Life is one of the most familiar concepts in biology and one of the most difficult to define. Biological accounts commonly characterise living systems through such features as metabolism, growth, reproduction, homeostasis, responsiveness, evolution, or particular forms of organisation. APS does not treat the presence or absence of any such list as sufficient to establish its own account. Instead, it asks what kind of organisation makes the continued functioning of a living system possible despite continual material and environmental change.
APS defines life as viability-oriented, constraint-closed organisation. This is a canonical commitment of the framework. It identifies living organisation in terms of the ongoing modulation of constraints through which a system actively maintains and re-establishes the conditions of its own persistence. The definition provides the starting point from which APS analyses biological activity, continuity, and organisation; it is not presented as a comparator-validated uniquely correct definition of life.
Living systems continually undergo change. Materials are replaced, structures are renewed, internal conditions fluctuate, environments vary, and developmental trajectories unfold. Yet living organisation can remain continuous through these transformations. APS describes this continuity as organised persistence: persistence achieved through ongoing organisation rather than through material or structural invariance.
Agency, Process, and Scale provide complementary analytic projections of this living organisation. Agency concerns what living systems do in maintaining and re-establishing viable conditions. Process concerns how continuity is maintained through change. Scale concerns where persistence is organised across spatial and temporal extents. These are analytic projections rather than separate components, ontological levels, or universally required explanatory steps. Their relevance and explanatory priority depend on the biological target under investigation.
Constraint closure is therefore not an additional feature appended to the APS definition of life. It is part of the definition itself. Living organisation depends on constraints that participate in maintaining the conditions under which the organisation that sustains them can continue. APS places this constraint-closed organisation in a viability-oriented formulation: what is maintained is not an unchanging state but an organisation capable of continuing to function within conditions compatible with its persistence.
This formulation also provides the APS starting point for analysing biological normativity. Conditions can differ in their consequences for the viability of an organised living system, creating viability-relative asymmetries between what sustains, disrupts, or threatens continued functioning. APS develops normativity, evaluation, function, and related concepts from this organisational starting point. Their inclusion within the wider APS conceptual system does not, by itself, establish their comparative explanatory superiority or independent empirical validation.
The APS definition of life should therefore be understood according to its present scientific status. It is a current APS framework commitment that organises subsequent analysis of agency, process, scale, viability, constraint closure, and organised persistence. Its conceptual integration within APS is not evidence that competing biological accounts have been displaced, nor does APS currently claim that this definition supplies a universal foundation or general explanatory grammar for biology.
Key Point
Life is viability-oriented, constraint-closed organisation. In APS, this organisation is maintained through ongoing activity that preserves and re-establishes the conditions of organised persistence. Agency, Process, and Scale provide complementary, target-sensitive analytic projections through which aspects of that organisation can be investigated.