimble are their minds and imaginations.
Other older people, more tragically to me, find the subtle volition of thought to be the first element surrendered, while volition of the physical body takes longer to decline. A human whose mind badly fails is often regarded, quite logically, as vegetative. Not human, not even animal like, but plant like.
All this may be the reason why I find myself taking more sheer pleasure from the act of movement, in all its forms, as I grow older. Imagination now strikes me as the richest, most wonderful of all forms of movement. And I also treasure each and every act of physical movement in the world, arthritic ankles notwithstanding. I started wearing a pedometer on my belt recently. In the first four days of this week, I've walked 23.84 miles, mostly because I find it such a precious delight to walk the early morning streets until a pending morning meeting forces me to finally hop onto the commuter bus. At lunch, I make a point of walking to the farthest restaurant or park bench that is reasonable within an honest lunch hour.
I won't always have the capacity for motion now available to me, so I'm relishing it. With any luck, dreaming will be the last to go.