Al-Ghazali's Book of the Lawful and the 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 of theologian-mystics of all time, The Book of the Lawful and the Unlawful is unlike other Islamic works concerned with legal issues.wara) 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. As in his other works, Ghazali bases himself on the Qur'an and narrations from the Prophet Muhammad, followed by examples from the Companions, the Successor generations and the pious Predecessors.