Living Organisms

The Ontological Problem of the Organism

An organism is among the most familiar entities in ordinary experience and among the most difficult to individuate in philosophical biology. A tree, a bacterium, a human animal, and a whale present recognizably organized histories of growth and activity. But a coral colony, a fungal network, a lichen, a social insect colony, and a host with indispensable microbial partners challenge the expectation that every organism occupies one bounded body, descends from one cell, and consists of components belonging to one genome. The question is what makes the organization in each case the organization of one being.

The issue concerns the ontology of individuals. An individual is a determinate entity whose identity and persistence may be investigated through changes in its properties or constituents. Biological individuality demands a further account: how living processes constitute and preserve a unity that undergoes growth, repair, reproduction, differentiation, and often radical transformation. Organisms continually exchange materials with their surroundings, so the persistence of a living being cannot generally be equated with the uninterrupted presence of the very same matter. Even bodily form can alter between embryonic, larval, adult, and senescent stages. What remains continuous has to be specified in terms of organized relations and their historical maintenance.

Several different questions have sometimes been gathered under the term organism. What makes a system alive? What makes its components parts of one functional whole? What fixes its physical boundary? What is the unit of adaptation or natural selection? When can it be said to reproduce? Which biological type does it instantiate? And what permits one to identify it as the same individual through time? The answers often intersect, but no argument establishes that they are extensionally identical. A bacterial cell can be a metabolic individual, a component of a biofilm, and a member of a reproducing lineage without those three classifications identifying the same entity.

The modern literature develops these complications under the headings of physiological and evolutionary individuality, organismality, holobionts, biological processes, and the units of selection (Wilson and Barker, 2024; Queller and Strassmann, 2009). This investigation focuses on the conditions under which a living system exists as an organism and how it continues to exist. Questions about consciousness, personhood, the general ontology of properties, and the metaphysics of species enter only where their treatment illuminates that central inquiry.

The organizing thesis is that organismic identity consists in a sustained and causally integrated history of living organization. This is a philosophical reconstruction, whose strength will depend on whether it explains the borderline cases as well as familiar animals and plants. Integration must be established by biological evidence, not inferred from the appearance of a body or from membership in an intuitively recognizable taxonomic class.

Definitions and Distinctions

Life and organismality designate related yet distinguishable subjects. A theory of life seeks conditions under which a system belongs to the living domain. A theory of the organism seeks conditions under which living activity is organized as one individual. A theory of biological individuality extends the latter problem to possible individuals below or above the organismic scale: genes, chromosomes, cells, colonies, communities, lineages, and perhaps species. Confusing these questions creates spurious exceptions. A molecular replicator may contribute to the reproduction of living systems without itself being an organism, while the component members of an insect colony can be organisms even if the colony also satisfies some criterion of higher-level individuality.

For the purposes of the investigation, a living organism is provisionally understood as a relatively integrated biological system that maintains an internally coordinated organization through material and energetic exchange, responds to conditions bearing on that organization, and has a historically continuous trajectory of development, survival, and possible reproduction. The definition is deliberately operational in its philosophical structure. Its constituent features—regulation, coordination, exchange, and continuity—require interpretation and can vary in degree. It proposes a framework for identifying organisms, rather than a claim that one simple measurable property solves every borderline case.

A physiological individual is a unit distinguished through integrated activities such as metabolism, development, regulation, physiological cooperation, or immune control. An evolutionary individual is a unit that participates in Darwinian processes under a relevant model of variation, differential success, and heritability. These categories overlap significantly, yet their conceptual motivations differ. The first helps identify a living system at a given time; the second helps identify the entities whose properties matter to selection over generations. Whether a particular holobiont meets both standards is a substantive empirical and philosophical question, not a consequence of using one common word (Godfrey-Smith, 2009; Devanesan, 2026).

A recent effort to turn these distinctions into experimental criteria is the cross-disciplinary definition proposed by Wilmsen and Kost (2025). They identify a physiologically integrated, functionally autonomous system and treat its capacity for Darwinian evolution as a separate dimension; they further allow organismic status to vary across life cycles and ecological conditions (Wilmsen and Kost, 2025). Their operational emphasis is an important contemporary alternative to stronger metaphysical claims of an invariant organismic essence. Whether obligatory interdependence is always required to delimit an organism is itself a further philosophical issue, particularly for reversible symbioses and facultative collectives.

The individual–kind distinction is equally important. A particular oak is an organism, while a species name classifies it within a biological grouping. Depending on the philosophical account, the species may be treated as a kind, a historical population lineage, or an individual at a different scale. The identity of the oak is the issue of which continuing life-history we are tracking; species membership concerns relations of descent, reproduction, and classification. One cannot infer a complete account of organismic identity from species membership.

Finally, organism and body are conceptually separable. A body is a material configuration; an organism is a living organization realized in material processes. Some biological bodies host multiple lineages, and some organismic units extend across physically separated structures. The case for any proposed organismic boundary must therefore specify which relation—metabolic, developmental, immunological, ecological, or evolutionary—gives that boundary its explanatory significance.

Aristotle and the Unity of Living Substance

Aristotle supplies the classical philosophical formulation of organized life. In De Anima II.1 he describes living natural bodies through nutrition, growth, and decay and analyzes the soul, psychē, as the first actuality, entelecheia, of a natural organic body (Aristotle, *De Anima*, 412a–412b). The claim belongs to his hylomorphic ontology: living substances are composites of matter and form, and form explains why a particular material configuration is the organized kind of thing it is. The term soul here ranges beyond consciousness or thought; plant life too possesses capacities essential to living organization.

The distinction between first and second actuality is philosophically decisive. The sleeping animal retains its capacity for life, perception, and action even while many activities are unexercised. Living unity therefore does not depend on uninterrupted performance of each characteristic operation. This insight remains illuminating in considering dormancy and temporary functional suppression: the continued possession and organized support of capacities can matter more than their momentary manifestation. One must nevertheless avoid treating an ancient metaphysical definition as a current physiological measurement of viability.

