The Book of the Lawful & the Unlawful is the 14th chapter of The Revival of the Religious Sciences, which is widely considered as the greatest work of Islamic spirituality. Written by one of the most famous of theologian-mystics of all time, The Book of the Lawful & the Unlawful is unlike other Islamic works concerned with legal issues; here, Abu Hamid al-Ghazali aims to teach his readers firstly the theory of what is lawful and what is unlawful and how to deal with dubious or ambiguous issues, and secondly how to apply the knowledge they have gained to their everyday lives. The main support for this application, especially in matters that are unclear, is caution or prudence, and Ghazali explains the degrees of prudence necessary, beginning with the prudence of the upright and ending with the prudence of the saintly. Thus the sincere practice of what is lawful and desisting from what is unlawful become integrated into one’s spiritual life.