Living systems do not merely sustain ongoing processes. They sustain the constraints that make those processes possible.
Enzymes regulate metabolic reactions while being produced through metabolism itself. Membranes constrain molecular exchange while being generated and repaired by cellular activity. Regulatory systems maintain the organisation upon which their own continued operation depends.
APS refers to this recursive organisation as constraint closure.
Constraint closure does not imply complete isolation from the environment. Living systems remain thermodynamically open and continuously dependent on external conditions and resources. What distinguishes them is that the organisation regulating their activity is maintained from within the system itself rather than being imposed entirely externally.
This distinguishes living systems from many non-living dissipative structures. A flame may sustain combustion under suitable conditions, but it does not actively regenerate the constraints required for its own continued persistence. Living systems, by contrast, continuously repair, reorganise, and regenerate the organisation that sustains them.
Constraint closure therefore provides the organisational basis for self-maintenance, biological agency, and viability-oriented persistence.