This priesthood became a closed group, able to control enormous wealth and incomes, and concerned very largely with the study of the solar and astronomical periodicities on which there influence was originally based. With the surplus thus created, the priesthood was able to command human labor in huge amounts and to direct this labor from the simple tillage of the peasant peoples to the diversified and specialized activities that constitute civilized living.

Chapter 7, Mesopotamian Civilization, p. 213 - The Evolution of Civilizations (1961) (Second Edition 1979)