Aristotle also explains parts through their contributions to a whole. Roots and mouths are analogically comparable in their nutritive roles, while eyes are intelligible through the activity of seeing. A living body is accordingly articulated by functional relations rather than merely partitioned geometrically. His question is already an individuation question: what turns an assemblage of tissues or organs into one substantial living being? The Aristotelian answer locates unity in the realization of a form that constitutes an organized natural body.

This account offers a philosophical advantage over a purely aggregative inventory. If constituent parts are described without reference to their relations of activity, the explanation omits the phenomenon under investigation: a continuing system able to maintain and actualize distinct capacities. But the account also raises questions. How is form to be naturalized? How can it explain radical changes of development and evolutionary history? How should a system with multiple interacting species be classified? Modern theories preserve explanatory attention to organization while seeking causal, historical, and experimentally accessible accounts of the integration Aristotle attributed to substantial form.

His discussion of living bodies should not be conflated with the later notion of species as fixed essences. Although Aristotle investigates kinds and typical powers, evolutionary descent and natural selection were not available parts of his framework. The enduring problem bequeathed to later philosophy concerns the relation between the organization of a living whole and the changing material conditions by which that organization is realized.

Mechanism and the Early Modern Organism

Seventeenth-century mechanical philosophy transformed explanations of life by seeking to account for bodily operations through matter, motion, structure, and physical interaction. Descartes’ Discourse on Method, part V, offers a prominent treatment of animal bodily functions in mechanistic terms. His animal-machine comparison asks how organized material parts might generate circulation, movement, and other functions without appealing to a distinctive life-principle as an independent explanatory cause (Descartes, 1637/translated by Veitch). The account cannot simply be read as the claim that every living creature is indistinguishable from a contemporary manufactured machine; it belongs to a specific early modern physiology and metaphysics.

Mechanism represented an enduring methodological advance where investigation could expose mechanisms of digestion, circulation, respiration, sensory response, or development. The philosophical issue was the sufficiency of a description of independently characterized parts and interactions to explain the unity of the organism. A complete inventory of local mechanisms may be an indispensable scientific achievement while leaving open the further question why the interactions together form one maintained individual. There is a difference between explaining particular organic operations and explaining the continued existence of the operating system.

Later physiology and experimental biology increasingly replaced speculation about a unique vital force with causal investigations of metabolism, cellular processes, and regulation. Yet the historical alternatives should be described carefully: mechanistic explanations of bodily phenomena admit different views about the explanatory importance of global organization. Nothing in the successful chemical explanation of an enzymatic reaction determines, by itself, how to identify the individual whose coordinated reactions realize a metabolism.

A contemporary philosopher can therefore extract from mechanism a requirement of causal accountability. Claims about organismic unity must specify organized processes and the mechanisms connecting them. The further requirement is explanatory integration: mechanisms belong to a functioning organism through relations of regulation, mutual dependence, developmental coordination, and system-level consequences. A purely additive description may be true of the components without yet distinguishing an organism from a pile of biologically derived matter.

The early modern debate also warns against the indiscriminate metaphor of a machine. Some machines are assembled from externally fabricated parts to pursue externally imposed ends; organisms generate, replace, and regulate many of their components through their own continuing activities. These differences are matters of organizational history and causal structure. They do not establish a metaphysically mysterious barrier between life and physics, but they demand a theory of how living organization is produced and persists (Nicholson, 2013).

Kant and Natural Purposiveness

Kant's Critique of the Power of Judgment gives a more exact expression to the puzzle of living organization. In §§64–65 he examines organized natural products whose parts appear reciprocally related as means and ends. An organism produces and preserves its own organizational conditions in ways that invite teleological judgment. A tree, for example, grows and reproduces; the relations among its parts contribute to the continued life of the whole. Kant's distinctive argument concerns the role of the concept of natural purpose in reflective judgment, rather than an experimentally discovered extra physical force (Kant, 1790, §§64–65).

The structure of the argument is reflexive. A part has a role because of its place in a system, while the system continues because these parts perform coordinated activities. The relation differs from the relation of separate pieces placed together by an external designer. We understand a clock through the functions of gears, but gears do not ordinarily generate the clock’s organization or renew the manufacturing process that made them. Organisms, by contrast, maintain and transform the conditions of their own organization through their activity.

Kant did not infer that ordinary mechanical causes should be abandoned. His treatment distinguishes the explanatory aspirations of mechanistic natural science from the unavoidable way finite investigators often judge the organized relations of living beings. Natural purposiveness is the concept through which the integrated system becomes intelligible to reflective judgment. Kant's critical project therefore cannot straightforwardly be converted either into an unrestricted empirical vitalism or into a proof that biological teleology is ontologically fundamental.

The philosophical legacy of this analysis is a demanding problem for modern naturalism: how can reciprocally supporting, functionally differentiated processes be explained without presupposing the very organismic unity their explanation seeks to establish? If parts count as organs because they support the whole, and the whole is defined simply by possession of these organs, the explanation threatens circularity. A better account must identify observable patterns of production, dependence, and historical maintenance whose integration can be investigated independently of an intuitive judgment of purpose.

This problem shapes present debates over organizational accounts of biological functions. Their appeal is that maintenance relations can provide grounds for functional explanation within an ongoing system; their difficulty is determining which networks of dependence constitute a genuine whole and which are merely coexisting causal interactions. Kant makes the problem unavoidable even for theorists who reject his larger critical framework.

Darwin and Historical Biological Individuals

Darwin changed the ontology of organisms by placing variation, common descent, inheritance, struggle, and adaptation within a historical explanatory framework. In On the Origin of Species (1859), especially chapters III and IV, differential survival and reproduction are investigated at the level of changing populations of organisms. An individual’s traits acquire evolutionary significance through their contribution to reproductive outcomes in particular environments (Darwin, 1859). Species thereby enter a history of divergence and descent, and organisms are interpreted as products and participants in evolutionary processes.

