Where this article fits: *The question What is life? occupies a foundational place within APS because it identifies the biological organisation that the framework takes as its principal object of investigation. Several APS articles approach this organisation from different explanatory perspectives.

Agency as the Defining Activity of Life examines the viability-oriented organisational activity through which living organisation is enacted and sustained. Persistence — Organised Continuity Through Time examines the problem of maintaining continuity despite continual change. The Continuity Architecture of Life explores forms through which biological continuity is organised across different spatial and temporal extents.

This article has a different role. It addresses the APS account of Life itself: what kind of organisation APS proposes as characteristic of living systems and how that proposal relates to other approaches to the biological question. It therefore serves as one of the principal conceptual entry points into the framework.

Together, these articles form an explanatory sequence:

  • What Is Life? — A Biological Question Revisited — What organisational account of Life does APS propose?

  • Agency as the Defining Activity of Life — What do living systems do as viability-oriented organisations?

  • Persistence — Organised Continuity Through Time — How is continuity maintained despite continual change?

  • The Continuity Architecture of Life — Where and through what forms is biological continuity organised?

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

At first sight, the question What is life? appears almost trivial. Living organisms seem immediately recognisable. Plants grow, animals move, microorganisms reproduce, and biological systems display remarkable forms of organisation. Modern biology has mapped genomes, reconstructed evolutionary histories, revealed molecular mechanisms, and transformed our understanding of living systems across biological inquiry. Few scientific disciplines have achieved comparable explanatory success.

Yet despite these extraordinary advances, biology still lacks a universally accepted answer to one of its most fundamental questions: what is life?

This continuing uncertainty does not imply an absence of empirical knowledge. Contemporary biology possesses an unprecedented understanding of metabolism, development, heredity, physiology, ecology, and evolution. The difficulty lies partly in determining what kind of question a definition of Life is intended to answer. Knowledge of the many processes exhibited by living systems does not by itself settle how those processes should be related within an account of living organisation.

The persistence of this question is reflected in the diversity of proposed definitions. Contemporary discussions include accounts centred on metabolism, replication, evolution, information, autonomy, autopoiesis, thermodynamics, artificial life, viral biology, and organisation (Zimmer, 2021; Koons, 2025; Nicholson, 2025). Such disagreement should not be confused with a general lack of biological knowledge: different proposals are designed for different purposes, and some make incompatible claims about what is constitutive of life. Viruses, sterile organisms, artificial systems, and unfamiliar possible life forms expose the limits of familiar examples and challenge the adequacy of proposed criteria.

This continuing disagreement suggests that the problem is not simply the absence of a sufficiently comprehensive list of biological characteristics. Different definitions can address different explanatory questions. Some identify properties associated with living systems, others specify criteria for classification, while others investigate distinctive forms of biological organisation. These are related but not identical enterprises.

APS approaches this issue by distinguishing recognition, definition, and explanation. Observable characteristics such as metabolism, growth, regulation, reproduction, responsiveness, and evolution provide important evidence about living systems. A biological definition identifies what is proposed to qualify a system as living. Biological explanation can additionally ask how the characteristic activities of living systems are organised and related.

The central claim of this article is that part of the enduring debate over Life concerns uncertainty about this explanatory target. If the aim is classification, operational criteria may be especially important. If the aim is to explain how characteristic biological activities belong together within a living system, the problem also becomes organisational.

This shift in emphasis does not reject existing biological knowledge. It asks how the many processes investigated throughout biology should be understood in relation to living organisation.

The question therefore becomes not simply Which properties distinguish living systems? but also What kind of organisation does APS propose as the target of this biological explanation?

The sections that follow examine principal approaches to the question before developing the APS proposal that:

Life is viability-oriented, constraint-closed organisation.

Organised persistence then concerns how this organisation maintains continuity despite continual material and organisational change.

From Definitions to Explanations: Major Approaches to Understanding Life

The enduring debate over the nature of Life has generated a remarkable diversity of scientific and philosophical approaches. Although these perspectives sometimes compete, they can also begin from different explanatory objectives. Some seek criteria for recognising living systems, others identify properties shared by organisms, while still others investigate the organisation through which living systems maintain themselves. Distinguishing these objectives helps clarify both the richness of the literature and the persistence of disagreement.

