For me archaeology is not a source of illustrations for written texts, but an independent source of historical information, with no less value and importance, sometimes more importance, that the written sources.

Social and Economic History of the Roman Empire (1926) and A Social and Economic History of the Hellenistic World (1941) were his pioneering works that transferred the attention of historians from military or political events to global economic or social problems that had been formerly hidden behind their surface. He summed up his vast knowledge on the subject in Iranians and Greeks in South Russia (1922) and Skythien und der Bosporus (1925).