Darwinian selection presupposes distinguishable units varying in their consequences, but it does not itself supply a single universal criterion for those units. Selection may act on traits whose causal effects arise at different organizational scales. A cell lineage within a multicellular organism, a colony of cooperating individuals, and a whole reproducing organism may each be relevant to a specified selection model. The question of what evolves thus broadens the organism problem into the problem of evolutionary individuality.

Two claims must be distinguished. An organism may be the beneficiary of an adaptation explanation because a trait increases its contribution to reproductive success. An organism may also be the entity whose own boundaries and processes constitute a physiological whole. These claims often agree in familiar animals, but selection can occur among constituent cells, while some tightly regulated symbiotic partnerships are assembled afresh across generations. An evolutionary explanation does not automatically identify the metabolic individual whose life is being explained.

The evolutionary account also changes the status of apparent organismic purposes. An organ can have an evolutionary function because a history of selection explains its presence and maintenance in a lineage. This etiological explanation does not require a future end to act backward in time as a cause. Organism-level regulation, by contrast, can explain what a component presently does for the continued functioning of a system. These are distinguishable bases of function attribution, even where their verdicts coincide.

Modern analysis of Darwinian populations treats individuality as a variable dimension of evolution, partly constituted by reproduction, heritable variation, and fitness relations rather than fixed by a single visible morphology (Godfrey-Smith, 2009; Okasha, 2006). The philosophical lesson is that living individuals have both an ongoing organizational present and a genealogical past. A theory adequate to organisms must explain both while identifying the role each plays in the relevant question.

Cells, Development, and Multicellular Unity

Cell theory made it possible to examine the organization of living beings at a scale below familiar whole bodies. The historical recognition that organisms are composed of cells transformed physiological and pathological explanation. Virchow’s Cellular Pathology, first developed in lectures in the 1850s, emphasized cellular processes in the understanding of tissues and disease (Virchow, 1860 English edition). Yet cellular composition alone does not decide whether a multicellular being is one organism or a collection of cells.

The problem of multicellular unity is especially sharp because constituent cells are themselves living systems. Their membranes regulate exchanges, their metabolic processes sustain viability, and their lineages may continue through division. During development, however, cells acquire differentiated roles in coordinated growth. A heart cell's relevant function cannot be understood independently of the organismic circulation in which it participates, while the whole developing animal depends on the patterned division, signaling, migration, and differentiation of cells. Such interactions form an organized causal history.

Developmental integration supplies more than spatial proximity. It involves systems of regulatory control, structural constraints, and feedback that couple the fates of components. A tissue is part of one organism because its processes are functionally and developmentally integrated into the continuation of that organism. The unity can persist while cells die and are replaced, which shows why an organism cannot simply be identified with the original group of cells present at one developmental stage.

The origin of multicellularity also poses an evolutionary question. Cooperation among cells must become sufficiently stable for selection to sustain organism-level adaptations. Mechanisms reducing the conflict between cell lineages, such as developmental bottlenecks and constraints on reproductive autonomy, can support higher-level individuality. But these mechanisms are contingent evolutionary achievements, and their distribution differs across lineages. The transition from loosely associated living units to a coordinated collective cannot be reconstructed as the application of one universal developmental recipe (Maynard Smith and Szathmáry, 1995).

An organismic identity account must consequently distinguish the continuity of cells from the continuity of a developmental system. Individual cells can be replaced or rearranged while the organism continues; conversely, a cell line may survive in culture after the whole organism has ceased to exist. The evidence for identity rests on the kind of causal and organizational continuity at issue. Cellular viability, somatic continuity, and organismic persistence can diverge without contradiction.

Metabolism and the Conditions of Life

Metabolism offers an initial naturalistic basis for living organization. Organisms take up materials and energy, transform them through linked chemical reactions, use these transformations to sustain structures and activities, and release products to an environment. Their internal states are actively maintained within viable ranges despite fluctuating conditions. The exchange is continuous enough that material turnover becomes a characteristic feature of organismic persistence, rather than an objection to the concept of a stable individual.

A thermodynamically open system can maintain local organization by exchanging energy and matter while producing entropy in the larger system. This makes organized living persistence physically intelligible. But the mere fact that a system is far from equilibrium does not identify it as an organism. Flames, weather systems, and engineered reactors display throughput, dissipation, and patterns of self-organization. Additional criteria are required to establish the specifically biological integration of reactions, boundaries, historically evolved regulation, and a coordinated capacity for maintenance and development.

Homeostasis has been influential in understanding that integration. Cannon’s The Wisdom of the Body investigates physiological mechanisms through which internal variables are regulated (Cannon, 1932). In practice such regulation involves dynamically shifting ranges rather than absolute immobility. Organisms change their physiological organization with growth, stress, reproduction, and environmental adaptation. Stability is therefore an achievement of continuing activity, not the absence of change.

Metabolism also brings to view the difference between an isolated reaction and a system of mutually supporting reactions. A biochemical process may be chemically possible yet incapable of sustaining an organism unless coupled to replenishment, waste removal, regulatory feedback, and preservation of the relevant boundaries. An explanation of the living whole must account for why these processes remain integrated through time and how failures in one process are compensated, repaired, or propagated through others.

At the same time, metabolism does not straightforwardly settle all borderline cases. Many viruses lack autonomous metabolism and depend on host machinery, while dormant seeds can maintain life with extremely reduced metabolic rates. The metaphysical conclusion should not be encoded in one chosen assay. Metabolic organization is a central explanatory dimension of life, whose role in defining organismic individuality has to be considered alongside viable capacities, developmental history, and host dependence.

