What Is Life? — A Biological Question Revisited

Where this article fits: The question What is life? occupies a special place within APS because it addresses the most fundamental question in biology. Before we can explain evolution, development, cognition, ecology, or behaviour, we must first understand what kind of organisation makes something alive. Several APS articles approach this question from different perspectives.

Agency as the Defining Activity of Life examines why living systems are active participants in their own persistence rather than passive objects shaped by external forces. Persistence — Organised Continuity Through Time explains why continuity is the central explanatory challenge of biology and why living systems must continually maintain themselves despite constant change. The Continuity Architecture of Life explores the major continuity architectures through which persistence is sustained across development, ecology, evolution, cognition, and social organisation.

This article occupies a different position. Its purpose is to answer the foundational question itself. Rather than asking how continuity is maintained or how agency operates, it asks what kind of organisation makes these phenomena possible in the first place. It therefore serves as one of the principal entry points into the APS framework.

Together, these articles form a conceptual sequence:

• What Is Life? — A Biological Question Revisited — What kind of organisation makes something alive?
• Agency as the Defining Activity of Life — Why living systems act to maintain themselves.
• Persistence — Organised Continuity Through Time — Why continuity is biology’s central explanatory challenge.
• The Continuity Architecture of Life — How continuity is maintained across the major domains of life.

Introduction - Why “What Is Life?” Remains an Open Question

At first glance, the question What is life? appears deceptively simple. Living things seem easy to recognise. Plants grow, animals move, microbes reproduce, and ecosystems display extraordinary forms of organisation. Modern biology has mapped genomes, revealed molecular mechanisms, traced evolutionary histories, and transformed our understanding of living systems across an astonishing range of scales.

Yet despite these achievements, biology still lacks a universally accepted answer to the question itself.

The difficulty is not a shortage of knowledge. Biology understands an enormous amount about metabolism, reproduction, regulation, development, and evolution. The problem is that knowledge of biological processes does not automatically reveal what makes those processes part of a living organisation. We may understand many of the activities performed by living systems while still disagreeing about what makes those activities expressions of life in the first place.

Many proposed definitions therefore rely on lists of characteristic traits. Life is said to involve metabolism, reproduction, homeostasis, information processing, adaptation, or evolution by natural selection. Such approaches often work reasonably well for familiar organisms, but they repeatedly encounter difficulties at the boundaries. Viruses, sterile organisms, artificial systems, and other borderline cases expose the limitations of definitions based primarily on traits.

The deeper problem is that trait lists describe what living systems do rather than what they are. They identify recurring features without explaining why those features belong together or what unifies them within a single biological phenomenon. As a result, they provide useful descriptions but often fall short of genuine explanation.

APS does not reject these familiar characteristics of life. Rather, it distinguishes between definition, diagnosis, and evidence. Metabolism, growth, responsiveness, reproduction, and evolutionary capacity may help diagnose living systems and provide evidence of living organisation, but they do not by themselves explain what life is. APS therefore seeks a definition of life in terms of the organisational condition that makes these biological phenomena possible in the first place.

Contemporary discussions increasingly recognise that disagreements about life often reflect deeper ontological questions rather than merely disputes over definitions. Competing accounts emphasise metabolism, reproduction, information, evolution, autonomy, process, or organisation, yet these perspectives frequently differ because they begin from different assumptions about the nature of living systems themselves. APS approaches the life question primarily as an ontological and explanatory problem: not simply how life should be defined, but what kind of organisation makes living systems possible.

The question therefore remains open not because biology lacks facts, but because it still requires a framework capable of making sense of those facts. The challenge is not merely to catalogue the properties associated with life but to identify the kind of organisation that makes those properties biologically meaningful.

This shifts the question from:

Which traits define life?

to:

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

Recent reassessments of Schrödinger’s influential What Is Life? suggest that the book was less an attempt to define life than an attempt to explain the source of biological order. Nicholson argues that Schrödinger’s central concern was how the remarkable orderliness of living systems could arise from physical processes, a question that subsequently helped shape molecular biology’s emphasis on hereditary information, genetic control, and cellular mechanism. The continuing absence of consensus about life may therefore reflect not merely disagreement about definitions but uncertainty about the explanatory target itself.

Why Trait Lists Are Not Enough

Traditional definitions of life frequently begin by identifying a set of characteristic features. Metabolism, growth, reproduction, responsiveness, and evolution are among the most common candidates. These features are undeniably important because they occur throughout the living world and contribute to the persistence of organisms.

The difficulty is that none of these features explains why a system is alive.