One influential strategy has been to define Life through characteristic biological properties. Lists commonly include metabolism, growth, reproduction, responsiveness to environmental change, homeostasis, and the capacity for evolution. Such approaches provide practical criteria for recognising many familiar organisms and have played important roles in biological education, taxonomy, and astrobiology.

Property-based definitions nevertheless encounter familiar difficulties. Individual characteristics admit recognised exceptions. Sterile organisms remain alive despite lacking reproductive capacity, dormant organisms may exhibit little observable metabolism, and viruses satisfy some but not all commonly proposed criteria. Extending a list of defining properties can therefore leave open the question of how those properties are related and why they should jointly bear upon living status.

A reconsideration of the widely used NASA formulation illustrates the legitimate role of operational criteria. Mix (2026) examines the familiar description of life as “a self-sustained chemical system capable of Darwinian evolution” and its difficulties in application. A working recognition criterion can guide observation, experimental design, and astrobiological searches without supplying a final account of what makes a system living. Operational definitions are therefore not simply failed ontological definitions: they can succeed at identifying evidence relevant to a particular investigation while leaving deeper questions unsettled.

Evolutionary approaches relocate emphasis from individual organisms to populations capable of heritable variation and natural selection. From this perspective, participation in evolutionary processes provides a powerful basis for explaining biological diversity, adaptation, and the historical transformation of Life.

Evolutionary criteria become more difficult when transferred without qualification from populations and lineages to the present living status of individual systems. A sterile animal or a non-reproducing individual plant does not cease to be alive because it makes no direct reproductive contribution. Conversely, a population-level capacity for Darwinian evolution does not, by itself, specify the ongoing organisation of each individual. Accounts of the origin of life may also investigate transitional systems before the emergence of fully developed Darwinian evolution. These are distinct explanatory targets, not objections to the established explanatory achievements of evolutionary theory.

A different family of approaches investigates biological organisation itself. Maturana and Varela (1980) characterise autopoietic systems through networks of production that regenerate the components and boundaries constituting their unity. Ruiz-Mirazo and Moreno (2004), and Ruiz-Mirazo, Peretó and Moreno (2004), develop basic autonomy and its relationship to the origins and evolution of life. Moreno and Mossio (2015) elaborate biological autonomy, while Montévil and Mossio (2015) analyse closure of constraints: the mutual dependence of constraints that contribute to maintaining the organisation in which they operate. These accounts are not interchangeable. They differ in their explanatory roles for production, regulation, autonomy, normativity and evolution.

These organisational approaches have significantly deepened theoretical biology by moving beyond lists of characteristics toward accounts of self-maintenance, autonomy, individuality, and organisational continuity. Differences nevertheless remain concerning organisational closure, metabolism, autonomy, evolutionary history, normativity, and the explanatory significance assigned to organisational concepts. Organisational biology therefore comprises a sophisticated family of related perspectives rather than a single universally accepted account of Life.

A more fundamental objection concerns whether a general definition should yet be attempted. Cleland and Chyba (2002) and Cleland (2019) argue that, without a sufficiently developed general theory of life, familiar examples and proposed criteria may be inadequate guides to unfamiliar forms. This is a substantive challenge to APS as well as to other definitions: an internally coherent organisational account is not thereby established as universally applicable. It remains possible that new discoveries will require the definition itself to be revised.

Taken together, these approaches show that contemporary biology possesses multiple explanatory traditions addressing different aspects of living systems. Property-based approaches identify characteristic features. Evolutionary theory explains historical transformation. Organisational theories investigate self-maintaining biological organisation. Each can contribute to biological understanding while addressing a different explanatory target.

The continuing diversity of these approaches therefore raises a comparative question: how should their explanatory achievements and targets be related?

APS enters this discussion as a comparative explanatory methodology. Rather than treating every approach as a rival answer to an identical question, APS asks whether their explanatory targets can be distinguished and related through an explicit analysis of living organisation.