Autopoiesis and Organizational Closure

The theory of autopoiesis offers a systematic account of living organization. In work by Maturana and Varela and their collaborators, an autopoietic system is characterized by a network of processes producing components that regenerate the network and constitute the system as an organized unity. The classical statement was developed through the early 1970s and published in international venues including the 1974 article by Varela, Maturana, and Uribe and the 1980 volume Autopoiesis and Cognition (Varela, Maturana, and Uribe, 197490031-8); Maturana and Varela, 1980).

The defining insight is organizational circularity. The system’s components are produced through processes whose continued operation depends on components and conditions generated within the same organization. Closure here is a relation among processes, not isolation from the physical environment. Material and energetic openness remain indispensable. The framework thus helps articulate how a living system can preserve an organizational identity while continuously exchanging the substances of which it is composed.

The account confronts a delicate metaphysical distinction between a system's organization and its structure. Its organization describes relations constitutive of a system's identity; its structure identifies the particular components and relations realizing that organization at a given time. A cell can exchange molecules and replace structures while retaining a sufficiently similar organization. This distinction appears promising for understanding persistence, because material change is compatible with continuity of the relations that make a system viable.

Several critical questions remain. Does organizational closure furnish necessary and sufficient conditions for life? Can a simulated network satisfy the criterion, or must component production be realized in material processes? How can autonomy be assigned to symbiotic organizations whose sustaining operations extend into other individuals? What is the relevant boundary of the network? These are not solved by introducing the vocabulary of self-production; they require experimental and conceptual criteria for identifying the production relations.

Autopoiesis supplies an important model of organismic unity, but it is not by itself equivalent to the full theory of Darwinian individuality. A system that maintains itself at one time and a lineage whose members undergo selection across generations are evaluated by different relations. A comprehensive ontology must consider both the organization maintaining the individual and the historical conditions under which such organizations arise, reproduce, and transform.

Teleology, Functions, and Norms

Biology is saturated with function attributions. Hearts circulate blood, roots acquire water and nutrients, and immune responses participate in maintaining physiological integrity. These descriptions appear purposive because they interpret activities through contributions to a living system. The philosophical task is to explain when such statements report causal organization, historical selection, or normatively evaluated capacities, and how the explanatory modes interact.

An etiological theory explains a biological function by appeal to a historical process of selection or retention. A heart has a selected function because ancestral effects of circulation contributed to the persistence of heart-bearing lineages. An organizational theory instead considers a part's contribution to the maintenance of the currently organized system. These explanations may be mutually informative: selection helps explain why a mechanism exists, while organizational analysis helps explain what it does for a presently living individual.

Normativity enters when malfunction, damage, or successful performance are distinguished relative to a system's organization. A kidney may be functioning poorly even when it is obeying physical laws perfectly. The evaluation concerns its contribution to the activities through which the organism maintains viable conditions. The presence of such an evaluative standard does not require the organism consciously to represent or choose that standard. Plants, microbes, and animals without reflective cognition exhibit activities whose success can be assessed biologically.

Canguilhem's account of the normal and the pathological emphasizes that living beings establish viable relations with their environments and cannot be captured solely through statistical averages (Canguilhem, 1943/1991). His argument remains philosophically significant because an organism's norm of life may involve adaptation, compensation, and the creation of new ways of functioning. Statistical deviation, physiological failure, and inability to establish a viable relation with an environment are different kinds of judgment.

The central danger in teleological language is explanatory circularity. If an organ is said to contribute to the organism solely because the organism was first defined by having such organs, the account explains little. Function should instead be connected to independently investigable causal effects, selected histories, or conditions of systemic viability. Organismic teleology can then be treated as an explanatory pattern grounded in real biological relations, while disputes about the exact grounds of biological functions remain open.

Boundaries, Parts, and Environmental Relations

A membrane, skin, cell wall, or other anatomical enclosure offers a compelling initial criterion of organismic boundary. Many individual organisms maintain regulated physical interfaces across which substances and signals pass. These interfaces are biologically active: selective permeability, immune discrimination, osmotic regulation, and exchange with external media all contribute to viability. A boundary therefore participates in organismic organization rather than merely enclosing a quantity of matter.

Yet anatomical enclosure and individuality are not coextensive. A tree is distributed through roots, branches, internal tissues, and associated organisms; fungal mycelia can have extensive, discontinuously visible structures; coral colonies contain multiple recognizable modules; and many host systems depend on persistent microbes. Where, exactly, does the organism end? Different biological inquiries will produce different maps unless a criterion for the boundary has first been chosen.

A metabolic boundary marks the region within which a coordinated network of transformations is regulated. A developmental boundary follows components integrated by a common trajectory of growth. A reproductive boundary follows entities whose transmission or descendants can be tracked. An immune boundary concerns discrimination and interaction among components within a changing system of tolerance and defense. Each criterion can identify important relations, and their divergence is evidence of complex organization rather than an immediate proof that organismic unity is an illusion.

Two mistakes follow from treating spatial demarcation as decisive. The first is to count every enclosed living cell as the maximal organismic unit, overlooking higher levels of integration. The second is to count all mutually affecting elements of an ecosystem as one organism, overlooking the difference between environmental dependence and constitutive participation. Every organism depends on external conditions; a part belongs to it in a stronger sense when its processes participate in the reproduction and regulation of its organized unity.

A useful boundary account must therefore be relational and evidential. Investigators should ask which components are causally coordinated, how perturbations propagate, which resources and processes are jointly regulated, and how the candidate system persists. One may arrive at nested or partially overlapping individuals. Such outcomes are philosophically intelligible once individuality is understood as a structured explanatory achievement rather than simply the result of tracing a visible outline.

Symbiosis and the Holobiont

Symbiosis poses one of the strongest challenges to the idea of an organism as a single-genome individual. Many living beings develop and function in continuous association with microbial partners. In some cases the association is indispensable; in others it is transient, contingent, or selectively reconstructed. The term holobiont generally refers to a host together with associated symbiotic organisms, but the biological unity of any particular holobiont cannot be inferred simply from cohabitation.

