Life is one of the most familiar concepts in biology and one of the most difficult to define. Traditional definitions often identify life through lists of characteristics such as metabolism, growth, reproduction, homeostasis, responsiveness, or evolutionary participation. While these features are important, APS argues that they do not by themselves explain what unifies living systems or why such features matter biologically.
APS approaches the question from a different direction. Rather than beginning with a checklist of traits, it begins with the organisational activity through which living systems continue to exist. Living systems are distinguished not primarily by what they are made of but by what they continually do. They regulate internal conditions, maintain boundaries, repair damage, adapt to changing circumstances, coordinate activity across scales, and reorganise themselves in ways that preserve viability despite continual transformation.
For this reason, APS defines life as viability-oriented organisation sustained through agency across process and scale. Agency identifies the activity through which living systems maintain the conditions required for their continued persistence. Process emphasises that this activity unfolds through continual transformation rather than static stability. Scale recognises that viability is maintained through interacting forms of organisation distributed across biological contexts and timescales.
This perspective helps explain why living systems differ fundamentally from non-living objects. A stone may persist because relatively little changes. A living system persists because change is continually organised. Materials are replaced, structures are renewed, environments fluctuate, and developmental trajectories unfold, yet continuity is maintained through ongoing viability-oriented activity. Life is therefore not simply persistence but organised persistence.
Constraint-closure remains an important part of this account because living systems maintain themselves through networks of mutually supporting constraints that contribute to their own continued existence. APS incorporates this insight within a broader framework centred on agency, viability, process, and scale. Constraint-closure helps explain how living organisation is maintained, while agency explains the activity through which that maintenance occurs.
The concept of life in APS is therefore inseparable from biological normativity. Conditions matter because they affect viability. Activities matter because they contribute to the maintenance or degradation of organised persistence. Living systems continually evaluate and respond to such conditions through their own organisation, giving rise to the normative structure characteristic of biological existence.
Key Point
Life is not defined by a checklist of traits. In APS, life is the organised persistence of viability-oriented agency across process and scale.