Modern historiography emerged in 19th century German universities, where Leopold von Ranke revolutionized historiography with his seminars and critical approach; he emphasized politics and diplomacy, dropping the social and cultural themes Voltaire had highlighted. Sources had to be hard, not speculations and rationalizations. His credo was to write history the way it was. He insisted on primary sources with proven authenticity. Hegel and Marx introduced the concept of spirit and dialectical materialism, respectively, into the study of world historical development. Former historians had focused on cyclical events of the rise and decline of rulers and nations. Process of nationalization of history, as part of national revivals in 19th century, resulted with separation of "one's own" history from common universal history by such way of perceiving, understanding and treating the past that constructed history as history of a nation. A new discipline, sociology, emerged in the late 19th century and analyzed and compared these perspectives on a larger scale.