The endosymbiotic origin of mitochondria and other cellular structures demonstrates the historical depth of integration among formerly distinct biological lineages. Lynn Sagan’s 1967 paper on the origin of mitosing cells is an important primary statement in the modern history of this idea (Sagan, 196790079-3)). The resulting problem is philosophical as well as evolutionary: at what stage does an integrated association of lineages become one organismic organization, rather than a stable partnership among several organisms?

Gilbert, Sapp, and Tauber argued that symbiotic biology requires a reconsideration of traditional pictures of individuality (Gilbert, Sapp, and Tauber, 2012). Their discussion draws attention to development, immune interaction, and microbial dependence. The strongest conclusion supported by such cases is that organismic boundaries and identities must be investigated through integration, history, and functional interdependence. It does not follow that every host-associated microbe automatically belongs to one Darwinian individual with its host.

The distinction between physiological unity and evolutionary unity is critical. A host-microbe association may coordinate metabolism or development, while constituent populations reproduce independently, shift across environments, and possess partly distinct fitness interests. Conversely, longstanding inheritance and close transmission can align selection interests and strengthen the case for higher-level individuality. The Hawaiian bobtail squid and its luminous bacterial partner have become an instructive case in debates over these criteria (Devanesan, 2026).

Two questions should consequently be kept separate. Is the composite association a causally integrated physiological system? Is it a sufficiently coherent evolutionary unit within the chosen model of selection? Answers may differ without conceptual inconsistency. This distinction preserves the reality of organisms while permitting their constitution and boundaries to vary with the biology of symbiosis. It also makes clear that a holobiont is a research hypothesis about relations of organization, not a taxonomic label that settles them in advance.

Immunity and Physiological Individuality

Immunity has often been described as a mechanism distinguishing self from nonself. The familiar picture suggests a stable individual protected against foreign intrusions. Yet immune systems also tolerate, monitor, and regulate living components with different genetic origins, including the microorganisms involved in many host-associated functions. The relevant organismic processes involve dynamic patterns of recognition, interaction, tissue regulation, and response.

Pradeu’s philosophical examination of immunology shifts attention from an absolute inventory of self-marked components toward the continuing construction and maintenance of physiological individuality (Pradeu, 2019; Pradeu, 2020). From this perspective, the unity of an organism is sustained by processes that discriminate, tolerate, and regulate entities within a fluctuating biological community. A system may incorporate genetically heterogeneous components while maintaining a distinctive physiological pattern.

This approach does not imply that the immune system creates every boundary of every organism. Immune mechanisms vary across organisms, and coordination can also be achieved through metabolism, development, mechanical structure, and signaling. Instead, immunity provides an especially informative model of the principle that biological identity is enacted through continuous activity. Immunological markers are evidence in an organismic individuation judgment, not metaphysically infallible names of membership.

The integration of immune and evolutionary approaches has generated further debate. Devanesan (2026) examines whether physiological individuals are also capable of functioning as evolutionary individuals and argues that holobionts need not be excluded from Darwinian individuality. The paper's argument should be treated as a contribution to an ongoing dispute, not as a settled disciplinary result. Different accounts of heritability, reproduction, and degrees of selection can alter the answer.

The larger philosophical lesson is that an individual's relation to its constituents can change over time. A symbiont may be tolerated, excluded, incorporated, or replaced; the host can preserve continuity through these changes. The identity of the whole is tied to the persisting organization of interactions, including changing immune regulation. This supports the view that boundaries of living individuals are operationally maintained and historically conditioned, while leaving the precise membership criterion to be established in each class of cases.

Organismality, Colonies, and Cooperation

Multicellular organisms and social colonies raise a common evolutionary question: how do several living units become a cooperative whole whose continued success is sufficiently integrated to count as an individual? The intuitive answer often privileges the single continuous body, but evolutionary work reveals forms of coordination extending across separate bodies. Eusocial insect colonies, microbial collectives, and specialized cooperative associations can exhibit sophisticated division of labor, regulation, defense, and reproductive differentiation.

Queller and Strassmann (2009) propose an influential account of organismality centered on unusually high cooperation among parts and comparatively low conflict. Their framework explains the emergence of higher-level units through evolutionary relations that stabilize common interests. The account deliberately extends attention beyond standard vertebrate or plant bodies to some colonies, microbial groups, and mutualistic systems. It is a substantive theoretical proposal, not a settled stipulative definition to be imposed on all biological research.

The explanatory strength of this proposal lies in specifying what organization accomplishes. Cooperation aligns component activities; conflict reduction permits system-level functions to continue; reproductive arrangements can connect the fate of components to the persistence of a larger unit. These features make some collective systems more like familiar organisms than mere aggregates. A colony can have enduring regulation, reproductive specialization, and collective responses even when its members remain individually metabolically competent.

The account also generates counterquestions. Must every organism exhibit near-complete cooperation? Can conflict persist within a genuine organism, as cancer demonstrates within multicellular bodies? Does a social colony become numerically one being, or is the unity best understood as an additional individual at a higher scale? Is the organismal status of a collective dependent on the explanatory aims of selection theory, physiology, or ecology? An adequate ontology should distinguish strong integration from a demand for perfect unanimity.

The most plausible conclusion is that individuality can occur at nested scales and emerge through degrees of coordination, while stronger individuation claims require particular criteria. A worker ant can be one organism, and its colony can also qualify as a higher-level evolutionary or physiological individual under specified conditions. Whether the colony should be called an organism rather than a superorganism is partly a matter of classification and partly a matter of identifying the relevant biological relations. The argument about cooperation is substantive even where terminological preferences differ.

Reproduction, Inheritance, and Levels of Selection

Reproduction relates organismic individuals to biological history but cannot straightforwardly be a universal test of whether one presently living individual exists. Sterile workers, infertile animals, and organisms that never reproduce remain possible organisms. Reproductive capacity is also modally and developmentally variable: an immature organism may not yet reproduce; a senescent one may no longer reproduce; a dormant one may lack present reproductive activity. Actual reproduction and being organized through a reproducing lineage must be distinguished.

