“If you have, you know that a definition is elusive. How are living things different from nonliving things? You may try to define living things as things that can move. But of course many living things do not move (most plants, for example), and some nonliving things, such as glaciers and Earth itself, do move. So motion is neither unique nor definitive to life. You may try to define living things as those things that can reproduce. But again, many living things, such as mules or sterile humans, cannot reproduce; yet are alive. In addition, some nonliving things can (in some sense) reproduce. So what is unique about living things?
One definition of life uses the concept of equilibrium – living things are not in equilibrium with their surroundings.
For example, our body temperature is not the same as the temperature of our surroundings. If we jump into a swimming pool, the acidity of our blood does not equilibrate with the surrounding water. Living things, even the simplest ones, maintain some measure of disequilibrium with their environment.
We must add one more concept, however, to complete our definition of life with respect to equilibrium. A cup of hot water is in disequilibrium with its environment with respect to temperature, yet it is not alive. The cup of hot water has no control over its disequilibrium, however, and will slowly come to equilibrium with its environment. In contrast, living things—as long as they are alive—maintain and control their disequilibrium. Your body temperature, for example, is not only in disequilibrium with your surrounding—it is in controlled disequilibrium. Your body maintains your temperature within a specific range that is not in equilibrium with the surrounding temperature.
So, one criterion for life is that living things are in controlled disequilibrium with their environment. Maintaining disequilibrium is a main activity of living organisms, requiring energy obtained from their environment. Plants derive energy from sunlight; animals eat plants (or other animals that eat plants), and thus they too ultimately derive their energy from the sun. A living thing comes into equilibrium with its surroundings only after it dies." (From Chemistry: A Molecular Approach, Tro 2007)
When I read this snippet a few years ago during my intro chemistry class, something clicked. "Life really doesn't get any easier," I thought. " By simply being alive we must constantly fight nature's tendency to bring us into equilibrium."
I was compelled by the concept of equilibrium, quickly embracing the idea that in nature, equilibrium is always a dynamic process. To maintain steady emotional or psychological equilibrium requires patience with yourself, others, and nearly everything in your life.