What is the definition of life? I remember a conference of the scientific elite that sought to answer that question. Is an enzyme alive? Is a virus alive? Is a cell alive? After many hours of launching promising balloons that defined life in a sentence, followed by equally conclusive punctures of these balloons, a solution seemed at hand: “The ability to reproduce—that is the essence characteristic of life” said one statesman of science. Everyone nodded in agreement that the essence of a life was the ability to reproduce, until one small voice was heard. “Then one rabbit is dead. Two rabbits—a male and female—are alive but either one alone is dead.” At that point, we all became convinced that although everyone knows what life is there is no simple definition of life.

If I were forced to rush in where angels fear to tread, I would offer “a living organism is an organized unit, which can carry out metabolic reactions, defend itself against injury, respond to stimuli, and has the capacity to be at least a partner in reproduction.” But I'm not happy with such a brief definition. When allowed more extensive reflection, however, I think the fundamental pillars on which life as we know it is based can be defined. By “pillars” I mean the essential principles—thermodynamic and kinetic—by which a living system operates.