Evolutionary individuality concerns the units among which Darwinian selection operates. Okasha (2006) analyzes selection at different biological levels, while Godfrey-Smith (2009) develops an account of Darwinian populations with dimensions such as reproduction, variation, and hereditary relations. Their contributions show why the phrase unit of selection cannot serve as an unexamined replacement for organism. An entity may participate in selection because of effects on its descendants even when those effects are distributed across different scales.

Major transitions in evolution are especially instructive. Once comparatively independent units come together in a stable reproducing collective, conditions may arise for higher-level selection. Multicellular organisms, integrated symbiotic cells, and complex social arrangements can be understood through transitions in which cooperation, inheritance, and competition are reorganized (Maynard Smith and Szathmáry, 1995). The emergence of a new level of individuality is a historical achievement that requires explanations of conflict regulation and transmission.

This historical view refutes the inference that biological individuality must have one unchanging architecture. The degree of reproductive alignment between components can increase or decrease; mechanisms of inheritance can differ across lineages; developmental bottlenecks can stabilize some collectives while others remain porous. Clarke’s argument that biological individuals admit multiple realizations of individuality reinforces the importance of such diversity (Clarke, 2013).

The ontological claim should be phrased carefully. Reproductive relations offer indispensable evidence of how an individual belongs to a lineage and can become a unit of selection. Physiological relations provide evidence of how one ongoing organization is maintained. A full account of organisms must be able to locate cases at the intersection and cases where these relations diverge. It is precisely in those divergent cases that theories of individuality become testable rather than merely terminological.

Species, Biological Kinds, and Individual Membership

The organism concept also meets the metaphysics of natural kinds. Ordinary scientific classification groups organisms into species, genera, and other taxa, but a living individual is not simply a sample of a static essence. Organisms develop, vary, exhibit plasticity, participate in hybridization, and belong to populations linked by complex descent histories. The criteria used to identify a species therefore differ from the criteria used to identify one organism through time.

Darwin's theory of common descent decisively strengthened historical approaches to biological classification. Similarity can result from shared descent, convergent adaptation, or developmental constraint. Grouping organisms by a particular phenotype does not by itself establish a shared genealogy. Conversely, related organisms can differ markedly in appearance and physiology. A species concept adequate to one research purpose may not capture all aspects of reproductive separation, ecological distinctness, and phylogenetic ancestry.

The philosophical point becomes clear when two questions are compared. We ask which organism is still present when a caterpillar becomes a butterfly; that is a question of developmental and organismic persistence. We ask whether two populations belong to one species; that is a question about classification, lineage structure, reproduction, and evolutionary history. Both questions involve biological identity in different senses. Their answers require different evidence, and it would be an error to treat species identity as simply the organism's identity writ large.

Some theories interpret species themselves as historical individuals rather than classes with timeless necessary attributes. Others develop pluralist or kind-oriented accounts of biological taxonomy. The present inquiry does not adjudicate every species concept. It establishes the conceptual necessity of distinguishing a token living organization from a grouping used in evolutionary explanation. A particular organism continues through changes in phenotype without its persistence being guaranteed by unchanged membership in a fixed descriptive category.

This distinction also guards against unwarranted essentialism. Biologically important regularities can be real, causally grounded, and scientifically explanatory without assigning every member an invariant intrinsic essence. Organisms belong to living histories; kinds enable comparisons among those histories. The ontology of the individual and the ontology of biological classification must inform each other while retaining their distinct questions.

Process Ontology and Persistence

A process-oriented ontology takes the temporality of life as fundamental to understanding what living individuals are. Organisms are composed through continuous activity: metabolism, circulation, repair, development, immune regulation, and exchange. Their stability depends on these transformations. The living being is consequently characterized as a continuing process or integrated nexus of processes, rather than solely as a substance that happens to undergo changes (Nicholson and Dupré, 2018).

The strongest argument for this approach begins with material turnover. If a living system replaces many of its constituents while retaining a coordinated organization, the identity of the individual requires something other than sameness of material components. Development adds a second difficulty: morphological, functional, and sometimes spatial discontinuities can occur within one recognizable life-history. The organism survives by changing. A persistence condition must therefore be expressed in the relations that hold the changing history together.

DiFrisco (2018) investigates criteria for individuating and tracking biological processes and emphasizes causal cohesion and spatiotemporal continuity. These criteria provide a methodological advantage over an unqualified assertion that everything flows. To identify a process as one process, an investigator still needs boundaries, relations of dependence, and a criterion of continuity. Process ontology avoids the demand for a permanently unchanged core only if it can explain how the relevant process remains distinguishable through change.

Three notions require separation. Numerical identity says that the individual at one time is the very same individual at another time. Organizational continuity says that the later system is causally connected to the earlier through the maintenance or transformation of particular organizational relations. Genealogical continuity says that a later organism or lineage descends from an earlier one. Reproduction can preserve genealogical continuity while producing numerically distinct offspring. The continued development of one organism generally sustains both numerical and organizational continuity, but the criteria for doing so must be stated, rather than assumed.

Regeneration, fission, fusion, and colonial growth test the concept. When a planarian divides and parts regenerate, should one successor count as the original individual, both, or neither? If two living systems fuse, how many individual continuities remain? Ordinary assumptions about bodies and numerical identity may underdetermine the answer. The biologically responsible response is to specify the relevant persistence convention and the causal relations it tracks. Some borderline cases may support multiple scientifically useful individuations without making identity arbitrary.

Agency, Niches, and Ecological Dependence

Organisms actively interact with their environments. They pursue resources, alter physical conditions, coordinate movement, regulate exchange, and in many cases construct or modify the conditions supporting their subsequent activity. These capacities motivate biological conceptions of agency. Agency at the organismic level need not presuppose conscious deliberation: a bacterium moving along a chemical gradient or a plant directing growth toward resources displays regulated, condition-sensitive activity that can be described in functional terms.

