Contemporary frameworks such as control theory and nonlinear dynamical systems theory provide powerful and indispensable accounts of living systems. They explain how systems regulate their behaviour, maintain stability, and exhibit complex dynamics across time.
APS does not replace these approaches, nor does it reduce them to a single underlying model. Instead, it addresses a different question: what kind of organisation makes a system a living system in the first place.
Multiple theoretical frameworks can therefore apply to the same system, but they do so in different ways and address different explanatory questions. A dynamical systems description may characterise the system’s trajectories, and a control-theoretic model may describe its regulatory structure. These accounts are often non-reducible to one another, and each can be independently valid.
However, such accounts do not by themselves distinguish living systems from non-living systems. Many non-living systems also exhibit regulation, feedback, and complex dynamics.
APS contributes by specifying the organisational conditions under which a system must actively maintain its own viability. A system is viability-oriented when its processes are organised such that their effects contribute, directly or indirectly, to maintaining the conditions required for the system’s continued existence.
Importantly, in APS, viability is not defined by reference to life itself. It is specified in terms of the persistence of an organised system within a bounded region of possible states. A living system is therefore not defined in a circular sense as “that which maintains viability”, but as a system whose organisation is structured around maintaining these conditions.
In this sense, APS provides a way of identifying what makes a system a living system, rather than a general account of everything such a system does.
This does not mean that APS provides a complete or self-sufficient account of living systems. Describing how such systems behave, regulate, and evolve requires additional theoretical frameworks. APS specifies conditions that are necessary for identifying living systems, but it does not by itself exhaust the explanations required to understand them.
A complete scientific understanding of living systems therefore requires multiple forms of explanation. Dynamical and control-theoretic accounts explain how systems behave and regulate. APS clarifies the conditions under which such systems count as living systems at all.
Key Point:
Explaining regulation, dynamics, or behaviour is not the same as explaining what makes a system a living system. APS addresses this latter question by specifying the organisational conditions under which viability is maintained.