A fire consumes energy, grows, spreads, and can even exhibit forms of self-propagation. Crystals grow and maintain organised structure. Machines process information, regulate internal conditions, and respond to changing inputs. Yet none of these systems is ordinarily regarded as living. At the same time, some unquestionably living systems may temporarily lack one or more of the supposedly defining traits. Sterile organisms do not reproduce. Dormant organisms may exhibit little visible activity. Viruses occupy a persistent grey area between life and non-life.

Trait lists therefore struggle because they focus on outcomes rather than organisation. They identify recurring characteristics without explaining the principle that unifies them. Even when several traits are combined, the result remains descriptive rather than explanatory.

The central question is not whether living systems metabolise, reproduce, regulate, or evolve. They unquestionably do. The deeper question is why these processes belong together and what organisational condition makes them part of a living system.

APS approaches the problem by shifting attention away from properties and toward organisation. Instead of asking which traits define life, it asks what kind of organisation generates and sustains those traits. From this perspective, metabolism, regulation, reproduction, and evolution are not definitions of life. They are consequences of a deeper organisational reality.

Life is therefore better understood not as a list of properties but as a distinctive mode of organisation that continually maintains the conditions of its own continued existence.

From Traits to Organisation

The move from traits to organisation represents a fundamental shift in biological explanation. Traditional definitions seek a set of observable features that distinguish living things from non-living things. Organisational approaches instead seek the underlying principle that makes those features biologically significant.

This distinction becomes clearer when we consider that the same physical components can participate in either living or non-living systems. The molecules that constitute a living cell are not intrinsically alive. What matters is the way those molecules are organised and the roles they play within a larger system. Life therefore cannot be reduced to matter alone, because the same matter may be alive in one context and inert in another.

Organisation provides what trait lists lack: an explanation of why biological processes belong together. Metabolism matters because it contributes to maintaining a larger organisation. Regulation matters because it helps preserve viability. Reproduction matters because it extends continuity through time. Each process derives its biological significance from its contribution to an organised whole whose continued existence is at stake.

The question of life therefore becomes an organisational question. Rather than asking which traits are present, APS asks what kind of organisation allows a system to maintain itself despite continual internal and external change. The focus shifts from identifying properties to understanding the organisational conditions that make those properties meaningful.

Life as Viability-Oriented Organisation

APS approaches this question through the concept of viability. A system is viable when it remains within the conditions necessary for its continued existence. These conditions differ across organisms and circumstances, but the underlying principle remains the same: living systems must continually maintain themselves within a range of states compatible with persistence.

Viability is therefore not an optional feature added to life. It is the condition that gives biological activity its significance. Regulation matters because it supports viability. Repair matters because it restores viability. Adaptation matters because it contributes to viability under changing circumstances. Across every domain of biology, the distinction between success and failure ultimately derives from whether a system remains viable.

This orientation toward viability transforms the meaning of biological organisation. Living systems are not merely organised. They are organised in ways that preserve the conditions of their own continued existence. Their activities are therefore not arbitrary collections of processes but components of an ongoing effort to sustain continuity in the face of change.

APS describes this form of organisation as viability-oriented, constraint-closed organisation. The constraints that regulate activity are generated, maintained, and supported by the organisation itself, producing a system whose continued existence depends upon the ongoing maintenance of its own organisational conditions.

Life, on this view, is not best understood as a substance, a mechanism, or a checklist of properties. It is a distinctive form of organisation oriented toward the maintenance of viability through time. Understanding life in these terms naturally raises a further question: how is such organisation maintained despite continual change? The answer leads directly to agency and organised persistence.

This framework provides the foundation for understanding how living systems actively sustain themselves through agency and organised persistence.

Agency and Organised Persistence

If life is viability-oriented organisation, a further question immediately arises.

How is viability maintained?

Living systems do not simply exist. They act. They regulate internal conditions, respond to disturbances, repair damage, acquire resources, and modify their behaviour in ways that contribute to continued existence. This capacity for self-maintaining activity is what APS describes as biological agency.

Agency, in this context, does not imply consciousness, intention, or deliberation. A bacterium moving toward nutrients, a plant adjusting its growth in response to light, and an animal regulating body temperature all exhibit forms of agency because their activities contribute to maintaining viability. What unites these examples is not intelligence but organisation. In each case, activity is directed toward preserving the conditions necessary for continued existence.

Agency therefore emerges naturally from viability-oriented organisation. Once a system must maintain itself within viable bounds, its activities acquire significance. Some outcomes contribute to persistence while others undermine it. Some actions succeed while others fail. Biological activity becomes meaningful because the continued existence of the system is at stake.

This relationship between agency and viability leads directly to the concept of organised persistence. Living systems do not maintain themselves through isolated actions or individual processes. Persistence is achieved through the coordinated organisation of many activities operating across multiple timescales and dimensions of biological organisation. Regulation, repair, resource acquisition, development, adaptation, and reproduction all contribute to a larger pattern of continuity.

