A civilization finds its coherence and continuity in remaining faithful to its governing spiritual principles. Continuity means essentially attachment to transcendent and immutable principles, which have formed the basis of all traditional societies. Therefore, to search for continuity in a civilization is to seek these principles along with their applications in the domain of contingency. Regarded in this manner the history of Persia is characterized by both continuity and discontinuity. It is discontinuous because it is marked by two periods, in each of which a distinct set of religious and spiritual principles has been dominant. The first period may be considered to stretch from the time of the migration of the Iranian tribes onto the Iranian plateau to the coming of Islam, and the second from the time of the integration of Persia into the Muslim world to the present day. Moreover, each of these periods is marked by phases in which different spiritual forces have been dominant.