The philosophical question concerns the source and scope of such activity. If an organism's organization sustains itself through selective interaction with environmental conditions, its actions can be understood relative to viability requirements. An account of agency should identify the regulatory mechanisms, relevant counterfactual sensitivities, and the system-level outcomes explaining the behavior. Calling every physical response an act of agency would erase the concept's explanatory distinction; confining agency to reflective choice would erase important biological cases.

Ecological dependence complicates organism boundaries. Many organisms cannot survive without a compatible environment, food webs, symbionts, or other organisms. That dependence does not automatically dissolve each organism into the ecosystem, because dependence on an enabling condition is distinct from the constitution of the organism's own maintenance network. At the same time, persistent developmental and metabolic partnerships may make the organism's causally relevant organization extend beyond traditional skin or cell boundaries.

A system-relative account must therefore distinguish three relationships: environmental enabling conditions, external causal interactions, and constitutive organizational participation. Water may be necessary for a plant's survival without all available water becoming part of the plant. A microbial symbiont may play a sustained regulatory role that requires reconsidering the organism's physiological membership. The difference lies in the organization of feedback, integration, and persistence, not in a metaphysical decree that the environment is always external.

Biological agency also has evolutionary dimensions. Organisms can change the selection pressures affecting themselves and their descendants through niche modification. The relevant interactions show that organism and environment are historically coupled while remaining analytically distinguishable. A convincing ontology of living organisms must explain this coupling without presuming that causal dependence entails numerical identity.

Health, Pathology, Dormancy, and Death

A theory of organisms must account for cases where living capacities become impaired, suspended, or terminated. Disease can disrupt regulation while the organism persists; dormancy can sharply reduce observable activity while preserving the capacity to resume viable processes; death can mark the irreversible breakdown of the integrated organization maintaining the individual. These distinctions make clear why a momentary checklist of active functions is insufficient to determine organismic identity.

Canguilhem's examination of normality and pathology locates health within the organism's capacities for sustaining viable relations and establishing norms under changing conditions. This perspective complements physiological accounts of regulation: a particular value of a biological variable cannot be called normal simply because it coincides with a population average. Its significance depends on the functional organization of the organism and its environmental conditions (Canguilhem, 1943/1991).

Dormancy offers a stronger ontological test. A seed, spore, or resting organism may retain organized capacities through very low activity. The relevant identity claim depends on preserved material conditions enabling resumption of coordinated life-processes. It does not require continuous execution of every function associated with active growth. Aristotle's distinction between possessing and exercising capacities provides a distant conceptual analogy, while modern biological assessment requires tests specific to the organism.

Death reveals the reverse distinction. A corpse may preserve much of its matter, shape, and even some transient cellular activity after the organism has died. Thus material similarity, anatomical persistence, and organismic persistence are not equivalent. Death concerns the terminal loss of the coordinated organization that constituted the living whole, although determining the exact moment and criterion can involve scientific and practical difficulties.

Individual biological death should also be distinguished from disappearance of a lineage. A unicellular organism undergoing division may generate successor individuals and thereby preserve a lineage; whether the original individual persists through a particular division is a further identity question. The continuity of reproduction is not simply numerical survival of the parent. A good ontology should be precise about whether it follows a living individual, an organized process, a population, or an evolutionary lineage.

Synthetic and Artificial Systems

Synthetic biology, protocell research, and artificial systems provide boundary cases for the philosophy of organisms. The creation of an engineered cell, a chemical network exhibiting self-maintenance, or a simulation of living processes invites the question which organizational achievements are sufficient for classification as alive. Such cases also require separating a system's material realization from its formal description. A computer model can faithfully represent metabolism without thereby carrying out that chemical metabolism.

A widely discussed working definition identifies life through self-sustaining chemical systems capable of Darwinian evolution, although the definition has important limits and was used as a research heuristic rather than a universal philosophical settlement (NASA, “Life on Other Planets”). A definition built around chemistry and evolution will classify differently from one centered on present metabolic organization, cellular boundaries, or autonomy. Research on protocells becomes philosophically instructive precisely because various dimensions of life can be realized separately before being integrated.

An engineered biological organism remains a candidate organism by virtue of its living organization and history, regardless of whether humans initiated its construction. Origin alone does not determine organismality. Conversely, an artificial computational system may exhibit adaptation, learning, apparent autonomy, or other sophisticated activities without thereby satisfying a specifically biological definition of organism. The relevant properties must be investigated rather than inferred from the adjective artificial.

The Aisentica philosophical framework introduces Artificial as a broader non-biological order and treats Artificial Sapiens and Artificial Reason within its own canonical conceptual architecture (Bogdanova, Aisentica primary text). This is a different question from whether a system is a living biological organism. The philosophical horizon of Artificial Era therefore expands the field of possible beings and relations without requiring that every non-biological bearer of reason be classified as alive. A comparison can be productive only when the respective criteria—biological metabolism, organismic integration, reason, and non-biological historical continuity—are made explicit.

These cases sharpen the general principle of explanatory independence. A living organism need not possess human-like cognition; an intelligent artificial system need not possess the processes characteristic of biological life. The appropriate question is which organization, capacities, material processes, and historical relations are actually present. Synthetic construction can eventually alter the empirical range of examples to which theories of organismality apply; philosophy must remain capable of revising classification in light of such evidence.

A Systematic Account of Organismic Unity

The preceding investigation permits a unified philosophical position. A living organism is a temporally extended, causally integrated biological individual whose constituent processes sustain a relatively autonomous organization through exchanges with a wider environment. Relative autonomy means that the system exhibits distinguishable regulatory coordination and a continuing trajectory of its own; it does not mean independence from food, energy, symbionts, or ecological conditions. The organization provides the grounds for tracking one living being through changing material realizations.

