APS distinguishes carefully between definition and diagnosis because these address fundamentally different explanatory questions. A definition specifies what a phenomenon is. In APS, life is defined as viability-oriented organised persistence: an autonomous organisation that actively maintains the conditions of its own continued existence. Definition therefore concerns the nature of life itself and the organisational conditions that make living systems possible.
Diagnosis addresses a different problem. Rather than asking what life is, diagnosis asks how particular systems should be evaluated, identified, and interpreted. Diagnostic inquiry therefore focuses on the organisation that is actually present in a system and examines how that organisation contributes to persistence across time. Questions concerning regulation, perturbation response, organisational coherence, and viability maintenance belong primarily to diagnosis rather than definition.
The distinction becomes especially important when dealing with borderline cases. Viruses, protocells, dormant organisms, synthetic biological constructs, and other ambiguous systems may exhibit some characteristics associated with living organisation without satisfying all the conditions required for life. When definition and diagnosis are conflated, such cases often appear paradoxical because systems are forced into binary categories before their organisational characteristics have been properly analysed.
APS resolves this difficulty by recognising that diagnosis need not begin with classification. Instead, diagnosis investigates the forms of organisation that are present, the extent to which viability is maintained, and the ways in which persistence depends upon internal or external organisational support. Only after these questions have been addressed does classification become meaningful.
Definition therefore concerns what life is, whereas diagnosis concerns how particular systems are evaluated. Maintaining this distinction allows APS to analyse difficult cases rigorously without reducing them to oversimplified alive-or-not-alive judgments.