In classical functionalist accounts, two systems may count as equivalent if they perform sufficiently similar causal or computational roles. APS argues that this criterion is insufficient for biological explanation because it abstracts away from the organisational conditions that make living activity possible.
In APS, realization is not understood primarily as abstract input-output equivalence. Rather, realization concerns participation within viability-oriented, constraint-closed organisation.
Biological systems are not merely collections of mechanisms performing isolated functions. They are temporally extended organisations whose processes contribute to sustaining the conditions of their own persistence. Functions therefore derive explanatory significance from their role within organised self-maintaining systems.
For this reason, materially distinct systems may realise comparable biological capacities only insofar as they participate in sufficiently similar forms of persistence-maintaining organisation. What unifies realizations is not identical material structure, nor abstract computation alone, but organisational participation within living systems.
APS therefore rejects both strict substrate essentialism and unrestricted substrate neutrality.
A system does not become biologically equivalent merely because it reproduces selected behavioural outputs or formal causal patterns. A computational simulation of metabolism, for example, does not thereby become metabolically alive, and a system that reproduces selected behavioural features associated with cognition does not necessarily instantiate biological cognition unless those activities contribute to the persistence-maintaining organisation of the system itself.
APS consequently treats realization as organisationally constrained. Biological capacities may be implemented through diverse mechanisms and structures, but those realizations remain governed by the requirements of viability, persistence, and constraint closure.