This is a methodological proposal. The coherence or explanatory gain achieved by that comparison must be assessed relative to particular targets and relevant alternatives; it is not guaranteed merely by organising different perspectives within APS.

The Explanatory Shift: From Biological Characteristics to Biological Organisation

The diversity of definitions of Life reflects more than disagreement over individual biological characteristics. It also reflects differing assumptions about explanatory objectives.

Property-based definitions, evolutionary accounts, and organisational theories can illuminate different aspects of living systems, but their differences cannot always be dissolved by assigning each a separate explanatory target. A claim that evolution is constitutive of life, for example, may conflict with a definition centred on individual organisational autonomy; competing organisational accounts may also disagree over what closure requires. Distinguishing explanatory purposes is therefore the beginning of comparison, not a presumption that all proposals are mutually compatible.

Characteristics such as metabolism, growth, regulation, development, responsiveness, reproduction, and evolution provide important evidence about living systems. Yet listing those characteristics does not by itself specify how they are organisationally related.

An organisational account therefore asks what relations make these activities parts of a continuing living system. The explanatory challenge is not merely to recognise familiar Life, but to investigate the organisation through which living systems maintain and re-establish the conditions of their own persistence.

From this perspective, the principal traditions discussed above can be treated as addressing distinguishable explanatory targets. Property-based approaches identify characteristic manifestations. Evolutionary theory explains historical transformation. Organisational approaches investigate self-maintenance and continuity. Their relations must be established by comparison rather than assumed in advance.

APS begins with that comparative problem.

Rather than asking which single biological characteristic should define Life, APS asks how explanatory perspectives can be related to the organisation of living systems. Its objective is neither to replace established biological theories with a master theory nor to reduce their distinctive explanatory contributions to one principle.

The methodological proposal is that an explicit organisational target can make comparison among these perspectives more disciplined by clarifying what each explains and where their explananda overlap or differ.

Biology already explains mechanisms, development, ecological interaction, and evolutionary change with extraordinary success. APS does not treat those explanations as incomplete merely because they do not employ APS terminology. It asks instead when an organisational perspective contributes to a particular explanatory problem and whether that contribution exceeds what the strongest relevant comparator already provides.

This distinction also clarifies debates about definitions of Life. Apparent competition can sometimes reflect differences in explanatory target rather than direct incompatibility. In other cases genuine theoretical disagreement remains. The task is to distinguish these possibilities rather than presume either convergence or conflict.

Within APS, the organisational question can therefore be stated as:

What organisation is APS proposing when it defines a system as living, and what explanatory work does that proposal perform?

Life as Viability-Oriented, Constraint-Closed Organisation

APS answers the organisational question with its canonical definition:

Life is viability-oriented, constraint-closed organisation.

This is the framework’s current conceptual commitment. It identifies a proposed mode of biological organisation rather than adding another isolated trait to metabolism, reproduction, evolution, information, or other familiar characteristics. The proposal builds upon established work on autopoiesis, autonomy, adaptivity and constraint closure (Maturana and Varela, 1980; Di Paolo, 2005; Moreno and Mossio, 2015; Montévil and Mossio, 2015). APS does not claim to have discovered self-maintenance, closure or organisational normativity. Its particular formulation foregrounds viability-oriented activity and uses Agency, Process and Scale to organise comparative questions; whether this yields a scientifically distinctive result remains open.

The formulation should be understood as an organisational unity.

Viability orientation and constraint closure are analytically distinguishable aspects of this organisation. They are not independently constituted components that are subsequently combined to produce Life.

Viability concerns the conditions under which a living system can continue functioning as the organisation it is. Viability orientation refers to the organisation of activity relative to maintaining and re-establishing those conditions. It is not equivalent to mere survival, reproductive success, evolutionary fitness, or indefinite persistence.

Constraint closure concerns the materially realised organisation of constraints through which biological activity is selectively channelled and mutually sustained. Constraints do not constitute a separate structural layer beneath Life, nor does closure considered abstractly establish living status by itself.

