The concept of historical philosophy developed by Spengler is founded upon two assumptions: the existence of social entities called 'Cultures' ("Kulturen"), regarded as the largest possible actors in human history, which itself had no metaphysical sense, and the parallelism between the evolution of those Cultures and the evolution of living beings. Spengler numbered nine Cultures: Egyptian, Babylonian, Indian, Chinese, Greco-Roman, 'Magic' or 'Arabic' (including early and Byzantine Christianity and Islam), Mexican, Western, and Russian–, interacted between each other in time and space but were distinct from each other by 'internal' attributes. According to him, "Cultures are organisms, and world-history is their collective biography."