What Is Life? — A Biological Question Revisited

Abstract

This article revisits the question of what life is, arguing within APS that life is best understood as viability-oriented, constraint-closed biological organisation rather than a list of defining properties.

Why “What Is Life?” Is Still an Open Question

At first glance, the question What is life? seems straightforward. We recognise living things easily: plants grow, animals move, microbes reproduce. Biology has mapped genomes, traced metabolic pathways, and explained evolution in extraordinary detail.

Yet despite this knowledge, biology still lacks a widely agreed definition of life itself.

Many definitions list properties such as metabolism, reproduction, evolution, homeostasis, and information processing. But such lists encounter familiar difficulties. Some living systems lack one or more of these features, while some non-living systems display several of them. Viruses, sterile organisms, artificial systems, and borderline cases continually challenge trait-based approaches.

The deeper difficulty is that most definitions describe what life depends on, not what life is.

The central question therefore remains:

What kind of biological organisation makes something a living system rather than a non-living process?

Why Trait Lists Are Not Enough

Traditional definitions of life often rely on characteristic features:

metabolism
growth
reproduction
response to stimuli
evolution by natural selection

These features are undeniably important. But none of them—alone or combined—explains why a system is alive rather than merely complex.

A fire grows, spreads, and consumes energy, but it is not alive. Crystals grow and maintain structure, but lack regulation. Machines process information and maintain stability, but only under external control.

Trait lists describe outcomes of living organisation, not its organising principle. They do not explain what unifies these features or why they matter biologically.

APS Box — Why Trait Lists Fail

Trait-based definitions of life attempt to identify living systems by listing observable properties such as metabolism, reproduction, growth, or homeostasis. In APS, this approach fails because it confuses expressions of life with the organisation that makes them possible.

No fixed list of traits reliably distinguishes living from non-living systems. Some living systems lack canonical traits under particular conditions, while some non-living systems exhibit several of them. What matters is not the presence of specific features, but whether a system sustains itself through viability-oriented organisation.

Trait lists can function as diagnostic tools, but they do not define life. APS resolves this by grounding definition in organisational principles and treating observable traits as evidence of underlying viability-oriented organisation.

Key Point. Trait lists fail as definitions because they describe observable features, not the organisational conditions that constitute life.

Life as Organised Activity

Life is not best understood as a thing, but as an activity.

A living system is composed of matter, yet it is not defined by that matter. The same molecules can be alive in one context and inert in another. What matters is how they are organised and what they are doing.

A dead organism retains its components. What it lacks is ongoing, organised activity.

The question therefore shifts from What is life made of? to:

What kind of activity must be organised for a system to count as living?

Agency: Why Living Systems Act

Living systems are not passive. They regulate, repair, and respond in ways organised around sustaining their own persistence.

This is captured by the concept of biological agency.

In biology, biological agency does not imply intention or consciousness. It means that a system’s activity is organised around maintaining its own viability. Some states matter more than others because the system’s continued existence is at stake.

Agency explains why malfunction is meaningful, regulation is intrinsic, and success and failure are biologically real. Without biological agency, biological processes would be indistinguishable from ordinary physical flows.

Process: Why Life Must Continue

Agency exists only insofar as activity continues.

Living systems are maintained through ongoing processes of metabolism, regulation, development, interaction, and repair. These processes are not fixed structures but continuous activities that must be sustained.

Life therefore cannot be reduced to a configuration of matter. It is an ongoing process of organised activity. When that activity stops, life ends—even if the structure remains.

Scale: Why Life Persists

For life to endure, activity must be coordinated across space and time.

Living systems integrate fast molecular reactions, slower physiological regulation, developmental trajectories, and evolutionary change across generations. This coordination allows persistence despite continual material turnover and environmental fluctuation.

Scale explains how life remains stable without being static, and why short-term regulation and long-term evolution belong to the same phenomenon.

Purpose and Function: Why Activity Matters

Living activity is not random. It is oriented.

Processes in living systems are organised around maintaining viability—the conditions required for continued existence. This orientation arises from biological normativity: the intrinsic distinction between what supports or undermines persistence.