Within the APS definition, therefore, viability orientation does not transform an independently constituted closure into Life, and constraint closure does not first produce a neutral organisation to which viability is subsequently added.

The canonical claim is instead about one viability-oriented, constraint-closed organisation.

This organisation consists in the ongoing modulation of constraints through which a system actively maintains and re-establishes the conditions of its own persistence.

Organised persistence identifies a related explanatory problem: how such living organisation maintains continuity despite material turnover, perturbation, development, environmental change, and internal reorganisation. It is not a third component added to the definition of Life.

This distinction helps organise relationships with familiar biological phenomena. Metabolism can contribute to renewal of material and organisational conditions. Regulation can contribute to maintaining viable activity under perturbation. Development reorganises living systems through time. Reproduction can contribute to continuity beyond individual organisms. Evolution explains historical transformation across generations.

None of these phenomena defines Life in isolation within APS. Their particular explanatory roles must be established in relation to the biological target being investigated.

Life is therefore not treated as a property added to otherwise neutral matter or as a single mechanism operating within organisms. APS proposes it as viability-oriented, constraint-closed organisation materially enacted through ongoing biological activity.

Borderline cases must be diagnosed against the proposed criteria rather than decided by verbal stipulation. Sterile organisms can sustain their own viable organisation without individual reproduction. Viruses raise a different issue: their replication and evolutionary history are well established, but the extent to which a virion or virus–host system constitutes an autonomous, viability-oriented, constraint-closed organisation is disputed (Gómez-Márquez, 2021). Artificial systems likewise cannot be classified by complexity, apparent purpose or simulated behaviour alone; their material organisation and dependence on external maintenance must be examined. These cases test APS rather than automatically confirming it. Organisational and autonomy traditions already supply substantial resources for this analysis, and comparative discriminatory gain remains unestablished.

The definition nevertheless provides the conceptual basis from which APS develops its own analytic architecture.

APS diagram showing Life as viability-oriented, constraint-closed organisation and organised persistence as the maintenance of continuity through change

What Makes Something Alive? APS defines Life as viability-oriented, constraint-closed organisation. Organised persistence identifies the problem of how that living organisation maintains continuity through change. Agency, Process, and Scale provide complementary analytic perspectives on the same living organisation.

The APS definition is an explanatory proposal. Viability orientation and constraint closure are introduced as analytically distinguishable aspects of one organisation, not as independent criteria whose conjunction automatically establishes biological status in every difficult case. Applying the definition requires evidence and diagnosis.

Agency, Process, and Scale: Three Complementary Perspectives on Living Organisation

If Life is understood within APS as viability-oriented, constraint-closed organisation, the next question is how that organisation should be investigated.

APS approaches this explanatory task through Agency, Process, and Scale.

These are not independent theories of Life, separate components of living systems, causal levels, or ontological divisions. They are analytic projections rather than components of reality: complementary, target-sensitive perspectives through which one living organisation can be investigated.

Agency asks what living systems do. It concerns the present-tense viability-oriented organisational activity through which living organisation is enacted and sustained.

Living systems regulate, repair, reorganise, and coordinate activities in ways that contribute to maintaining and re-establishing viable conditions. Biological agency does not require conscious intention or deliberative choice.

Within APS:

Biological agency is viability-oriented organisational activity.

Process asks how continuity is maintained despite change.

Living systems undergo continual material turnover, developmental transformation, physiological regulation, and environmental interaction. Process directs explanatory attention toward how organisation persists through these changes rather than toward preservation of unchanging material structure.

Scale asks where persistence is organised across spatial and temporal extents.

Biological organisation can be investigated across different domains and extents according to the explanatory target. Scale does not establish a hierarchy in which wider domains supersede local causation. It directs attention to where relevant organisation is materially realised and to how explanatory priorities vary with the target under investigation.

Agency, Process, and Scale therefore provide complementary analytic projections of one viability-oriented, constraint-closed organisation.

Their use is target-sensitive. A particular explanation need not deploy all three equally, and invoking the three perspectives does not by itself demonstrate explanatory gain.

