What Is Life, Really? Why That Simple Question Is So Hard to Answer

Imagine you're explaining life to an alien. Not your life—just life, in general. You might say it moves, eats, reproduces, and evolves. You might mention DNA, cells, or metabolism. But the moment you try to pin it down with a neat, one-line definition, something wriggles out. Fire grows and spreads—alive? Crystals replicate—life? A virus evolves but can't survive on its own—undead?

It turns out that defining life is like trying to draw a line around a cloud; it shifts as you look closer. Yet, across science—from biology to space exploration to synthetic chemistry—the need to define life is very real. We use it to search for aliens, decide what counts as a living being, and guide experiments that may one day build life from scratch.

So what really matters? Let’s explore the most significant definitions. They’re not mere technicalities—they’re various lenses through which we perceive our place in the universe. And like all good lenses, each one sharpens some details while blurring others.

The List Approach: Biology’s Greatest Hits

Textbooks often begin with the “checklist” definition: life is made of cells, uses energy, maintains internal balance, responds to stimuli, grows, reproduces, and evolves. This approach resembles a dating profile for living things: it highlights desirable traits, but no single element seals the deal.

The strength here lies in breadth. These lists capture the sheer richness of life as we know it. The problem? Not every living thing ticks every box. Mules and worker bees don’t reproduce. Viruses lack cells. Fire responds and grows. The list is helpful but somewhat too Earth-centric—and a bit too tautological: it defines life by what life does, not what life is.

The Evolution Test: Darwin to the Rescue?

NASA’s working definition, crafted to guide the search for alien life, reads: “Life is a self-sustaining chemical system capable of Darwinian evolution.”

It’s short. Elegant. Evolution, after all, explains the endless variety of forms that life takes. Anything that evolves—adapting through variation and natural selection—surely counts as alive, right?

Maybe. But what about a single sterile organism, like a solitary panda or a petri dish cell line? They can’t evolve as a population—are they not alive? This definition works beautifully for species and lineages, but less so for individuals.

It also insists on chemistry. That’s practical when hunting for Martian microbes, but it rules out digital life—those simulated “organisms” evolving in computer code. They might behave like they’re alive, learn, even mutate. But they're not “chemical,” so NASA says: nope.

Metabolism and Thermodynamics: Life as a Rebel Against Decay

Another definition views life as an entropy-fighting machine. Living beings, as physicist Erwin Schrödinger put it, “feed on negative entropy.” In simpler terms, they maintain order in a world that tends toward chaos.

This angle makes sense. A living cell maintains itself, consumes energy, repairs damage, and stays far from equilibrium—unlike, say, a rock. The problem? So does a hurricane. Or a flame. They, too, use energy and maintain patterns. But we don’t invite them to dinner.

This view requires fine-tuning. The key isn’t just using energy; it’s about utilising energy to self-maintain a complex, bounded structure. We’re getting closer now—and here’s where biotropy enters the picture. Biotropy describes life’s statistical resistance to entropy: not just avoiding decay, but exporting disorder to maintain internal coherence. It’s what makes living systems stay organised when everything else is falling apart.

The Cell Theory: Life Has Walls

A popular definition suggests that to be alive, a thing must be made of cells. This aligns well with Earth biology. Every known life form—from bacteria to blue whales—is cellular.

It’s also convenient: cells are self-contained, membrane-bound mini-factories that manage their own business. However, problems arise. Viruses? No cells. Some synthetic life? Maybe no “membranes” at all. This definition may be robust for Earth but struggles with hypothetical life elsewhere.

Enter Autopoiesis: Life as Self-Made, Self-Organised, and Self-Correcting

Now for the showstopper: autopoiesis. The word’s clunky, but the concept is elegant.

Coined by Chilean biologists Humberto Maturana and Francisco Varela in the 1970s, autopoiesis means “self-production.” They argued that a system is alive if it continually produces and maintains itself—its parts, its processes, its boundary. Think of a cell: it makes the very molecules that keep it running and builds the membrane that holds it all together. It’s a self-sustaining factory, yes—but also something more subtle: a self-organising, self-correcting system.

Here’s where self-organisation and self-correction come into play. Life isn’t just a jumble of molecules following orders. It’s a network of reactions that spontaneously arrange into a structured whole. No master controller coordinates the enzymes, membranes, and genetic machinery. They emerge from local interactions—each reacting, folding, fitting, and feeding back into the others. Like bird flocks or city traffic, life’s coherence arises from within.

But structure isn’t enough. Systems drift. Errors accumulate. This is where self-correction becomes essential. Whether through enzyme inhibition, immune responses, or behavioural recalibration, life detects deviation and adjusts. Without self-correction, production would veer off course and organisation would disintegrate.

Why the Triad Wins?

So, which definition takes the crown?

None are perfect. Each reveals a different truth: evolution explains change, metabolism explains energy flow, cells explain structure, and autopoiesis explains autonomy. But if we had to choose a foundation, a base upon which to stack the rest, it’s hard to beat the triad: self-organisation, self-production, and self-correction.

Why? Because these principles reveal how something becomes an individual: not just a temporary configuration of matter, but a bounded, ongoing process that constructs itself from within, detects its own errors, and resists dissolution.

In this view, life is less a category and more a verb. It’s something you do, moment to moment: organise, respond, repair, produce, adapt. Lose that dynamic, and you’re just a memory.

A Final Thought: Definitions as Mirrors

Defining life is akin to defining love, art, or intelligence. The response reveals as much about the one defining as it does about the thing being defined.

Biologists see cells. Evolutionists see lineages. Systems theorists see networks. Astronomers see chemistry. And philosophers? Well, they see all the problems.

Autopoiesis doesn’t end the debate but offers a profound perspective on life: a dance of parts that give rise to the whole, and a whole that remakes its parts. It’s not merely an explanation; it’s an invitation—to view life not as a thing but as a dynamic, self-sustaining mystery.

Perhaps that’s the real point. Every definition is a frame, not a fence. And in the quest to understand life, the journey may matter more than the answer.