Borderline cases are often taken to show that life cannot be clearly defined. In APS, they do not undermine the definition of life but test its explanatory adequacy. The question is not whether such systems possess particular traits, but whether they exhibit viability-oriented organisation.
Viruses, for example, do not sustain their own persistence independently. Their activity depends on host systems that provide the organisational conditions required for replication, and they therefore exhibit dependent rather than fully realised organisation. Sterile organisms, by contrast, may lack reproductive capacity but maintain themselves through internally organised processes and therefore remain fully within the domain of life.
Artificial systems present a different case. Some exhibit complex or adaptive behaviour, but unless they establish constraint-closed organisation through which they sustain their own persistence, they do not meet the definition of life.
Borderline cases therefore clarify the boundary of life by revealing where viability-oriented organisation is present, absent, or dependent, rather than exposing a failure of definition.
Key Point. Borderline cases do not weaken the definition of life—they reveal how viability-oriented organisation distinguishes living, dependent, and non-living systems.