Purpose does not imply foresight. It expresses the fact that some outcomes support persistence while others undermine it.

Function is the operational expression of this orientation at the level of parts and processes. A function can succeed or fail because it operates within a viability-oriented biological organisation.

APS Box — From Viability to Meaning

In biology, meaning is often treated as something added to life through cognition, language, or representation. In APS, meaning is not a separate domain but arises directly from viability-oriented organisation.

Living systems regulate their activity in relation to conditions that affect their persistence. Because these conditions make a difference to viability, they are not merely physical states but become biologically significant. Differences that matter for persistence are registered and acted upon within the system’s organisation.

This establishes a continuity from viability to meaning. Normativity—distinguishing what supports or undermines persistence—is the basis from which meaning emerges. As organisation becomes more complex, systems can differentiate, anticipate, and modulate their activity in increasingly structured ways, but the underlying logic remains the same.

Key Point. Meaning in APS is not added to life—it emerges from viability-oriented organisation as the structured significance of what matters for persistence.

Units of Life: Individuals, Organisms, and Beyond

Once life is understood as viability-oriented biological organisation, questions about biological units can be reframed.

Cells, organisms, collectives, and even ecological systems can count as living individuals when they regulate themselves, maintain functional coherence, and sustain viability through their own activity.

Genes and other components are indispensable explanatory constructs, but they are not living units in themselves. They participate in living biological organisation rather than constituting it.

Life, on this view, is graded and scale-relative rather than confined to a single privileged unit.

The APS Perspective: Life as Viability-Oriented Organisation

The APS framework integrates these insights into a single account:

Life is viability-oriented, constraint-closed biological organisation enacted through ongoing process and sustained across interacting scales.

This account avoids trait lists, avoids appeals to intention or design, integrates physiology, development, and evolution, and explains normativity, function, and purpose.

Life is not a substance or a single mechanism. It is a mode of organisation—a way in which matter and energy are coordinated to sustain their own persistence.

On Necessity and Sufficiency

APS does not offer a checklist definition. It specifies a structural condition.

Viability-oriented, constraint-closed biological organisation is necessary for life in the biologically relevant sense. Without it, no system-relative standards of persistence, malfunction, or regulation obtain.

It is also sufficient in principle: any system that genuinely instantiates such organisation qualifies as minimally living within the framework.

Borderline cases test the precision of constraint closure, not the coherence of the criterion.

Why This Definition Matters

A definition of life should do more than classify cases. It should guide explanation and clarify what biology is about.

Understanding life as viability-oriented biological organisation explains why regulation and malfunction matter, clarifies the continuity between life and mind, grounds evolutionary theory in organismal activity, and provides criteria for synthetic and artificial systems.

Most importantly, it makes explicit what biological explanation already presupposes when it succeeds.

APS Box — Borderline Cases: Viruses, Sterile Organisms, Artificial Systems

Borderline cases are often taken to show that life cannot be clearly defined. In APS, they do not undermine the definition of life but test its explanatory adequacy. The question is not whether such systems possess particular traits, but whether they exhibit viability-oriented organisation.

Viruses, for example, do not sustain their own persistence independently. Their activity depends on host systems that provide the organisational conditions required for replication, and they therefore exhibit dependent rather than fully realised organisation. Sterile organisms, by contrast, may lack reproductive capacity but maintain themselves through internally organised processes and therefore remain fully within the domain of life.

Artificial systems present a different case. Some exhibit complex or adaptive behaviour, but unless they establish constraint-closed organisation through which they sustain their own persistence, they do not meet the definition of life.

Borderline cases therefore clarify the boundary of life by revealing where viability-oriented organisation is present, absent, or dependent, rather than exposing a failure of definition.

Key Point. Borderline cases do not weaken the definition of life—they reveal how viability-oriented organisation distinguishes living, dependent, and non-living systems.

Current Outlook

Biology increasingly moves away from static, component-based views toward dynamic, organisational ones. Systems biology, developmental biology, theoretical biology, and philosophy all reflect this shift.

The APS framework articulates this convergence in a unified explanatory grammar. It does not replace biology—it clarifies the conceptual structure on which biological explanation already rests.