Scientists have collected at least 123 different definitions of life
Everyone recognises a living thing, yet nobody agrees on what life is. More than 123 definitions exist. NASA's favourite, based on a suggestion by Carl Sagan, has an awkward flaw: under it, a single rabbit does not count as alive, because one animal on its own cannot evolve.
Part of the trouble is that life is a process, not a substance. Biologists therefore describe it by traits: keeping a steady internal state, as when sweating cools the body; being built of cells; running a metabolism that turns energy into cell parts; growing; adapting over generations; responding to stimuli, like leaves turning toward the Sun; and reproducing. Every living thing also carries proteins and nucleic acids, the molecules that store instructions and carry them out.
Other definitions come from physics and systems theory. One casts an organism as an open system that uses energy gradients in its surroundings to make imperfect copies of itself. The NASA version, drawn up for the search for life beyond Earth, calls life a self-sustained chemical system capable of Darwinian evolution. Stuart Kauffman described living things as autonomous agents that can reproduce and complete at least one thermodynamic work cycle. Viruses sit awkwardly across every line. They have genes and evolve by natural selection, but they do not metabolise and can only multiply inside a host cell, earning the label organisms at the edge of life.
The question is ancient. Empedocles, around 430 BC, held that everything, living things included, was a mixture of earth, water, air and fire. Democritus thought the soul was made of fiery atoms, linking life with heat and motion. Aristotle proposed that every living thing has a soul as its form, in three grades: a vegetative soul for plants, an animal soul that adds movement and feeling, and a rational soul he reserved for humans.
Later thinkers pictured organisms as machines. René Descartes treated animals and people as assemblies of working parts, and Leibniz wrote in 1714 that living bodies are machines even in their smallest parts. Around 1900 Stéphane Leduc argued that growth could be explained by chemistry and compared it to crystals forming in sodium silicate solution; he was widely dismissed then, but his ideas have lately drawn fresh interest.
Source: Life
