Classification Without Reduction: The Epistemology of Clinical Taxonomy Examined Through TCM Constitution Theory.

RATIONALE, AIMS AND OBJECTIVES: Clinical classifications operate at different explanatory levels-from molecular subtypes to phenotypic syndromes-yet the epistemological standing of a classification is often treated as though it depends on which level it occupies. This conflation of explanatory level with epistemological standing is unfounded. This essay identifies the minimal epistemological conditions that any clinical classification must satisfy and examines whether these conditions depend on explanatory level. METHODS: Philosophical analysis drawing on natural kinds theory (Boyd's homoeostatic property cluster theory), operationalism (Bridgman) and interventionist causation (Woodward). Traditional Chinese Medicine (TCM) constitution theory, developed by Wang Qi over four decades, is used as a developing top-down case study whose three core tenets-classifiable, measurable and modifiable-map onto the three philosophical frameworks. RESULTS: Three conditions were identified as necessary for a clinical classification to be scientifically tractable: ontologically grounded categories, operationalized measurement and testable causal propositions. These conditions are level-independent: they can be met by bottom-up molecular taxonomies and top-down phenotypic classifications alike. The TCM constitution case demonstrates uneven progress across the three conditions-operationalization is relatively advanced, ontological grounding has partial empirical support and causal validation remains nascent-illustrating the framework's utility as a diagnostic tool for assessing research programme maturity. CONCLUSION: The epistemological standing of a clinical classification depends not on the explanatory level at which it operates, but on whether it satisfies the three identified conditions. This analysis is consistent with explanatory pluralism: molecular investigation and phenotypic classification address different questions along different axes, and evidence-based medicine's evaluative framework applies to both without prejudice.