In APS, biological function is present-tense viability-relative organisational contribution.
A process, structure, or activity is functional in the APS sense where its present activity contributes to maintaining or re-establishing conditions relevant to the viability of the living organisation in which it occurs.
Function therefore depends on organisational context. The same mechanism or activity may contribute to viability under one set of conditions while making no contribution, or even undermining viability, under another.
This account does not exclude evolutionary explanations of function. Evolutionary history may explain why a trait exists, how it was selected, or why a capacity has been retained. APS addresses a distinct question: what contribution does the process, structure, or activity make to the viability of the organisation in the present explanatory context?
Organised persistence may be one explanandum to which functional contributions are relevant, because it concerns how living organisation maintains continuity through material and structural change. It does not define function or provide the principle from which function derives.
Key Point: In APS, function is present-tense viability-relative organisational contribution. Its explanatory relevance must be established for the particular system and explanandum rather than inferred from APS terminology alone.