Organised persistence therefore provides a unifying perspective on life. It explains why biological activity is directed, why regulation matters, why function can succeed or fail, and why continuity remains central to biological explanation. Living systems are not merely collections of processes occurring together. They are organised systems whose activities collectively sustain their own continued existence.

The question of life is therefore inseparable from the question of persistence. To understand what life is, we must understand how living systems maintain continuity despite continual change.

Process and Scale

Organised persistence cannot be understood without recognising the importance of process and scale.

Life is fundamentally processual. Organisms persist despite continual turnover of their material components. Cells replace molecules, tissues remodel themselves, physiological states fluctuate, and organisms develop across time. Continuity is therefore not achieved by preserving fixed structures but by maintaining organised activity through ongoing change.

This insight helps explain why living systems cannot be reduced to static arrangements of matter. The persistence of an organism depends upon processes that must continue. When these processes cease, life ends even if many structural components remain temporarily intact. A living organism is therefore not merely a thing but an organised process sustained through time.

At the same time, persistence depends upon organisation across multiple scales. Molecular interactions contribute to cellular activity, cells contribute to tissues and organisms, organisms interact within ecological systems, and evolutionary processes extend continuity across generations. These scales are not isolated domains but interconnected dimensions of a single continuity-producing organisation.

Scale explains how living systems remain stable without being static. Short-term physiological regulation, developmental change across a lifetime, and evolutionary transformation across generations all contribute to continuity, even though they occur across very different temporal and organisational scales. What appears as change at one scale may contribute to persistence at another.

Process and scale together reveal why life cannot be understood through any single event, mechanism, or level of analysis. Living systems persist because continuity is maintained through organised processes operating across interacting scales. The challenge of biology is therefore not simply to identify components but to understand how these processes and scales become integrated into viable forms of organisation.

The APS Definition of Life

The preceding sections lead to a definition of life that differs significantly from traditional trait-based approaches.

Instead of identifying a list of characteristic properties, APS begins with the organisational condition that makes biological activity meaningful. Living systems are distinguished not by possessing particular traits in isolation but by maintaining themselves through viability-oriented organisation.

The APS definition can therefore be stated simply:

Life is viability-oriented, constraint-closed organisation sustained through organised persistence.

Each element of this definition performs an important explanatory role. Viability identifies the conditions under which a system can continue to exist. Constraint closure describes the organisational relationships through which the system maintains the conditions of its own activity. Organised persistence captures the ongoing continuity produced by this organisation through time.

Together these concepts explain why biological processes belong together as parts of a single phenomenon. Metabolism, regulation, development, adaptation, reproduction, and evolution are not separate definitions of life. They are different expressions of a deeper organisational reality whose central challenge is the maintenance of continuity.

The definition is therefore explanatory rather than merely classificatory. Its purpose is not simply to separate living systems from non-living systems but to identify the organisational principle that makes biological explanation possible. It provides a framework within which concepts such as function, agency, normativity, persistence, and evolution become intelligible parts of a coherent account of life.

Borderline Cases and the Question of Life

One advantage of organisational definitions is that they illuminate rather than obscure difficult cases.

Traditional trait-based definitions often struggle when confronted with systems that possess some characteristics of life while lacking others. Viruses remain the classic example. They evolve, contain genetic material, and participate in biological processes, yet they depend upon host organisms for many of the activities required for persistence. Debates about whether viruses are alive therefore persist because trait lists provide no clear principle for resolving the question.

From an APS perspective, the issue is not whether viruses possess enough traits but whether they instantiate viability-oriented, constraint-closed organisation in their own right. The question becomes organisational rather than classificatory. Attention shifts away from counting properties and toward understanding the nature of the organisation involved.

A similar shift occurs when considering sterile organisms. Such organisms may be incapable of reproduction, yet they clearly participate in viability-oriented organisation and maintain themselves through biological activity. Their status as living systems is therefore unproblematic despite the absence of a trait often regarded as essential.

Artificial systems present another challenge. Machines may regulate themselves, process information, and perform sophisticated tasks. Yet these capacities alone do not establish life. The relevant question is whether such systems genuinely maintain themselves through viability-oriented, constraint-closed organisation or merely execute externally designed functions. Once again, the issue becomes one of organisation rather than appearance.

Borderline cases therefore do not undermine organisational approaches. On the contrary, they reveal why organisational criteria are needed. Difficult cases become opportunities to clarify the nature of living organisation rather than exceptions that threaten the definition itself.

Why This Definition Matters

A definition of life should do more than classify cases. Its deeper purpose is to guide explanation by identifying what biology is fundamentally trying to understand.

