Sufism is the esoteric aspect of Islam. Its purpose is to convey direct knowledge of the eternal. The Sufis impart knowledge through lineages that go back to the Prophet Muhammud. In these various Sufi orders, the zikr, the repetition of "la illaha illa'llah" (There is no God but God), is part of initiation ceremonies. In fact, the method of the Sufis is zikr, and the manner in which zikr is performed is the essential difference among the various orders. The Dervishes repeat their zikr as they turn. They empty their hearts of all but the thought of God and whirl in the ecstatic movements of His breath.