In The Mysteries of Purification (Kitab asrar al tahara); the third of the forty books of the Revival of the Religious Sciences (Ihya' 'ulum al-din); Abu Hamid al-Ghazali explains the fundamentals of the purification that is necessary in order to perform the five daily prayers. The book begins with an introduction to the general topic of purity. Al-Ghazali explains the hadith “Purification is half of faith;† and reminds readers that; for the earliest Muslims; inner purification was much more important than outer purification. In chapter 1; he considers purification from impurities: What are the impurities and how should they be removed? Over the following chapters; al-Ghazali examines the lesser ablution (wudu); the greater ablution (ghusl); and the “dry ablution† (ablution without water; or tayammum). He discusses the etiquette of the bathhouse and outlines exactly how to clean the body of external things and natural growths; finally; he discusses the beard and practices related to it.