Introduction
What does biological explanation explain, and how must it be structured to do so?
The APS framework addresses these questions in two stages. The article Biological Explanation — What Needs to Be Explained establishes the target of explanation as viability-oriented, constraint-closed biological organisation. It shows that biological inquiry must account not only for how systems behave, but for how they sustain themselves as living systems.
This article develops the second stage. It specifies the structure of explanation required once that target is identified.
If biological systems are organised around sustaining their own viability, then explanation must be organised accordingly.
This article forms part of a two-step account of biological explanation in APS. For the full entry sequence, see:
→ How APS Explains Life — A Two-Step Guide
From Target to Structure
Identifying viability-oriented organisation as the target of explanation establishes the conditions that explanation must satisfy.
Such systems maintain themselves through ongoing activity, reorganise under changing conditions, persist across time while undergoing transformation, operate across interacting spatial and temporal scales, and exist only through continuous interaction with their environments.
An adequate explanation must therefore account for all of these features together.
APS organises this requirement into a coherent explanatory structure.
The Core Explanatory Dimensions
Biological explanation in APS is structured around three co-constitutive dimensions: agency, process, and scale.
Agency refers to the viability-oriented activity through which systems sustain themselves. Process refers to the dynamic organisation through which this activity unfolds. Scale refers to the spatial and temporal extent across which processes are coordinated.
These are not separate components or causal layers. They are analytic projections of a single, integrated organisation.
Explanation proceeds by showing how viability-oriented organisation is enacted through the continuous interaction of these dimensions.
The Temporal Structure of Biological Organisation
Biological organisation unfolds across time in a structured way.
Persistence refers to the present-time maintenance of viability. Adaptation refers to the reorganisation of activity under changing conditions. Evolution refers to the long-term transformation of organisation across generations.
These are not distinct domains but temporally differentiated expressions of the same system.
Persistence is the condition for adaptation. Adaptation enables persistence across change. Evolution is the historical accumulation of adaptive reorganisation.
Biological explanation must therefore integrate immediate regulation, ongoing reorganisation, and long-term transformation within a single framework.
System–Environment Coupling
Biological organisation does not exist in isolation.
In APS, systems and environments are co-constituted through ongoing coupling. The environment is not a fixed external domain but a relational field defined by its integration into viability-oriented organisation.
Environmental conditions matter only insofar as they are taken up into biological organisation. Systems actively modify the conditions that sustain their own persistence. Adaptation involves the reorganisation of system–environment relations.
Explanation must therefore account not only for internal processes but for the relational dynamics through which systems and environments co-produce one another.
Multi-Scale Causation
Causation in biological systems is distributed across scales.
Processes at molecular, cellular, organismal, and ecological scales are not arranged in a hierarchy of levels, but are dynamically coupled within a single organisation.
APS replaces level-based explanation with multi-scale causation, in which causal influence propagates across scales, processes constrain and enable one another reciprocally, and no single scale has intrinsic explanatory priority.
Biological explanation must therefore trace how organisation is maintained through coordinated activity across scales.
Integrating Mechanism, Function, and Evolution
APS does not replace existing explanatory approaches. It clarifies their roles within a broader structure.
Mechanistic explanations describe how processes are organised. Functional explanations describe how those processes contribute to viability. Evolutionary explanations describe how such organisation is transformed across time.
These are not competing accounts but complementary aspects of a unified explanatory system.
Their integration depends on recognising viability-oriented organisation as the common reference point.
Explanatory Adequacy
An explanation is biologically adequate when it accounts for how a system maintains its own viability, how its processes contribute to that maintenance, how it reorganises under changing conditions, how its organisation is sustained across scales, how it is transformed across generations, and how it is coupled to its environment.
Explanations that omit these dimensions may remain useful, but they are incomplete as biological explanations.
APS makes these requirements explicit.
APS as Explanatory Grammar
Taken together, these elements constitute a unified explanatory grammar.
APS does not add new entities to biology. It clarifies how explanation must be organised to account for the systems biology already studies.
In this sense, APS is both descriptive and normative. It describes how biological systems are organised and specifies what counts as a complete explanation of such systems.
The framework mirrors its subject matter. Just as living systems are constraint-closed organisations, APS is a conceptually integrated system in which explanatory elements support and constrain one another.
Conclusion
The APS framework distinguishes between what biological explanation must explain and how it must be structured to do so.
The first task is to identify viability-oriented, constraint-closed organisation as the defining feature of living systems. The second is to articulate the explanatory structure required to account for such organisation.
By integrating agency, process, scale, temporal dynamics, and system–environment coupling, APS provides a framework in which biological explanation becomes coherent and complete.
Key Point
APS defines the target of biological explanation as viability-oriented organisation and provides the explanatory grammar required to account for how that organisation is sustained and transformed across time.