Many biological concepts already presuppose distinctions that make sense only within a viability-oriented framework. Function, regulation, adaptation, malfunction, fitness, and agency all depend upon the fact that some states support persistence while others undermine it. Without this orientation toward viability, biological explanation would lose the standards that allow success and failure to be distinguished in the first place.

This is why the question of life cannot be separated from the question of biological explanation. If life is understood merely as a collection of processes or components, it becomes difficult to explain why those processes matter biologically. Metabolism, development, signalling, and reproduction become descriptions of activity rather than explanations of organisation. APS instead provides a framework in which these activities are understood in relation to the continued existence of the living systems that perform them.

The implications extend across biology. Function becomes intelligible because biological structures and processes contribute to viability. Regulation becomes intelligible because living systems must maintain themselves within viable conditions. Adaptation becomes intelligible because persistence occurs within changing environments. Evolution becomes intelligible because continuity is extended across generations through inherited organisational capacities. In each case, biological explanation derives its coherence from the organisational conditions that make persistence possible.

This perspective also clarifies the relationship between life and mind. APS does not treat cognition as the defining feature of life, nor does it regard every living system as cognitive in the same sense. However, the capacities associated with cognition emerge from organisational foundations already present in living systems. Evaluation, responsiveness, and the distinction between biologically significant and insignificant conditions all arise within viability-oriented organisation. The continuity between life and mind therefore reflects a continuity of organisation rather than the appearance of an entirely new principle.

The definition also provides a framework for approaching emerging questions in synthetic biology, artificial life, and life detection. Rather than asking whether a system resembles familiar organisms, APS asks whether it instantiates viability-oriented, constraint-closed organisation sustained through organised persistence. This shifts attention away from superficial similarities and toward the organisational conditions that make life possible.

Most importantly, the definition makes explicit something that biological explanation already presupposes. Biologists routinely explain traits, behaviours, and processes in relation to the persistence of living systems. APS does not introduce a new biological reality. It clarifies the organisational principle that gives coherence to biological explanation itself.

Current Outlook

The question of life remains open not because biology lacks evidence but because evidence alone cannot determine the conceptual framework through which life is understood. Advances in molecular biology, systems biology, developmental biology, ecology, and evolutionary theory have transformed our knowledge of living systems, yet they have also revealed the limitations of definitions based solely on components, mechanisms, or trait lists.

Across many areas of contemporary biology, attention has increasingly shifted toward organisation. Researchers investigate networks rather than isolated parts, processes rather than static structures, and system-level dynamics rather than individual components. Although these approaches differ in emphasis, they share a growing recognition that life cannot be understood solely by cataloguing what organisms contain. It must also be understood in terms of how those components are organised and sustained through time.

The persistence of disagreement over definitions of life reflects not simply empirical uncertainty but the continuing absence of agreement about the appropriate explanatory target. Contemporary discussions increasingly suggest that the question is less which traits define life than what kind of organisation those traits collectively express.

Recent historical and philosophical analyses suggest that contemporary biology is increasingly moving beyond the deterministic and gene-centred explanatory framework that dominated much of the twentieth century. Rather than viewing organisation as the execution of information contained in hereditary structures, current research increasingly emphasises dynamic organisation, stochastic processes, cellular architecture, self-organisation, and context-dependent activity. APS participates in this broader shift while arguing that these developments become fully intelligible only when interpreted through viability-oriented, constraint-closed organisation.

APS contributes to this broader movement by providing a unified explanatory framework centred on viability-oriented organisation and organised persistence. It does not replace established biological disciplines. Instead, it offers a conceptual structure capable of integrating their insights into a coherent account of what life is and why biological systems exhibit the distinctive properties they do.

The enduring significance of the life question lies in the fact that it remains inseparable from every other question biology asks. Understanding evolution requires understanding what persists and changes. Understanding development requires understanding what maintains continuity through transformation. Understanding cognition requires understanding why some conditions matter for living systems and others do not. In every case, the question of life remains present, whether explicitly recognised or not.

For this reason, What is life? is not merely a historical question inherited from earlier generations of biology. It remains one of the central conceptual questions of the discipline. The answer proposed by APS is that life is best understood as viability-oriented, constraint-closed organisation sustained through organised persistence. This definition does not reduce life to a trait, a mechanism, or a substance. It identifies the organisational condition that makes biological explanation possible.

Where to Go Next

• Agency as the Defining Activity of Life
• Biological Agency as the Activity of Self-Maintenance
• Persistence — Organised Continuity Through Time
• The Continuity Architecture of Life
• Viability
• Biological Explanation
• How Do We Know Something Is Alive?
• Life Detection in APS — Biosignatures, Organisation, and Evidence

Together these articles develop the central APS claim that life is not best understood as a list of properties but as a form of viability-oriented organisation sustained through organised persistence.