In the The Mysteries of Purification (Kitāb asrār al-ṭahāra), the third of the forty books of the Revival of the Religious Sciences (Iḥyāʾ ʿulūm al‐dīn), Abū Ḥāmid al-Ghazālī 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-Ghazālī explains the ḥadīth “Purification is half of faith,” and reminds readers that, for the earliest Muslims, inner purification was much more important than outer purification.