This architecture also clarifies APS’s relationship with established biological disciplines. Physiology, development, ecology, evolution, cognition, and behaviour address distinctive explanatory problems using their own empirical methods and theoretical resources.

APS does not seek to replace those disciplines or reduce them to one explanatory principle. Its comparative role is to clarify how particular explanatory targets relate to living organisation and to assess whether an APS analysis adds anything relative to the strongest relevant comparator.

Life persists, evolution transforms that persistence, and biological explanation clarifies the organisation that makes both possible.

What APS Contributes to the Biology of Life

If APS neither replaces established biological theories nor constitutes a new empirical discipline, its contribution must be stated carefully.

Its primary contribution is integrative and methodological.

APS provides a comparative framework for asking how concepts and explanatory perspectives relate to living organisation. It can therefore organise questions concerning function, development, evolution, ecology, cognition, and agency without assuming that all such questions have the same explanandum or that APS automatically improves their established explanations.

Consider biological function. Within APS, function is interpreted in terms of present-tense viability-relative organisational contribution. Historical processes can explain how a functional trait arose, while its current function concerns what contribution it makes within ongoing living organisation.

Development presents a different explanatory problem. It concerns transformation through the life history of an organism while forms of organisational continuity are maintained and re-established.

Evolution concerns historical transformation. APS interprets this as transformation of organised persistence across generations while leaving the established causal and historical work of evolutionary theory intact.

Ecology investigates relations among organisms and environments. APS can ask how those materially implemented relations alter conditions relevant to viable organisation without treating ecological domains as higher levels that supersede local biological causation.

Cognition requires another distinction. APS does not identify cognition with Life. Life does not require cognition; cognition presupposes Life. Cognition concerns a further form of organisation in which biological significance is integrated across a temporal field of viability-relevant possibilities.

These interpretations illustrate how APS can organise relations among biological concepts.

They do not demonstrate, simply by being available, that established biological explanations become more coherent or that APS provides additional explanatory gain in every domain.

That stronger conclusion requires comparison.

The APS framework therefore proposes a comparative explanatory methodology, whose value must be assessed against particular targets, evidence and the strongest relevant comparator. Bich and Bechtel (2022), for example, already provide a substantive account of integrated biological control; an APS interpretation must show what it clarifies beyond that account rather than merely restating integration in new terms.

Implications for Biological Explanation

The APS interpretation of Life has implications for how the framework approaches biological explanation, but those implications should remain bounded.

Modern biology employs concepts including function, development, regulation, physiology, ecology, evolution, behaviour, cognition, and agency. These concepts operate within specialised disciplines with their own methods, evidence, explanatory priorities, and theoretical traditions.

APS preserves that explanatory plurality.

It asks whether particular explanations can be compared more clearly when their explananda, organisational targets, and relevant scales or temporal extents are made explicit.

This can help distinguish genuine theoretical disagreement from cases in which different approaches are answering different questions. It can also identify cases in which two frameworks genuinely address the same target and therefore require comparative adjudication.

The organisational perspective may likewise assist interdisciplinary work by providing a common vocabulary for asking how specialised findings bear upon living organisation. But a shared vocabulary is not itself evidence that explanatory integration has been achieved.

Theoretical biology therefore has both empirical and conceptual work to perform. Clarifying explanatory targets, relations among concepts, comparator structures, and conditions of explanatory gain can complement empirical investigation without replacing it.

APS contributes to this work as a comparative explanatory methodology.

Its canonical Life definition provides one organisational proposal to be investigated and tested. Its Agency–Process–Scale architecture provides complementary analytic projections through which living organisation can be examined. Neither commitment is exempt from scientific comparison.

The question What is life? consequently retains its significance. APS does not claim to terminate that inquiry. It proposes a particular organisational answer and a disciplined methodology for determining what explanatory work that answer can perform.

Conclusion: Revisiting the Question What Is Life?

The question What is life? continues to resist universally accepted resolution.

