The Book of the Lawful and Unlawful is the fourteenth 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 theologian-mystics of all time, The Book of the Lawful and Unlawful is unlike other Islamic works concerned with legal issues. Here, Abu Hamid al-Ghazali aims to teach his readers firstly the the theory of what is lawful and unlawful and how to deal with dubious and ambiguous issues, and secondly how to apply the knowledge 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 unlawful become integrated into one's spiritual life. As in his other works, Ghazali bases himself on the Quran and the narrations from the Prophet Muhammad (s), followed by examples from the Companions, the Successor generations and the pious predecessors.