In The Mysteries of the Prayer and Its Important Elements , book four of the 40 books of the Revival of the Religious Sciences, Abu Hamid al-Ghazali explains the inner and outer meanings of the prayer, a central practice for Muslims. He explains the acts involved in the outward elements of the prayer and outlines which of these acts are obligatory and which are sunna. He then discusses the inward requisites of the prayer--the practices of the heart--...