Any ontological theory commits us to accept and classify a number of phenomena in a more or less specific way – and vice versa: a classification tends to reveal the theoretical outlook of its creator. Objects and their descriptions and relations are not just “given” but determined by theories. Knowledge is fallible and consensus is rare. By implication, knowledge organization has to consider different theories/views and their foundations. Bibliographical classifications depend on subject knowledge and on the same theories as corresponding scientific and scholarly classifications. Some classifications are based on logical distinctions, others on empirical examinations, and some on mappings of common ancestors or on establishing functional criteria. To evaluate a classification is to involve oneself in the research which has produced the given classification. Because research is always based more or less on specific epistemological ideals (e.g. empiricism, rationalism, historicism or pragmatism), the evaluation of classification includes the evaluation of the epistemological foundations of the research on which given classifications have been based. The field of knowledge organization itself is based on different approaches and traditions such as user-based and cognitive views, facet-analytical views, numeric taxonomic approaches, bibliometrics and domain-analytic approaches. These approaches and traditions are again connected to epistemological views, which have to be considered. Only the domain-analytic view is fully committed to exploring knowledge organization in the light of subject knowledge and substantial scholarly theories.