TL;DR
Life seems obvious… until you try to define it. From viruses and artificial organisms to alien biosignatures and ethical debates, no single definition fits all cases. After centuries of science and philosophy, “life” remains a flexible, contested concept - less a fixed boundary and more a set of overlapping ideas that shift with context, purpose, and discovery.
The Full Story
What Is Life?
Biology textbooks often present life as a neat checklist: metabolism, reproduction, evolution, organisation. Yet, the moment scientists look closely, that clarity dissolves. Fire uses energy. Crystals organise themselves. Viruses evolve but do not metabolise. Parasites cannot survive alone. The boundary between life and non-life is blurry, and it always has been. Despite centuries of debate, no definition cleanly captures everything we intuitively call “alive” without also including things that clearly are not.
This difficulty is not academic nit-picking. Modern science increasingly needs a working notion of life. Astrobiologists must decide what counts as life when searching other planets. Synthetic biologists ask whether a lab-built system is genuinely alive or merely lifelike. Origins-of-life researchers face the puzzle of how chemistry crossed an invisible threshold into biology. Each field pulls the concept of life in a different direction, revealing that “life” may not be a single thing at all.
From Souls to Systems
Historically, life has been understood in radically different ways. Plato divided it into vegetal, animal, and rational life. Aristotle saw it as self-directed organisation resisting disruption. Descartes treated animals as sophisticated machines while reserving inner life for humans. Vitalists later argued that life required a special force beyond chemistry, a view eventually weakened (but never fully erased) by advances in biochemistry.
Modern definitions reflect this diversity. Some focus on metabolism and energy flow, others on cells, information, evolution, or interaction with the environment. Thousands of definitions exist, none universally accepted. Attempts to impose a single, rigid definition increasingly give way to skepticism: perhaps defining life is not only difficult, but unnecessary—or even misleading. Some philosophers argue that insisting on one definition risks blinding us to unfamiliar forms of life.
Life Beyond Organisms
The problem becomes sharper at the edges. Are viruses alive? What about prions, protocells, or digital organisms? At larger scales, could ant colonies (or even Earth itself) count as living systems? The famous Gaia hypothesis pushed this idea to its limit, suggesting that life might operate at planetary scale. Whether convincing or not, such ideas show how flexible the concept becomes when stretched beyond familiar organisms.
Ethics and politics further complicate matters. Questions about when life begins or ends underpin debates on abortion, euthanasia, and emerging technologies like transhumanism. Yet biology alone cannot resolve these issues, because “life” is often conflated with sentience, personhood, or moral value—concepts that do not neatly align.
Where Does This Leave Us?
After centuries of effort, there is no consensus—and perhaps there never will be. Future discoveries may force a reorganisation of our concepts, just as atomic theory reshaped chemistry. Alternatively, science may settle on practical, context-specific meanings of life, much as medicine did with the definition of death. Or we may accept that “life” is a family of related ideas: metabolic life, evolutionary life, informational life - each useful in its own domain.
For now, life remains one of science’s most productive uncertainties: a concept everyone uses, no one fully agrees on, and one that continues to shape how we understand ourselves, our planet, and the possibility that we are not alone.
The BioLogical Footnote
If we still cannot agree on what it means for cells to be alive, the real question may not be whether AI will ever become alive—but whether we will even recognise it when it does.