This persistence need not indicate scientific failure. It partly reflects the diversity of questions that definitions and theories of Life have been asked to answer.

Property-based approaches identify characteristic features. Evolutionary approaches explain historical transformation. Autopoietic, autonomy, and other organisational approaches investigate forms of self-production, self-maintenance, closure, and biological organisation.

APS enters this landscape with a specific organisational proposal:

Life is viability-oriented, constraint-closed organisation.

The definition identifies the organisation that APS takes as its biological target. Its coherence makes it a candidate for investigation, not a demonstrated universal criterion. Cleland’s challenge is especially relevant: unfamiliar life, borderline cases and competing organisational theories may expose limitations that require further testing or revision. The definition does not replace established biological theories, and its scientific value cannot be inferred from its formulation alone.

Within this framework, organised persistence concerns how living organisation maintains continuity despite change.

Agency, Process, and Scale then provide complementary analytic projections of that same organisation. Agency concerns what living systems do as viability-oriented organisations. Process concerns how continuity is maintained despite change. Scale concerns where persistence is organised across spatial and temporal extents.

These perspectives are not independent causes, entities, stages, or levels.

APS’s principal contribution is therefore methodological and integrative. It provides a structured way of relating explanatory questions to living organisation while preserving the distinct achievements of established biological disciplines.

Whether a particular APS analysis supplies additional explanatory gain remains an empirical and comparative question.

That requirement is especially important for the Life definition itself. Viability orientation and constraint closure are substantive APS commitments, but their ability to provide additional discrimination or explanation relative to established organisational, autonomy, evolutionary, or other accounts must be demonstrated on particular targets.

APS therefore does not claim to end the investigation of Life.

It proposes an organisational answer, clarifies the explanatory questions associated with that answer, and makes the proposal available for comparative scientific assessment.

Explanatory Architecture

Central Question

What organisational account of Life does APS propose, and how should that proposal be related to the distinct explanatory achievements of established biological approaches?

Architectural Role

This article establishes the canonical APS definition of Life:

Life is viability-oriented, constraint-closed organisation.

Its role is to identify the living organisation that subsequent APS concepts investigate while distinguishing that conceptual commitment from claims of demonstrated comparator-relative explanatory or discriminatory gain.

Preceding Explanatory Dependencies

This article depends upon:

  • the distinction between recognising, defining, diagnosing, and explaining Life;

  • established biological evidence concerning the characteristic activities of living systems;

  • the plurality of property-based, evolutionary, autopoietic, autonomy, and organisational approaches to Life;

  • organisation as materially realised activity rather than a static structural description; and

  • the distinction between conceptual coherence and demonstrated explanatory gain.

Subsequent Explanatory Developments

The Life account developed here provides the APS conceptual basis for subsequent work on:

  • biological agency;

  • viability;

  • organised persistence;

  • Constraint Closure;

  • biological function;

  • Biological Evaluation;

  • Process;

  • Scale;

  • cognition; and

  • biological explanation.

These developments analyse different questions concerning living organisation. Their dependency upon the Life account does not imply causal, temporal, hierarchical, or explanatory priority among them.

This article connects directly to questions including:

  • What is biological agency?

  • What is viability?

  • How does living organisation maintain continuity through change?

  • What does Constraint Closure contribute to an account of Life?

  • How should borderline systems be diagnosed?

  • What distinguishes Life from cognition?

  • How should biological explanations with different targets be compared?

  • What would demonstrate explanatory gain for the APS Life account relative to its strongest relevant comparator?

Position Within APS

Within APS, Life identifies the organisation whose activity, continuity, and spatial-temporal organisation the framework investigates.

  • Agency asks what living systems do: viability-oriented organisational activity.

  • Process asks how continuity is maintained despite change.

  • Scale asks where persistence is organised across spatial and temporal extents.

Agency, Process, and Scale are therefore complementary analytic projections of one viability-oriented, constraint-closed organisation rather than components of reality, independent causes, or hierarchical levels.

This article is consequently a principal conceptual foundation of APS, but not an assertion that the APS definition has already demonstrated universal or comparator-superior explanatory power.