Burgeoning acquisition of information about the workings, scope and diversities of the cosmos put serious pressure on 19th-century European intellectuals to classify branches of human knowledge. A challenge presented itself not only to order different subject-areas and disciplines intelligently, or assess them according to apparent degrees of certitude, but also to discover some synthesizing principle by which all the distinctive methods of approaching the world might be viewed in interrelationship. This review shows that such endeavours to classify and unify were traditional procedures, with deep roots going back to antiquity, and they brought coherence to academic programmes through the centuries. As a mark of European modernity, there was a tendency to establish more rational, scientific and secular principles of order, and the consequent tensions between positivistic and holistic styles of approach to science have continued since. Since the beginning of the twentieth century, it is also recognized, the constant subdividing of academic agendas has made the work of classification much less manageable and attractive. If traditional principles to express the unity of knowledge were philosophical, or, in the case of the medieval universities, evoked the oneness of the divine Creation, it is intriguing how planetary survivalism in the present time has pushed environmental science centre-stage as a pivotal point of activity encouraging interdisciplinary collaboration. If varying consideration has been granted to practical subjects (for example agriculture) in the history of knowledge classification, their importance has been clarified by current biospheric predicaments.