Borderline cases are often taken to show that life cannot be clearly defined. In APS, they provide demanding cases against which the applicability and explanatory adequacy of a definition of life can be assessed. The relevant question is not simply whether such systems possess particular traits, but what organisation they materially realise and whether that organisation warrants classification as viability-oriented and constraint-closed.
Viruses, for example, depend extensively on host organisation for replication and continued propagation. This dependence makes them an important boundary case for APS, but dependence alone does not settle their living status. Their classification requires examination of what organisation is realised by the virus–host relation and what conclusions the available evidence supports.
Sterile organisms illustrate a different point. Reproductive capacity is not by itself necessary for living status. An organism may be unable to reproduce while continuing to maintain the viability-oriented, constraint-closed organisation characteristic of its ongoing life.
Artificial systems provide another important test. Artificial origin does not determine biological status. An engineered system should be assessed by the same organisational criteria as any other candidate system: what matters is the organisation materially realised, including whether the system itself maintains and re-establishes the constraints and conditions required for its continued functioning. External dependence is likewise not automatically disqualifying, since living systems also depend upon environmental conditions and resources.
Borderline cases therefore test how the APS definition of life applies where biological status is scientifically difficult to determine. They can clarify which organisational dependencies matter to a proposed boundary without establishing that viability orientation or constraint closure provides a uniquely APS or comparator-superior discriminator.
Key Point. Borderline cases test the APS definition of life by asking what organisation is materially realised and what classification that organisation warrants; they should not be treated as demonstrating in advance that APS provides a uniquely decisive boundary between living and non-living systems.