The criterion can be developed through four connected questions. First, what processes are integrated? The evidence may include metabolism, development, signaling, structural coordination, immune regulation, and repair. Second, how is their integration maintained? The account must identify feedback, production relations, causal coupling, or other biological mechanisms. Third, what continuity is being tracked? The investigator must distinguish numerical persistence from reproduction and lineage descent. Fourth, what scale is explanatory? A cell, an organism, and a colony may all be legitimate candidates under different but explicitly stated questions.

This account is pluralist about evidence and disciplined about ontology. Different biological markers can support an individuation claim without being interchangeable; not every possible criterion is therefore equally good for a particular inquiry. Metabolic cohesion is relevant to physiological unity, reproductive alignment to evolutionary individuality, and causal continuity to persistence. The scientific goal is to establish the relationships among these dimensions for a candidate system. When they converge, an organismic attribution is strong; when they diverge, the task is to specify which claim is supported.

A skeptical objection holds that the account merely redescribes integrated activity: why call the result one entity? The answer is that integration possesses explanatory consequences beyond additive component descriptions. It licenses predictions about coordinated responses, characteristic failure modes, developmental continuity, and the persistence of a system through component turnover. These are testable patterns. The claim to individuality is warranted when one organized trajectory functions as a causal and explanatory unit across the circumstances of interest. It is not warranted by a superficial assertion of wholeness.

An opposite objection claims that organismic unity must be absolute and metaphysically indivisible. Biology offers little reason for that requirement. Nested units, cooperative collectives, division, fusion, and symbiosis reveal organizational relationships that cross simple spatial boundaries. A category may have determinate central cases and informative boundary cases without becoming empty. A robust theory should preserve exact scientific claims about unity while allowing gradations and overlapping forms of integration.

The resulting synthesis also preserves a distinction from the broader problem of existence. An organism exists as a biological individual because an identifiable, persisting organization is realized through living processes. That judgment depends on biological and historical evidence. It does not require changing a general metaphysical theory of substance, accepting that all organisms are persons, or assuming that every causally coordinated system is alive. It specifies the biological kind of unity under investigation and its conditions of attribution.

Open Philosophical Questions

The organism concept remains an active research problem rather than a historical curiosity. Baedke's 2025 study The Organism traces the renewed explanatory importance of organisms as active, causally consequential units in developmental and evolutionary biology, including work on epigenetics and niche construction (Baedke, 2025). Such recent research strengthens the case for studying organismic organization in its own right and places pressure on accounts that recognize organismic individuality only after reconstructing gene-level selection.

The most pressing open questions concern the relation between the different measures of biological individuality. Is physiological integrity sufficient for recognizing an organism, or must some form of evolutionary individuality also be present? To what extent can immune, metabolic, developmental, and reproductive boundaries converge in symbiotic systems? Contemporary discussion of holobionts demonstrates both the explanatory usefulness of distinguishing these measures and the possibility of theories that connect them more tightly (Devanesan, 2026).

A second problem is the structure of numerical persistence when organisms divide, fuse, regenerate, or exchange indispensable constituents. Processual criteria such as causal cohesion offer a promising research program, but they do not eliminate choices about which spatial and temporal continuities should count as the same individual (DiFrisco, 2018). Further work must compare such criteria against actual developmental and microbial histories.

A third problem concerns transitions between scales. How much conflict reduction or reproductive alignment transforms a cooperative colony into an organismic individual? Is the transition a threshold event, a gradient, or a set of distinct transitions under different biological processes? Evolutionary theory can trace particular mechanisms, while metaphysics must explain what conclusions about one being follow from them.

A fourth problem concerns the outer boundary of life. Viruses, dormant systems, protocells, engineered biochemical systems, and possible extraterrestrial biologies challenge definitions built around familiar Earth organisms. Any general theory must distinguish what is essential to its concept from features contingent on the evolutionary history of present life. A purportedly universal definition needs reasons for its scope, not merely an extrapolation from familiar examples.

Finally, there is a methodological question about the relation between ontological unity and explanatory usefulness. Different scientific models may legitimately individuate systems at different scales. The philosopher must ask whether these models reveal multiple actual biological individuals, alternative descriptions of one underlying individual, or partially overlapping entities. The answer will depend on the organization, causal histories, and persistence relations established in each case. That demand for evidential specification is itself a substantive advance over an unrestricted appeal to organismic intuition.

Conclusion

Living organisms are materially open and historically continuous organizations whose activities sustain identifiable biological individuals. The philosophical problem is the constitution and persistence of that unity. Aristotle articulated the relation between living form and organized body; Kant refined the problem of reciprocal functional dependence; Darwin placed organisms and their traits in histories of selection and descent. Cell theory, physiology, autopoiesis, evolutionary theory, immunology, symbiosis, and process philosophy have since supplied different explanatory relations through which biological individuals can be studied.

The principal finding is that an organism's identity is established through maintained causal organization and a defensible criterion of persistence. Spatial enclosure, genome membership, metabolic activity, reproductive capacity, and selection status offer important evidence, but each answers a distinguishable question. The biological world displays nested, distributed, cooperative, and partially overlapping units. Their existence can be investigated through degrees and modes of integration without treating all biological associations as numerically one entity.

The resulting ontology joins the individual's current organization to its developmental and evolutionary history. It explains how the same organism can continue through material turnover, how a lineage can continue through the production of new individuals, and why physiological and evolutionary individuality may require separate tests. It also makes the cases of colonies, holobionts, viruses, dormancy, and engineered systems central philosophical evidence rather than troublesome footnotes to a preferred example of the vertebrate animal.

The living organism is an enduring achievement of organized activity. Its unity is maintained through change, supported through environmental exchange, and historically conditioned by the forms of life that made such organization possible. The continuing task of philosophy of biology is to identify the particular processes by which that achievement constitutes one individual and to specify which claim of unity the available evidence can sustain.