The constitution of the Byzantine Empire was based on the conviction that it was the earthly copy of the Kingdom of Heaven. Just as God ruled in Heaven, so the Emperor, made in his image, should rule on earth and carry out his commandments. This was the theory, but in practice the state was never free from its Roman past, particularly the Roman law, and its heritage of Greek culture. Sir Steven Runciman's Weil lectures trace the various ways in which the Emperor tried to put the theory into practice - and thus the changing relationship between church and state - from the days of the first Constantine to those of the eleventh. The theocratic constitution remained virtually unchanged during those eleven centuries. No other constitution in the Christian era has endured for so long.